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Rev
iew
Anew
eraforliq
uid
crystalresea
rch:Applic
ationsof
liquid
crystals
insoftmatter
nan
o-,b
io-an
dmicrotech
nolog
y
JanP.F.
Lage
rwalla
,b, *,G
iusy
Scalia
a,b, *
aSeou
lNationa
lUniversity,
Gradu
ateScho
olof
Conv
ergenc
eScienc
e&
Tech
nology,8
64-1
Iui-do
ng,Y
eong
tong
-gu,
Suwon
-si,Gyeon
ggi-do
443-27
0,Rep
ublic
ofKorea
bAdv
ancedInstitutes
ofCo
nvergenc
eTech
nologies,D
epartm
entof
Nan
oscien
ce&
Tech
nology,8
64-1
Iui-do
ng,Y
eong
tong
-gu,
Suwon
-si,Gyeon
ggi-do
443-27
0,Rep
ublic
ofKorea
ar
ticle
in
fo
Article
history:
Received
14Fe
bruary20
12Accep
ted13
March
2012
Ava
ilableon
line30
March
2012
Keywords:
Liqu
idcrystal
Lyotropic
Thermotropic
Nan
otechnolog
ySo
ftmatter
Com
posites
Colloids
Biomem
bran
es
abstr
act
Liqu
idcrystals
constitute
afascinatingclassof
soft
conden
sedmatterch
aracterize
dby
theco
unterin-
tuitiveco
mbinationof
fluidityan
dlong-range
order.T
oday
they
arebe
stkn
ownfortheirex
ception
ally
successfula
pplication
inflat
pan
eldisplays,b
utthey
actually
exhibitaplethoraof
uniquean
dattractive
properties
that
offertrem
endou
spoten
tial
forfundam
entalscience
aswellas
innov
ativeap
plic
ations
wellbe
yond
therealm
ofdisplays.To
day
this
full
brea
dth
oftheliq
uid
crystallinestateof
matteris
beco
mingincrea
singlyreco
gnized
andnumerou
snew
andex
citinglin
esof
research
arebe
ingop
ened
up.
Wereview
this
excitingdev
elop
men
t,focu
singprimarily
onthephy
sics
aspects
ofthenew
research
thrusts,
inwhich
liquid
crystals
ethermotropic
aswellas
lyotropic
eoftenmee
tother
types
ofsoft
matter,su
chas
polym
ersan
dco
lloidal
nan
o-or
microparticledispersion
s.Becau
sethefieldis
oflarge
interest
also
forresearch
erswithou
taliq
uid
crystalb
ackg
roundwebe
ginwithaco
ncise
introd
uctionto
the
liquid
crystalline
state
ofmatteran
dthe
key
concepts
ofthe
research
field.W
ethen
discu
ssaselectionof
promisingnew
direction
s,startingwithliq
uid
crystals
fororga
nic
electron
ics,follo
wed
bynan
otem
platingan
dnan
oparticleorga
nization
usingliq
uid
crystals,liq
uid
crystalco
lloids(w
herethe
liquid
crystalc
anco
nstitute
either
theco
ntinuou
sphaseor
thedisperse
phase,
asdroplets
orsh
ells)an
dtheirpoten
tial
ine.g.
photon
icsan
dmetam
aterials,liq
uid
crystal-functionalized
polym
erfibe
rs,liq
uid
crystalelastomer
actuators,
ending
with
abriefov
erview
ofactivities
focu
sing
onliq
uid
crystals
inbiolog
y,food
science
andpharmacolog
y.�
2012
Elsevier
B.V.A
llrigh
tsreserved
.
Contents
1.Introd
uction
.................................................................
....
....
....
....
....
....
....
....
....
....
....
....
....
.138
82.
Liqu
idcrystals
inanu
tshe
ll........................................................
....
....
....
....
....
....
....
....
....
....
....
....13
882.1.
Twoliq
uidcrystalclasses,
man
yliq
uidcrystalph
ases
..........................................
....
....
....
....
....
....
....
....
..13
882.2.
Thean
isotropicph
ysical
prop
erties
ofliq
uidcrystals
........................................
....
....
....
....
....
....
....
....
...1
390
2.3.
Deformations
andsing
ularitiesin
thene
matic
director
field
.......................................
....
....
....
....
....
....
....
..13
913.
New
directions
forap
pliedan
dfund
amen
talliq
uidcrystalscienc
e......................................
....
....
....
....
....
....
....
....13
933.1.
Liqu
idcrystals
fororga
nicelectron
icsan
den
ergy
conv
ersion
applications
..............................
....
....
....
....
....
....
...1
393
3.2.
Liqu
idcrystaltemplatingforcrea
ting
nano
structured
materials
ororde
ring
nano
-an
dmicropa
rticles..................
....
....
....
...1
396
3.2.1.
Solid
ifyingliq
uidcrystalph
ases
forcrea
ting
regu
larna
nopo
rous
solid
s............................
....
....
....
....
....
....
1396
3.2.2.
Synthe
sizing
nano
particlesin
liquidcrystalline
templates
....................................
....
....
....
....
....
....
....
1397
3.2.3.
Collo
idal
particlesorga
nize
dby
liquidcrystals
andliq
uidcrystalph
ases
from
collo
ids
....................
....
....
....
....
..13
973.3.
Compo
site
materials
containingthermotropicliq
uidcrystals
asfunc
tion
alad
ditive
............................
....
....
....
....
....
..14
003.3.1.
Emulsion
sof
cholestericdrop
lets
fortuna
bleom
nidirectiona
llasing
applications
.........................
....
....
....
....
..14
003.3.2.
Liqu
idcrystalline
shells
.....................................................
....
....
....
....
....
....
....
....
....
....
.140
13.3.3.
Liqu
idcrystal-filledelectrospu
nmicro-/na
nofib
ers
.......................................
....
....
....
....
....
....
....
...1
403
*Corresp
onding
authors.
Seou
lNational
University,Graduate
Schoo
lof
Con
vergen
ceScience
&Te
chnolog
y,86
4-1
Iui-don
g,Ye
ongton
g-gu
,Su
won
-si,
Gye
ongg
i-do
443-27
0,Re
public
ofKorea
.E-mailad
dresses:
jan.la
gerw
all@
lcsoftmatter.c
om(J.P.F.L
agerwall),g
iusy.scalia
@solcan
ta.com
(G.S
calia
).URL:
http://w
ww.snm.lcsoftmatter.c
om,h
ttp://w
ww.solcanta.com
Contents
lists
available
atSciVerseScienceDirect
Curren
tApplie
dPh
ysics
journalhom
epage:www.e
lsevie
r.c
om
/locate
/cap
1567
-173
9/$e
seefron
tmatter�
2012
Elsevier
B.V.A
llrigh
tsreserved
.doi:10.10
16/j.cap
.201
2.03
.019
Curren
tApp
liedPh
ysics12
(201
2)13
87e14
12
3.4.
Actua
tors
mad
efrom
liquidcrystalelastomers
.............................................
....
....
....
....
....
....
....
....
....
1404
3.5.
Liqu
idcrystals
inbiolog
y,biotechn
olog
y,food
scienc
ean
dph
armacolog
y.................................
....
....
....
....
....
....
.140
53.5.1.
Biod
etection
withliq
uidcrystals
................................................
....
....
....
....
....
....
....
....
....
...1
405
3.5.2.
Drug/nu
trient
deliv
erywithliq
uidcrystalcu
bosomes
andhe
xosomes
orliq
uidcrystalline
crem
es.................
....
....
...1
405
3.5.3.
Lipido
micsan
dtheliq
uidcrystalph
ases
ofcellmem
bran
es..................................
....
....
....
....
....
....
....
1406
4.Co
nclusion
san
dou
tloo
k...........................................................
....
....
....
....
....
....
....
....
....
....
....
....14
07Ack
nowledg
men
ts............................................................
....
....
....
....
....
....
....
....
....
....
....
....
....
1407
Referenc
es................................................................
....
....
....
....
....
....
....
....
....
....
....
....
....
...1
407
1.Introduction
Itis
almost12
5ye
arssince
thePrag
uescientist
Friedrich
Rein-
itze
rin18
88ob
served
acu
riou
sbeh
aviorw
ithdou
blemeltingpoints
ofch
olesterolb
enzo
ate,adisco
very
that
today
iswidelyreco
gnized
asthebirthof
liquid
crystalscien
ce.Thedisco
very
andthenew
area
ofresearch
that
ittrigge
red,en
gaging
phy
sicistsan
dch
emists
arou
ndtheworld,rem
ained
ofalmostpure
acad
emic
interest
for
some75
years,untiltheidea
occu
rred
toutiliz
ethesimplest
liquid
crystalp
hasee
thene
matic
ein
displaydev
ices
[1].Th
eim
pactof
thisidea
was
hardly
clea
rat
thetime,bu
ttoday
wecanlook
back
atthedev
elop
men
tofliquid
crystald
isplays
(LCDs)
andco
ncludethat
this
applic
ationof
thecu
riou
smaterials
that
liquid
crystals
consti-
tute
hav
een
tirely
revo
lution
ized
ours
ociety.Theex
plosive
progress
indisplaydesignan
dco
nstructionhav
emad
eold-style
cathod
eray
tube
displays
dinosau
rs,an
dit
has
mad
epossibleanumbe
rof
entirely
new
dev
ices
such
assm
artphon
es,laptop
and
tablet
computers,d
igital
projectorsan
ddyn
amic
inform
ationbillb
oards.
When
thepromiseof
liquid
crystald
isplays
becameclea
rin
the
1970
sthisstim
ulatedgrea
tactivity
amon
gacad
emican
dindustrial
research
ersalike,
andforqu
itealongtimethis
singleap
plic
ation
stronglydom
inated
theinternationalresearch
effortsin
thefield.A
saresu
ltof
this
longen
gage
men
t,stim
ulated
bytheco
mmercial
success,liq
uid
crystald
isplaytech
nolog
yistoda
yso
adva
ncedthat
theco
ntinued
research
anddev
elop
men
tisco
nducted
primarily
inindustry
labs
.Academ
icliq
uid
crystalscien
cehas,inco
ntrast,during
thelast
few
yearssh
ownaclea
rtren
dof
mov
ingaw
ayfrom
display
research
andinstea
dfocu
singon
topicsthat
canbe
very
different
andin
man
yresp
ects
even
morestim
ulating[2],forinstan
cein
new
usesin
optics,nan
o-/m
icro
man
ipulation
,nov
elco
mposites
and
biotechnolog
y.Th
eacad
emic
research
field
ispresently
experi-
encing
ave
rycrea
tive
and
explorative
phase,
giving
the
field
arebirthinto
ametam
orphosed
statein
whichother
uniqueaspects
ofliq
uid
crystalsarebrou
ghtforw
ardan
dwherethecu
riou
sliq
uid
crystallinestates
ofmatterarestudiedin
new
contexts.
Intheiren
counterwithother
very
releva
ntfieldsof
softmatter
science,mostco
nsp
icuou
sly
the
science
ofpolym
ers,
collo
ids,
nan
otechnolog
yan
dbiotechnolog
y,highly
interestingan
dattrac-
tive
research
fieldsareem
erging,man
ywithco
nsiderab
lepoten
tial
forap
plic
ationsof
entirely
new
kinds.
Theuniquesu
btle
balance
betw
eenorder
andfluiditythat
characterize
sliq
uid
crystals
isnot
only
dee
ply
fascinatingfrom
ascientificpointof
view
,butitgive
srise
toabroa
drange
ofsp
ectacu
larphen
omen
athat
arefarfrom
fully
explored.Itistheaim
ofthisreview
articleto
give
asn
apsh
otof
theex
citingdev
elop
men
tthat
thefieldiscu
rren
tlyex
periencing,
with
anem
phasis
onthe
phy
sics
and
materials
science
issu
esinvo
lved
.Th
erecentdev
elop
men
tsof
liquid
crystalch
emistry
(which
are
also
very
impressive
and
innov
ative)
hav
ebe
enreview
edelsewhere,
seee.g.
[3e12
].Wehav
etriedto
give
abird’s
eyeview
ofthemostex
citingcu
rren
tactivities
inliq
uid
crystal-
relatednan
o-,m
icro-an
dbiotechnolog
ynot
relatedto
displays
orsimila
relectroo
ptic
dev
ices,p
rovidingalso
acritical
analysisof
the
keyou
tstandingch
allenge
sin
each
field.S
ince
thetopicsco
vered
are
highly
interdisciplin
ary
and
thusof
interest
also
toread
ers
outside
the
liquid
crystalco
mmunity
we
begin
with
ave
ryco
nden
sedintrod
uction
toliq
uid
crystals
withex
planationsof
the
keyco
ncepts(an
extended
version
can
befound
inon
eof
the
authors’
hab
ilitation
thesis
[13]),allowingalso
non
-liquid
crystal
experts
toen
joythescience
described
inthefollo
wingch
apters.
2.Liquid
crystals
inanutshell
2.1.
Twoliq
uidcrystalclasses,man
yliq
uidcrystalph
ases
What
mak
esliq
uid
crystals
uniqueis
that
they
arefluids,
yet
they
exhibitlong-range
orde
r:either
theorientation
orposition,o
rbo
th,of
the
phase
build
ing
bloc
ksare
correlated
over
along
distance.In
the
nem
atic
phase
(abb
reviated
N)the
long-range
order
isof
purely
orientation
altype:
the
anisom
etric
build
ing
bloc
ks(often
rodsor
discs)arerough
lyorientedalon
gon
ean
dthe
same
direction
,referred
toas
the
director
(abb
reviated
n),
asillustratedin
Fig.
1.In
theab
sence
ofaligningforces
thedirector
varies
smoo
thly
through
outa
nem
atic
sample
withou
tdiscrete
chan
ges(exc
eptin
defects),hen
ceon
eoftensp
eaks
ofthedirector
fieldn(r),whereris
thesp
aceco
ordinate.
Two
main
classesof
liquid
crystalex
ist.
Thermotropicliq
uid
crystals
are
built
up
byindividual
molecules
and
no
further
Fig.
1.Ca
rtoo
nof
ane
matic
phasebu
iltfrom
rod-sh
aped
build
ingbloc
ks,d
rawnwith
ave
rtical
director
n.F
rom
[13].
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1388
molecularsp
ecies(specifically
solven
tmolecules)
arerequ
ired
for
theliq
uid
crystalp
haseform
ation.Tem
perature
isthefundam
ental
thermod
ynam
icco
ntrol
param
eter
determiningthephase,thetw
oke
ytemperature
values
beingthosedefi
ningthebe
ginningan
den
dof
liquid
crystallineorder,themelting
pointT m
from
thecrystalline
solid
andtheclea
ring
pointT c
into
anisotropic
liquid.T
heclea
ring
pointow
esitsnam
eto
thefact
that
abu
lkliq
uid
crystalscatters
light,wherea
stheisotropic
liquid
isclea
r.Becau
seof
theirinter-
med
iate
state
betw
een
solid
san
dliq
uids,
liquid
crystals
are
sometim
esreferred
toas
mesom
orph
icsystem
san
dmoleculesor
other
entitites(nan
oparticles,m
olecule
aggreg
ates,.
)capab
leof
form
ing
liquid
crystalphases
arecalle
dmesog
ensor
said
tobe
mesog
enic.In
addition
toan
isom
etry,mesog
enicityam
ongther-
motropic
moleculeshas
itsorigin
inaco
mbinationof
astiffco
re(often
alin
earor
circulararrange
men
tof
arom
atic
rings)an
dflex
ible
term
inal
chains,
generally
aliphatic
hydrocarbon
s,cf.the
exam
plesin
Fig.
2.Lyotropicliq
uid
crystals
form
only
upon
additionof
asolven
t,mostoftenwater.M
oreo
ver,thebu
ildingbloc
kof
alyotropic
phase
isoftennot
onebu
tman
ymolecules(typ
ically
ontheorder
of10
0),
orga
nized
into
anag
greg
atecalle
damicelle.T
hemicelle
form
ation
isaresu
ltof
theam
phiphilicch
aractero
ftheco
nstituen
tmolecules,
generally
surfactants,cf.thean
-,cat-
and
non
ionic
exam
plesin
Fig.
2def,
resp
ective
ly.Micelles,
inwhich
the
amphiphile
sare
orga
nized
with
thenon
-polar
tails
inwards,
thusprotected
from
water
contact
bytheou
twards-directed
polar
hea
dgrou
ps,start
form
ing
beyo
nd
alim
iting
amphiphile
concentration
calle
dthe
Critical
Micelle
Conc
entration,
abbrev
iatedCM
C.Ty
pical
CMCva
lues
liein
the1e
10mM
range
(abo
ut0.01
e0.1wt.%).Th
eCMCdoe
snot
constitute
theon
seto
fliquid
crystallineorder:themicellarphasein
thevicinityof
theCMCan
dqu
iteasu
bstantial
regimebe
yondis
isotropic
inallproperties.Another
very
importantex
ample
of
amphiphile
sform
ing
liquid
crystals,
althou
ghge
nerally
not
micelles,
isgive
nby
theam
phiphiliclip
ids(e.g.phospho-,glyco-
andsp
hingo
lipids)
that
build
upthemem
bran
esof
thecells
inou
row
nbo
dies.
The
prototypic
phospholipid
DMPC
issh
own
asex
ample
inFig.
2g.Alsoam
phiphlic
bloc
kco
-polym
erscan
form
liquid
crystalp
hases.
Lyotropic
liquid
crystals
can
form
also
byan
isom
etric
non
-am
phiphilicmacromoleculesor
particles
such
asvirusesor
inor-
ganic
rodsor
discs
inco
lloidal
susp
ension
.Th
issu
bclass
oflyo-
trop
icsis
attracting
increa
sing
interest
today
[14e
21].
Polymers
that
form
liquid
crystals
ofthis
typemay
besynthetic
such
ascarbon
nan
otube
sor
thepolya
ramidech
ainsthat
areprocessed
via
aliq
uid
crystallinestateinto
Kev
lar�
fibe
rs,or
they
may
bebio-
logical,e.g.
DNA.
Thenem
aticisthesimplest
liquid
crystalp
hase,ex
hibitinglong-
range
orientation
albu
tnopositional
order.Ifw
ead
d1D
positional
orde
r,i.e
.aperiodicdistribution
ofthemoleculesalon
gon
esp
ecific
direction
,wege
ttheso-calledsm
ectic(thermotropic)or
lamellar
(lyo
trop
ic)phases,w
herethemolecules(typ
ically
rod-shap
ed)are
orga
nized
into
laye
rs.Insm
ectics
thedeg
reeof
positional
order
isstill
not
very
highan
dmoleculesfreq
uen
tlymov
efrom
onelaye
rto
the
nex
t,bu
tin
the
lamellarphases
the
amphiphile
sbu
ildup
bilaye
rsthat
areseparated
byan
aqueo
usregion
that
canbe
much
thicke
rthan
thebilaye
r,mak
ingthelaye
ringmuch
moredistinct.
Within
thelaye
rsthemoleculesmay
beon
theav
erag
eperpen
-dicularto
thelaye
rs(w
ecallthis
asm
ectic-A,or
SmA,statefor
thermotropics,an
dan
L astateforlamellarlyotropics)
orthey
can
beinclined
withacertainan
gle(thermotropic
SmCor
lyotropic
L0 b;
thedifference
betw
eenL a
andL0 b
how
ever
goes
farbe
yondtheissu
eof
molecule
tilt[13]).
Thenex
tstep
inlong-range
positional
orde
rin
liquid
crystal
phases
isen
countered
amon
gthe
columnar
phases,where
the
Fig.
2.So
metypicalrod-sh
aped
thermotropicmesog
ens(aeb),a
classicdiscotic
thermotropicmesog
en(c)an
dfour
common
lyoc
curringam
phiphiles(deg).
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1389
build
ingbloc
ksarepositioned
ona2D
lattice,
butwithou
tlong-
range
positional
correlationin
thethirddirection
.Inthethermo-
trop
iccase
the
columnar
orde
ris
typically
form
edby
disco
tic
mesog
ens,thearom
atic
coresstackingupas
inpile
sof
coins,ea
chpile
beinglocatedon
apointof
alatticethat
isoftenhex
agon
al.In
thelyotropic
columnar
phases
wehav
e‘in
finitely’
longrod-shap
edmicellespositionally
correlated
ona2D
latticein
aplaneperpen
-dicularto
therods.
Ifwead
dch
iral
moleculesto
aliq
uid
crystalphase,
either
asnon
-mesog
enic
dop
ants
orby
usingch
iral
mesog
ens,
themacro-
scop
icliq
uid
crystalphaseis
generally
also
chiral.T
hepresence
ofstereo
isom
erically
enrich
edch
iral
moleculesis
forthermotropic
liquid
crystalsge
nerally
indicated
byad
dingastar
tothesh
orthan
dof
theco
rrespon
dingachiral
phase,
e.g.
N*forach
iral
nem
atic.In
lyotropicsthesameindicationis
usedforch
iral
nem
aticsbu
tthe
conve
ntion
has
not
been
adop
tedforlamellaran
dco
lumnar
phases.
The
best
studied
chiral
liqu
idcrystalis
the
chiral
nem
atic
phase,
oftenreferred
toas
thech
olestericphase.
Thech
iralityhere
expresses
itself
asahelical
mod
ulation
ofthedirec
toralon
gan
axis
perpen
dicularto
n,cf.Fig.
3.In
smec
ticphases,ahelical
directormod
ulation
perpen
dicularto
nwou
ldbrea
kthelaye
rsan
disthusnot
normally
admitted(anex
ception
istherare
classof
twist-grainbo
undaryphases).Th
eSm
A*phasethereforehas
the
samestru
cture
asan
achiral
SmAphase,bu
tits
phy
sicalp
roperties
arequ
itedifferentfrom
itsachiral
analog
[22].Inco
ntrasttoSm
A*,
thech
iral
SmC*phasedoe
sallow
ahelical
superstru
cture,b
utit
applies
tothedirec
tion
alon
gwhichthemolec
ulestiltaw
ayfrom
thelaye
rnormal.Inother
words,nin
SmC*doe
snot
twista
sin
N*,
butthetiltingdirec
tion
precesses
helically
alon
gthesm
ecticlaye
rnormal.T
heSm
C*phaseis
differentfrom
itsnon
-chiral
analog
,as
wellas
from
anynem
atic
phase,
also
inthat
itex
hibitsasp
onta-
neo
uspolarization
and
can
thussh
owferroe
lectric
switch
ing
properties
[22].
Theperiodicityof
thehelix
(thepitch
p)canbe
anyw
herefrom
afew
hundrednan
ometersto
infinity,
forch
olesterics
aswellas
SmC*-typephases.Ifitis
shortthephy
sicalproperties
canch
ange
dramatically
compared
tothe
non
-chiral
phases
[23].
The
mechan
ical
prope
rtiesof
ash
ort-pitch
N*phasealon
gthehelix
can
startresemblingthoseof
awea
ksolid
andforpon
theorde
rof
visible
lightwav
elen
gthsthe
periodicity
lead
sto
visible
Bragg
scattering,
from
N*as
wellas
from
SmC*,
referred
toas
selective
reflection
.Thesech
iral
liquid
crystals
then
beco
meself-assem
bled
photon
iccrystalswithaba
ndga
pthat
canbe
tuned
fairly
easily,e.g.
bytemperature
variation.
2.2.
Thean
isotropicph
ysical
prop
erties
ofliq
uidcrystals
Themacroscop
ican
isotropy
ofliq
uid
crystals,a
property
shared
with
no
other
fluid,is
theba
sisfortheirsu
ccessfulco
mmercial
exploitation.T
hean
isom
etry
ofthebu
ildingbloc
kstoge
ther
with
thelong-range
orientation
alorde
rge
nerally
render
liquid
crystals
optically
anisotropic,i.e
.theirrefractive
index
n kparallelto
nis
differentfrom
that
perpen
dicularto
thedirector,n t
.Fo
ranon
-ch
iral
uniaxial
material,thesetw
oindices
aretheex
trem
erefrac-
tive
index
values
andthedirectoriseq
uivalen
ttotheop
ticax
isof
the
liquid
crystal(pleasereferto
moresp
ecializ
edtexts,e.g.[23],for
the
specialcase
ofch
iral
liquid
crystals
with
short-pitch
director
mod
ulation
).Th
ebirefringe
nce
ofthe
material
isthen
Dn
¼n k
�n t
.Con
sequ
ently,ifn k
isthemax
imum
refractive
index
,Dn>
0an
dwecalltheliq
uid
crystalpositiveuniaxial
(this
isthe
generalcase
forrod
-shap
edmesog
ens),w
herea
sifitistheminim
um
thephaseisneg
ativeuniaxial(thisisthestan
dardcase
ford
isco
tics).
Since
the
effect
onpolarized
lightof
abirefringe
ntmaterial
dep
endscritically
ontheorientation
oftheop
ticax
isitisim
portant
tosp
ecify
how
aliq
uid
crystalis
aligned
with
resp
ectto
the
direction
oflig
htp
ropag
ation.A
dev
icein
whichtheliq
uid
crystalis
usedforitsan
isotropicop
ticalp
roperties,e.g.a
display,isge
nerally
athin
flat
film
,referred
toas
a‘cell’in
thege
neral
case.In
the
stan
dardco
nfigu
ration
lighten
ters
perpen
dicularto
thecellplane.
When
nis
aligned
perpen
dicularto
the
cell
the
lightray
thus
trav
ersesthecellalon
gtheop
ticax
is,lea
dingto
noeffectivebire-
fringe
nce
(any
polarization
ofthelig
htex
periencestheordinary
refractive
index
)andthisdirectoralignmen
tisthereforereferred
toas
homeo
trop
ic,aterm
that
can
beunderstoo
das
mea
ning
like
isotropic.Ifnis
instea
dperpen
dicularto
thecellplanewecallthis
planar
geom
etry.Atnormal
inciden
ceto
thecell,
lightwill
now
Fig.
3.Ca
rtoo
nsof
helic
aldirector
mod
ulationin
thech
iral
nematic
(cho
lesteric)ph
asewithrod-
anddisc-sha
pedbu
ildingbloc
ks,respe
ctively.
Each
rodor
disc
represen
tseither
amesog
en(the
rmotropics)or
micelle
(lyo
trop
ics)
withtheco
rrespo
ndingsh
ape.
Notethat,for
clarity,thepitchha
sbe
endraw
norde
rsof
mag
nitude
smallerthan
inrealityan
dthe
localde
gree
oforde
ris
muc
htoohigh
.From
[13].
Fig.
4.Sche
matic
draw
ingof
homeo
trop
ican
dplan
aralignm
ent,resp
ective
ly,h
ereillus
trated
forthecase
ofan
SmAph
ase(lay
erno
rmal
kpa
ralle
ltodirector
n)co
nfine
dbe
twee
npa
ralle
lflat
subs
trates.N
ormal
light
incide
ncemea
nsthat
thelig
htbe
amisprop
agatingve
rtically
inthepicture.
Notethat
thedraw
ingisno
tto
scale:
asm
ecticlaye
rissomethree
orde
rsof
mag
nitude
thinne
rthan
thedistan
cebe
twee
nthesamplesu
bstrates.F
rom
[13].
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1390
experience
themax
imum
birefringe
nce.Fo
rcu
rved
samples,
e.g.
liquid
crystald
ropletsor
shells,theterm
shom
eotrop
ican
dplanar
arestill
used,then
referringto
thedirectoralignmen
twithresp
ect
tothe
interface
with
the
continuou
sphase,
althou
ghthe
optic
effectsaremoreco
mplexdueto
thecu
rvature.In
Fig.
4thetw
oprincipal
alignmen
ttypes
areillustratedforthecase
ofasm
ectic-A
phasebe
twee
nflat
subs
trates.
Inaplanar-alig
ned
nem
atic
sample
wheren(r)has
norestric-
tion
toan
yparticu
lardirection
inthesample
plane(w
ereferto
this
geom
etry
asde
gene
rate
plan
aralignmen
t),thedirectoran
dthusthe
optic
axis
vary
random
lythroug
hou
tthe
sample
area
.W
hen
observing
the
sample
betw
een
crossed
polarizers,
alllocation
swherenis
parallelto
oneof
thepolarizerswill
appea
rblacksince
then
either
theordinaryor
theex
trao
rdinaryco
mpon
entva
nishes.
Since
these
location
sare
continuou
sly
connected,
form
ing
random
lycu
rving‘thread
s’or
‘strea
ks’,wege
tatexture
that
isve
rych
aracteristic
forthenem
atic
phase,
cf.theex
ample
inFig.
5.Th
istexture
iscalle
da
schlieren
texture
afterthe
German
word
for
streak
;Schliere.
Theab
ility
tomod
ulate
aliq
uid
crystald
isplayelem
entb
etwee
ndark
and
bright
isnormally
due
tothe
dielectric
anisotropy
Dε¼
εjj�εt,with
εjjan
dεt
beingthepermittivities
alon
gan
dperpen
dicularto
n,respe
ctively.
Byap
plyingan
electric
fieldthis
give
susahan
dle
forsw
itch
ingthedirectororientation
betw
een
differentdefi
ned
geom
etries,g
ivingdifferenttran
smission
.When
anelectricfieldEisap
plie
dov
eradielectricmed
ium
such
asaliq
uid
crystal,thefield
will
polarizethemed
ium,i.e
.an
electric
dipole
mom
entP
will
beinduced,p
roportion
alto
thefieldstrengthan
dto
thepermittivity
ofthemed
ium.Since
aliq
uid
crystalisdielectrically
anisotropicthepolarizationan
dthefieldwill
bedirecteddifferently
foran
ydirectororientation
that
isnot
alon
gE.
The
resu
ltis
adielectrictorqueGε¼P�Eon
thedirector,actingto
alignnalon
gðD
ε>
0Þor
perpen
dicularðD
ε<
0Þto
thefield.For
asimple
nem
atic
phasethetorquecanbe
calculatedto
be[23]:
Gε¼
�1 2ε0DεE2
sin2f
y(1)
Hereε0z8:85
$10�
12C/Vm
istheva
cuum
permittivity
andfis
the
angle
ofthe
directorwith
resp
ectto
the
electric
field.Th
edirectorisco
nsidered
tobe
inthex�
zplanean
dthefieldisap
plie
dalon
g�x
.In
adev
iceor
inatypical
research
sample
cellthereishow
ever
also
astronginfluen
cefrom
thesample
subs
trates,im
posingthe
startingco
nfigu
ration
(normally
planar
with
apositiveDεliq
uid
crystal).W
hen
anelectric
field
isap
plie
dwethustypically
get
aco
mpetition
betw
een
the
dielectric
contribution
tothe
free
energy
,promotingareorientation
ofthedirector,an
dtheelastic
contribution
,co
unteracting
any
chan
gefrom
the
initialplanar-
aligned
state.
Atwea
kfieldstheelasticco
ntribution
‘wins’
and
thedirectorremainsunaffected
bythefield,atstrongfieldsitisthe
other
way
arou
ndan
dthesample
issw
itch
ed.T
hebo
rderlin
ecase
iscalle
dtheFred
eriksthresholdafterthePo
lish-R
ussianphy
sicist
Vsevo
lod
Fred
eriks,
who
was
the
firstto
study
the
switch
ing
phen
omen
onin
detail[1].
The
switch
ing
process
isnow
aday
scalle
dtheFred
erikstran
sition
.Th
ean
isotropy
ofliq
uid
crystals
isalso
reflectedin
theirmag
netican
dco
nductiveproperties
aswella
sin
theirmultiple
viscosities,seee.g.
[23]
fordetails.
2.3.
Deformations
andsing
ularitiesin
thene
matic
director
field
Theundeformed
elasticgrou
ndstateof
anachiralnem
aticison
ewherethedirectorfieldistotally
uniform,i.e.n
points
inthesame
direction
everyw
herethroug
hou
tthesample.A
nelasticdeforma-
tion
inthecase
ofanem
atic
resu
ltsfrom
alocalreo
rien
tation
dnof
thedirector,lead
ingto
adirectorfielddistortiontran
smittedov
eralarger
scale.Since
distortionsin
n(r)c
oste
nergy
,thenem
atictries
toreestablishtheuniform
directorfield,n
otby
aforceas
insolid
elasticity
butby
anelastictorque
.Thestrengthof
thetorquecanbe
calculated
bydifferentiating
the
elastic
free
energy
.Neg
lecting
surfaceeffects,an
yge
neric
deformationof
n(r)canbe
described
asa
linea
rco
mbination
ofthree
elem
entary
elastic
deformations,
defi
ned
grap
hically
inFig.
6:splay,
twistan
dbe
nd.A
saresu
ltthe
fullelasticfree
energy
ofabu
lknon
-chiral
nem
atic
canbe
written
Fig.
6.Graph
ical
definition
sof
thethreeelem
entary
director
fieldde
form
ations
splay,
twistan
dbe
nd,tog
ethe
rwiththeirco
rrespo
ndingterm
sin
theelasticfree
energy
andtheir
resp
ective
elasticco
nstantsK1,K2an
dK3.Th
edraw
ings
illus
trateho
wea
chde
form
ationcanbe
prod
uced
inpractice
byen
caps
ulatingane
matic
betw
eenflat
subs
trates,the
inside
sof
which
areprep
ared
such
asto
ensu
reun
iform
plan
aror
homeo
trop
icdirector
orientationat
thesu
bstrate(the
preferreddirector
isindicatedwithathickdo
uble-h
eade
darrow).
Redraw
nafterde
Gen
nesan
dProst[23].
Fig.
5.Anex
ampleof
thech
aracteristic
schlierentextureof
nematic
liquidcrystals
inde
gene
rate
plan
aralignm
ent.Th
emicrograp
hisof
alyotropicne
matic
form
edby
disc-
shap
edmicelles,
butthesametype
oftexturecanbe
form
edby
anytype
ofachiral
nematic
inde
gene
rate
plan
arge
ometry.F
rom
[13].
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1391
asasu
mof
threeterm
s,ea
chgivingthetotalelasticfree
energy
contribution
due
tosp
lay,
twistan
dbe
nd
deformationsin
the
sample,respective
ly:
Gdeform:¼
1 2K1ðV
$nÞ2þ1 2K
2ðn
$V�nÞ2þ1 2K
3ðn
�V�nÞ2
(2)
Thisex
pressionisreferred
toas
theOseen
-Frankex
pressionfor
the
free
energy
,afterthe
Swed
eCarlW
ilhelm
Oseen
and
the
Englishman
SirCharlesFran
k.Ea
chdeformationis
described
bytheap
propriateco
mbination
oftheve
ctor
operatorsdiverge
nce
(V$n
)an
dcu
rl(V
�n)squared
andtheen
ergy
cost
foracertaindeformationisproportion
alto
the
resp
ective
elasticco
nstan
t,K1forsp
lay,K2fortw
ista
ndK3forbe
nd.
Theseco
nstan
tsareve
ryim
portantmaterialparam
eters,critically
dictatingthebe
hav
iorof
anem
atic.Th
eyhav
ethedim
ension
ofaforce,
themag
nitudetypically
inthepN
range
,andthey
must
bepositive
(req
uired
forstab
ility).
Theirva
lues
are
sosm
allthat
thermal
motioninducesco
nsiderab
lefluctuationsin
n(r)in
abu
lknem
aticat
room
tempe
rature.O
ften
K1an
dK2hav
eab
outthesame
valuewherea
sK3is
abou
ttw
icethat
value.
Itcanbe
conve
nientto
approximatethem
alleq
ual
toasingleva
lueK,theso-called‘one-
constan
tap
proximation’.
Thedirectorfieldssh
own
inFig.
6areallsm
ooth
andlocally
defect-free
despitetheirdeformed
state.
Ifweprepareanem
atic
sample
withdeg
enarateplanar
alignmen
twewill
how
ever
almost
alway
shav
edefects
inthedirectorfield,i.e.p
oints
wheren(r)isnot
defi
ned
.Su
chadefectis
calle
dadisclin
ation.
Someex
amplesof
disclinations
are
show
nin
Fig.
7toge
ther
with
exem
plifying
polarizingmicrograp
hsof
how
themostco
mmon
ones
may
appea
rvisu
ally.Th
edrawings
are
two-dim
ension
alas
isthe
plane
of
observation
inthe
microscop
e.Th
elin
esillustrate
n(r)in
the
sample
planean
dtheblackdot
indicates
thedisclinationitself(the
pointwherethedirectoris
undefi
ned
).As
can
beseen
inthe
figu
re,a
disclination
can
begive
nadefi
ningparam
eter
s,givingitacertainmag
nitudean
dsign
.This
resembles
electrical
charge
san
d,in
fact,theinteraction
ofdis-
clinationsisin
man
yresp
ects
analog
ousto
electrostatics,w
ithlik
e-sign
eddisclinationsrepellin
gea
chother
wherea
sdisclinationsof
oppositesign
sattract.Disclinationswithjsji1
arerarely
observed
since
theen
ergy
den
sity
inthesu
rrou
ndingfieldof
asingu
larity
increa
sesas
s2.E
nergy
isthusge
nerally
gained
bysp
littingan
jsj¼
2disclinationinto
twojs j
¼1,
oron
ejs j
¼1into
twojs j
¼1=
2.In
three
dim
ension
sthe
2Ddrawings
discu
ssed
here
correspon
deither
toadefectlin
ethrough
thebu
lk(valid
forhalf-intege
rdis-
clinations)
orto
apointdefectstuck
atthesu
rface(s
intege
r).F
orarich
erdiscu
ssion
oftheseissu
es,seee.g.
thetext
book
byde
Gen
nes
andProst[23].
Disclinationsareim
portantintoda
y’sresearch
onliq
uid
crystals
foremostfortw
omain
reason
s.In
onecase
thetopolog
yof
the
sample
requ
ires
disclinationsto
form
,e.g.inplanar-alig
ned
drop-
lets
orsh
ells
ofliq
uid
crystal.In
theseco
nd
case
wecrea
tedis-
clinationsby
addingco
lloidal
particles
totheliq
uid
crystal,su
chas
glassor
polym
erbe
adsor
isotropic
liquid
droplets.B
othcaseswill
bediscu
ssed
below.
The
elasticity
theo
rydescribed
abov
ewas
dev
elop
edfor
nem
aticsbu
titc
anbe
extende
dan
dmod
ified
tophases
withpartial
positional
order.F
orea
chparticu
larphaseon
emust
how
ever
take
into
acco
untthesp
ecialrestriction
sim
posed
byitssymmetry.Itis
forinstan
cerelative
lyea
syto
realizethat
twoof
thethreeelem
en-
tary
deformationsarenot
allowed
inSm
Adueto
itsstructure.Since
thedirectoralso
defi
nes
thelaye
rsthroug
hitsdou
blerole
aslaye
r
Fig.
7.Ex
amples
ofdisclin
ations
ofmag
nitude
1/2,1an
d2,
ofpo
sitive
andne
gative
sign
,inaplan
ar-alig
nedne
matic.For
thes¼
�1/2
ands¼
�1cases,po
larizing
micrograp
hsare
prov
ided
asillus
trations
how
thesedisclin
ations
canbe
reco
gnized
inthepo
larizing
microscop
e.Notethat
thesign
ofthedisclin
ationcann
otea
sily
bede
term
ined
from
asing
lemicrograp
h.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1392
normal,a
twistin
thedirectorfieldalso
entails
atw
istin
thelaye
rstructure,som
ethingthat
wou
ldrequ
ireabrea
kingupof
thelaye
rs.
This
cann
othap
penin
ordinarySm
Aphases
andthustw
istdefor-
mationsdonot
occu
rin
SmA.A
sfordirectorbe
nd,a
lsothis
give
sproblem
swiththelaye
rstructure
since
abe
ndin
n(r)effectively
correspon
dsto
asp
layin
thelaye
rs.T
helaye
rthickn
esswou
ldhav
eto
chan
geco
nstan
tly,asituationthat
doe
snot
occu
rin
realitysince
thelaye
rthickn
essisdefi
ned
bytheeffectivelengthan
dinclination
ofthe
molecules.
We
are
thus
left
with
only
directorsp
lay
asapossibledeformationin
SmA,corresp
ondingto
smecticlaye
rben
d.Althou
ghSm
Cis
less
symmetrican
dthusin
somesense
more
ordered
than
SmA
(occurringat
lower
tempe
ratures)
itactually
allowsmorefluctuationsin
thedirectorfield
than
SmA.Th
isis
because
thead
ditionof
atiltingdirection
give
sanew
free
dom
for
deformationswhich
do
not
interferewith
thelaye
red
structure,
allowingnto
bendas
wella
stw
istin
SmCan
dSm
C*.
We
end
this
briefintrod
uction
toliq
uid
crystalelasticity
bypointingou
tthat
this
isacontinuu
mdescription
:it
relie
sfund
a-men
tally
ontheassu
mption
that
theliq
uid
crystal’s
true
discrete
molecular
nature
can
beneg
lected
,the
fluid
being
trea
ted
asaco
ntinuum
instea
d.Th
isassu
mptionholds
aslongas
theob
jectsan
dphe
nomen
aund
erdiscu
ssion
have
sizesthat
are
very
large
onamolecular
scale.Sinc
ethisisindee
dthecase
inmostp
racticalusesof
liquid
crystals,the
elasticity
theo
ryismosto
ften
valid
andex
trem
ely
useful.Th
ereareho
wev
ersituations
whe
rethetheo
rymay
brea
kdow
n,in
particu
larin
theresearch
onna
noparticles
inliq
uidcrystals.
3.New
direc
tion
sforap
plied
andfu
ndam
entalliquid
crystal
science
Intheremainder
ofthis
articlewewill
highlig
htafew
ofthe
major
curren
tdirection
sinto
whichliq
uid
crystalscien
ceishea
ding
today.O
ther
authorsmighthav
emad
easlightlydifferentselection
from
the
very
rich
playg
roun
dthat
research
onliq
uid
crystals
constitutes,
butwebe
lieve
that
theex
amplesgive
ndorepresent
some
ofthe
mostex
citing
activities,an
dthey
certainly
attract
considerab
leinterest
across
theinternational
research
community.
3.1.
Liqu
idcrystals
fororga
nicelectron
icsan
den
ergy
conv
ersion
applications
Anarea
wherethesp
ontaneo
uslong
-ran
georder
ofliq
uid
crys-
tals
andtheea
seof
controllingtheiralignm
entov
erlargearea
sis
very
attractive
isorga
nicmaterials
forsemicon
ductor
dev
ices
and
energy
conv
ersion
.Inparticu
lartheuse
ofliq
uid
crystalsforb
uild
ing
efficien
torga
nic
pho
tovo
ltaic
and
tran
sistor
dev
ices
byself-
assemblyhas
been
attracting
attention
forsometime[7,24e
27],
butrecentlytherewerealso
impressivedem
onstration
sof
mecha-
noe
lectricalen
ergy
conv
ersion
utiliz
ingthehighflex
oelectricco
ef-
ficien
tsof
bent-corenem
atics[28e
31].Herewewill
firstg
iveabrief
review
oftheactivities
intheform
erfieldan
dthen
ween
dthis
subsection
withasn
apsh
otof
thestateof
theartof
thelatter.
Theinterest
inorga
nic
electron
icshas
been
very
strongon
anintern
ational
scalean
dov
erarelative
lylongperiodof
time,
andas
aresu
ltalargenumbe
rof
arom
atic
moleculesthat
arepoten
tially
usefulfororga
nic
semicon
ductorsor
solarcells
[32]
hav
ebe
endesigned
.Th
esingle-molecule
properties
arehow
ever
only
one
factor
indeterminingtheperform
ance
ofadev
iceem
ployingthese
materials.In
orde
rto
ensu
rehigh
charge
carrier
mob
ility
onadev
icescale,
themoleculesmust
bepacke
din
awellordered
arrange
men
tproviding
good
overlap
ofthe
porbitals
ofthe
arom
atic
moieties.
Long-range
orientation
alan
dat
leastpartial
positional
orde
rallows
the
orbitalov
erlap
betw
een
adjacent
moleculesto
extend
over
macroscop
icdistances,
thuslead
ingto
theco
mmon
picture
ofba
nd-likeco
nduction,cf.Fig.
8.Th
etypeof
conductionis
how
ever
ofthehop
pingtype.
Amorphou
smaterials
areob
viou
slyinap
propriatein
this
resp
ectsince
themoleculesare
disordered
alread
yon
asm
allscale.C
rystallin
estructuresmighton
the
other
han
dseem
tobe
idea
lat
first,
buton
emust
then
remem
berthat,fordev
iceap
plic
ation,theordered
arrange
men
tmust
extenduniformly
over
distancesco
rrespon
dingto
theinter-
electrod
edistancesof
thedev
ice.
This
typically
range
sfrom
some
hundredsof
nan
ometersto
seve
ralmicrons.
Iftheorde
rbrea
ksdow
nlocally
,asin
agrainbo
undary,
thego
odco
nductionpathis
brok
en.B
ecau
seitisve
rych
allengingto
achieve
perfect
crystals
ofthedesired
materials
over
thedistancesrequ
ired
,this
mak
esthe
route
viathecrystallinestateless
than
optimal.
By
using
molecules
that
are
suitab
lefororga
nic
electron
icap
plic
ationsat
thesametimeas
they
exhibitmesog
enicproperties,
onecansolvetheproblem
oflarge-scalealignmen
tin
aneleg
ant
andsimple
way
[24,25
].A
liquid
crystalphaseis
much
easier
toalignthan
acrystallinephasean
dgrainbo
undariescanbe
annea
led
rather
easily.Moreo
ver,they
arehighly
suitab
leforsolution
pro-
cessing.
The
mostco
mmon
lyem
ploye
dliq
uid
crystals
inthis
context
aretheco
lumnar
phases
ofdisco
ticmesog
ens,
providing
good
charge
carriermob
ility
andba
nd-typ
eco
nductivityalon
gthe
Fig.
8.Ca
rtoo
nillus
tratingthetran
sition
from
individu
alen
ergy
leve
lsto
energy
band
sas
theex
tens
ionof
pelectron
overlapgrow
salon
gtheco
lumns
ofaco
lumna
rdiscotic
liquid
crystalph
ase.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
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columnswhichthuseffectivelybe
comeakindof
‘molecularwire’
[24e
26].
Butalso
smectic
and
nem
atic
phases
ofrod-shap
edmesog
enshav
ebe
enex
ploredin
this
context.
Han
naan
dco
-worke
rsfunctionalized
moleculesop
timized
for
field
effect
tran
sistor
applic
ationswith
alky
lch
ains,
givingthem
smecticphases
betw
een
the
crystalline
and
isotropic
states.By
dissolvingthematerialin
anap
propriatesolven
tan
dsp
inco
ating
thesolution
sat
atemperature
wherethepure
materialis
inthe
smecticstate,
they
achieve
dex
celle
ntfilm
swithve
ryhighunifor-
mityan
dlong-range
order
[33],cf.Fig.9.
Ifthematerialw
asinstea
dsp
inco
ated
atlow
temperature,such
that
acrystallinestateform
swhen
thesolven
tev
aporates
they
achieve
darather
inhom
oge-
neo
uspolycrystallin
efilm
witharough
surface.Th
eresu
ltco
uld
beim
prove
dby
raisingthetemperatureto
theliq
uid
crystallinerange
aftersp
inco
atingan
dthusan
nea
lingthefilm
,butthefilm
did
not
reachthequ
alityof
thefilm
spin
coated
intheliq
uid
crystalline
state.
Using
arelated
non
-functionalized
materialwithou
tthe
capab
ility
toform
liquid
crystalp
hases
theresu
ltswereev
enworse
andthean
nea
lingop
tion
was
obviou
slynot
available.
Adrawba
ckof
using
liquid
crystalline
system
sfororga
nic
electron
icsis
that
themolecule
mob
ility
intheliq
uid
crystalline
stateis
still
rather
high.T
his
lead
sto
less
strongp-p
-stack
ingan
dthusto
alim
ited
hop
pingco
nduction,con
siderab
lypoo
rerthan
in
acrystallinestate.
Inge
neral,theea
sier
thephaseis
toalign,the
grea
terthisproblem
.Nem
aticsthushav
ethelowestch
arge
carrier
mob
ility
amon
gliq
uid
crystalphases
and
even
SmA
and
SmC
phases
hav
eunsatisfactory
values.T
herefore,
themostpromising
approachis
generally
topreparethelarge-scaleordered
arrange
-men
tin
theliq
uid
crystallinephase,
inwhichthealignmen
tcanbe
controlledea
sily,an
dthen
quen
chthesystem
tolower
tempe
ra-
turesso
rapidly
that
crystallization
ispreve
nted,instea
dgiving
aglassy
statein
whichtheliq
uid
crystallineorder
isfroz
enin.In
thisway
thego
odlong-range
order
withlittleor
nodefects
that
can
beachieve
dby
processing
intheliq
uid
crystallinestatecan
beco
mbined
with
smallmolecular
fluctuations,
and
hen
cehigh
charge
carriermob
ilities
over
longdistancescanbe
expected.
An
orga
nic
solarcell
consistsprimarily
ofa
heterojunction
betw
eenan
electron
-don
atingan
dan
electron
-accep
tingmaterial.
The
energy
provided
when
aphoton
isab
sorbed
lead
sto
the
crea
tion
ofan
exciton,i.e
.abo
und
electron
eholepair.
Forthe
function
ofthesolarcell
theex
citon
shou
lddiffuse
toward
the
interfacewhereitcandissociateinto
free
charge
carriers
inthetw
oresp
ective
materials,that
can
then
beharve
sted
asan
electrical
curren
t.In
addition
tothe
charge
carriermob
ility,alread
ydis-
cussed
,ake
yparam
eter
forthephotoe
lectricalco
nve
rsion
effi-
cien
cyof
orga
nic
solarcells
istheex
citondiffusion
length,i.e.h
ow
Fig.
9.Po
lycrystalline
film
sof
ado
ubly
alky
lterminated
terthiop
hene
spin
coated
from
isotropicsolution
sat
differen
ttempe
ratures;
a,dan
dg:
tempe
rature
ofcrystalline
phase;
b,ean
dh:
tempe
rature
ofsm
ecticliq
uidcrystalline
phase;
c,fa
ndi:tempe
rature
ofisotropicph
ase.
Optical
microscop
ytextures
(aec,scaleba
rin
aeb:
10mm
;inc:
100mm
),co
nfoc
allaserscan
ning
microscop
yprofi
les(def),A
FMtopo
grap
hicalimag
esan
dco
rrespo
ndinghe
ight
analyses
(gei).N
otetheve
rydifferen
tve
rtical
height
axes
inthelatter
diag
ramsfor
thedifferen
tsamples.F
rom
referenc
e[33].C
opyright
Wile
y-VCH
VerlagGmbH
&Co
.KGaA
.Rep
rodu
cedwithpe
rmission
.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1394
longan
excitoncantrav
elwithin
thematerialbe
fore
theelectron
andholereco
mbine.
Thedistance
from
theex
citoncrea
tion
site
tothe
heterojunction
must
besm
allerthan
the
exciton
diffusion
length
inorder
toharve
stthe
energy
.Disco
ticco
lumnar
liquid
crystals
hav
eturn
edou
tto
hav
eparticu
larlyhigh
values
ofthis
crucial
param
eter
(>70
nm
[7])
aswellas
high
charge
carrier
mob
ility
(1.4
cm2V�1
s�1[34]),an
dthishas
ledto
astrongfocu
sof
attention
thelast
yearson
usingtheseliq
uid
crystals
forphoto-
voltaics
[7,26].
These
materials
clea
rly
hold
promise,
asdem
onstrated
with
experim
entalan
dtheo
reticalwork,
butim
portantch
allenge
sstill
remainto
bead
dressed
.Itis
forinstan
cenot
trivialto
ensu
rethe
alignmen
tof
theco
lumnsthat
isrequ
ired
foraparticu
larap
plic
a-tion
.For
aphotov
oltaic
dev
iceba
sedon
disco
ticliq
uid
crystals,o
ne
must
crea
teauniformly
hom
eotrop
ic-alig
ned
heterostructure
oftw
odifferen
tco
lumnar
phases,whereon
eis
designed
forgo
odelectron
-don
atingproperties,theother
oneforbe
ingago
odelec-
tron
acceptor.Th
isis
quitech
allenging,
butrecentlysomehop
eful
reports
appea
red,
sugg
esting
that
carefulmolecule
design
and
optimization
ofthedev
iceprepa
ration
strategy
may
lead
tothe
passingalso
ofthis
hurdle,see
e.g.
[35].
While
hom
eotrop
icalignm
entis
requ
ired
fordiscotic
liquid
crystalsolar
cells
,for
anorga
nicfieldeffect
tran
sistor
(FET
)based
ondiscoticpha
sestheco
lumnssh
ould
liein
thesample
planean
dbe
aligne
duniform
lyalon
gasp
ecificdirection
.Th
isis
perhap
sev
enmorech
allenging,be
caus
etrad
itiona
lalig
nmen
ttech
niques
such
asrubb
ingof
polyimideco
atingor
pho
toalignm
enthavelittleor
no
impacton
theorientation
ofthedirectorin
disco
ticliq
uid
crystal
pha
ses.
Tothis
end,a
nex
perim
entby
Scalia
andco
-worke
rsmay
how
ever
show
some
promise
[36].Th
eysp
inco
ated
atoluen
esolution
oftheco
mmon
lyused
columna
rphase-form
ingdisco
tic
material
hex
akis(pen
tyloxy
)-triphe
nylene,
abbrev
iated
HAT-5
(Fig.2
c),a
ndstudied
theprodu
cedfilm
sby
polarizingmicroscop
yan
datom
icforcemicroscop
y(A
FM).Th
eAFM
studyreve
aled
very
interesting
textures,
cf.Fig.
10,highly
reminiscentof
nem
atic
directorfields
withafew
disclinationsbe
twee
nfairly
largearea
swithin
whichfibrils
ofco
lumnswereuniform
lyaligned
.The
authors
propo
sedthat
thego
odalignmen
tco
uldbe
aneffect
ofalyotropic
nem
aticpha
seof
shortd
iscco
lumns
form
ingduringthesp
inco
ating
process,as
the
solven
tqu
ickly
evap
orated
.Th
edirectorof
this
columna
r-nem
aticpha
sewou
ldthen
alignprimarily
alon
gtheradial
shea
rflow
active
duringthesp
inco
atingprocess.Itis
notew
orthy
that
HAT-5doe
snot
exhibitanem
atic
pha
seon
itsow
n,hen
cethe
prope
rco
mbinationof
solven
tcon
centration
andtempe
rature
(note
that
thetemperaturerapidly
coolsduringsp
inco
atingdueto
the
evap
orationof
solven
t)wou
ldbe
critical
forob
tainingthenem
atic
order.Further
inve
stigations
areclea
rlynee
ded
toco
nfirm
theex
act
origin
oftheuniform
columnalignmen
tbu
tthis
tech
niquemay
bepromising
foraligning
disco
ticco
lumna
rpha
sesuniform
lyov
erarea
sthat
areve
rylargeon
thescaleof
typicalorga
nicFE
Tstructures.
Morerecenta
ndmuch
less
exploredistheco
ncept
ofharve
sting
mechan
ical
motionas
electrical
energy
usingliq
uid
crystals
that
displayhighflex
oelectrican
d/orpiezo
electric
coefficien
ts.T
heidea
was
proposed
in20
09by
AntalJaklian
dco
-worke
rsat
Ken
tState
University,U
SA[28,30
,31].T
heprinciple
isillustratedin
Fig.11
for
thecase
ofanem
atic
phasemad
eou
tof
bent-co
remolecules.Su
chphases
displaythehighestflex
oelectricco
efficien
tskn
ownso
far.If
theco
ntainer
oftheliq
uid
crystalb
ends,thesemoleculeswill
pack
moreefficien
tlyifthey
allbe
ndin
thesamedirection
.Since
they
hav
eahighpolarizationalon
gthebe
nddirection
,ben
dingof
the
sample
thuspolarizes
it,an
dasu
rfacech
arge
resu
lts.
Flex
ingan
electrod
e-eq
uipped
sample
back
and
forth
then
generates
analternatingcu
rren
t.Under
practical
applic
ationco
nditionsliq
uid
crystals
areactu-
ally
foundto
outperform
man
ysolid
statealtern
atives
interm
sof
energy
conve
rsion
efficien
cy[28],since
one
must
take
the
max
imum
strain
grad
ientinto
acco
unt(w
hich
issu
bstantially
higher
fortheliq
uid
crystalcase)an
das
theliq
uid
crystal-ba
sed
materials
can
bemad
eve
rythin,en
suring
high
efficien
cy.Fo
r
Fig.
10.Atomic
forcemicroscop
yim
ages
ofasampleof
thediscotic
liquidcrystalH
AT-
5sp
inco
ated
from
toluen
esolution
,formingfibe
rsorga
nize
din
ane
matic-likeorde
red
arrang
emen
t.From
referenc
e[36].
Fig.11.Abe
nt-coreliq
uidcrystalcon
fine
din
aflex
ible
containe
rwill
prod
uceaflex
oelectricsu
rfacech
arge
dueto
theim
prov
edpa
ckingefficien
cywhe
nthebe
ndof
allm
olecules
isalon
gthemacroscop
icsamplebe
nddirection.
Ifthesampleis
flex
edba
ckan
dforthan
delectrod
esareattach
edto
theou
tsides,a
naltern
atingelectrical
curren
tcanbe
pick
edup
.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1395
experim
ents
theJakliteam
contained
thebe
nt-sh
aped
mesog
enphasebe
twee
nflex
ible
subs
trates
orinsidean
elastomer,a
llowing
them
tova
rythemacroscop
icsample
shap
ebe
twee
nco
nve
xan
dco
ncave
.Indee
d,this
flex
ingresu
lted
inacu
rren
tthat
was
easily
detectable
byfitting
the
sample
with
two
brasselectrod
esan
dco
nnectingthem
toastan
dardoscillo
scop
e.Itwill
beve
ryex
citing
tostudythefurther
dev
elop
men
tduringthenex
tfew
yearsof
this
very
recentlyintrod
ucedco
ncept,withco
nsiderab
lepoten
tial
for
new
low
cost
mea
nsof
energy
harve
sting.
3.2.
Liqu
idcrystaltem
platingforcreating
nano
structured
materials
ororde
ring
nano
-an
dmicropa
rticles
With
today
’sfocu
sin
materials
science
onnan
ostructured
materials
theab
ility
ofliq
uid
crystals
ethermotropic
aswellas
lyotropic
eto
self-assem
bleinto
structuresthat
exhibit
specific
nan
oscale
arrange
men
tsex
tending
inan
ordered
fash
ion
over
amuch
longe
rrange
isex
trem
elyattractive
.Se
veralap
proaches
hav
ebe
endev
ised
totake
adva
ntage
oftheliq
uid
crystallineorder
forge
neratingnew
functionalmaterials,either
bymak
ingtheliq
uid
crystal-ge
nerated
order
perman
entviapolym
erization,g
elationor
glassform
ation,or
bypreparingaliq
uid
crystallinesample
such
that
aregu
lararrayof
defects
form
s,whichcanthen
beusede.g.for
positioningnan
o-or
micropa
rticlesov
erlargearea
s.Herewebriefly
review
thecu
rren
tstateof
theartof
thesetw
omainap
proaches.
3.2.1.
Solid
ifyingliq
uidcrystalph
ases
forcrea
ting
regu
lar
nano
porous
solid
sOne
ofthe
mostim
portantcu
rren
tex
amplesof
how
liquid
crystalsareusedas
templatesistheproduc
tion
ofnan
oporou
ssilic
amaterials
forcatalysisap
plic
ationsfrom
orvialyotropic
hex
agon
alor
cubicliq
uid
crystalphases
[37].Very
often
the
term
‘meso-
porou
s’is
used
instea
d,follo
wing
anon
-metric
term
inolog
yco
mmon
inthis
particu
larresearch
field,bu
twehereuse
‘nan
o-porou
s’,refl
ectingthefact
that
thepores
arein
thenan
ometer
size
range
.Anorga
nosilicate
compou
ndsu
chas
tetrae
thylorthosilicate
(TEO
S)isad
ded
asasilic
aprecu
rsor
tothelyotropic
phasean
dthis
isthen
hydroliz
edinto
SiO2.
Asthereaction
take
splace
inthe
aqueo
usch
annelsof
thelyotropic
liquid
crystalhostphasethe
produced
silic
aad
opts
theinternal
shap
eof
thenan
ostructured
template,cf.Fig.12
.Thesoftan
dsensitive
liquid
crystalstructure
isthereb
ytran
sferredto
ahardan
ddurableinorga
nicco
py.A
fterward
thesu
rfactantis
remov
edby
thermal
decom
position(calcination),
leav
ingon
lythenan
ostructuredsilic
awhichnow
has
thehex
ag-
onal
orcu
bicstructure
imprintedby
theliq
uid
crystal.
Insomecasesitisof
adva
ntage
tofollo
wadifferenta
pproachfor
solid
ifyingthelyotropic
structure,instea
dreplacingtheordinary
surfactants
withsu
rfactants
withreactive
hea
dgrou
ps[38e
41].In
this
way
one
can
polym
erize
the
surfactantco
mpon
entof
the
lyotropicliq
uid
crystala
ndrather
than
mak
ingan
inorga
nicco
pyof
the
template,the
liquid
crystalline
structure
isthusfixa
ted
byturn
ing
itinto
anorga
nic
solid
.Th
eresu
lting
materials
are
ofinterest
forinstan
cein
water
filtration
applic
ationsor
assynthetic
biod
egradab
lematerials
formed
ical
applic
ations.
Alsothermotropicliq
uid
crystalscanbe
very
usefula
stemplates
inva
riou
sco
ntexts,
realizingasolid
either
bypolym
erizingthe
liquid
crystallinephase[42,43
]orby
polym
erizingan
other
material
insidetheliq
uid
crystal[44
,45].A
particu
larlybe
autifule
xample
ofthelatter
approach
isthesu
ccessfulproduction
ofhelical
poly-
acetylen
efibe
rsby
polym
erizingacetylen
ega
sin
ach
iral
nem
atic
reaction
field,a
strategy
pionee
redby
Hidek
iShirak
awaan
dKazuo
Aka
gian
dco
-worke
rsat
KyotoUniversity,Japan
[46].Th
ech
ole-
steric
liquid
crystalline
environmen
tforces
the
polya
cetylene
chainsto
twistas
they
grow
,into
fibe
rsthat
mim
icthestructural
featuresof
the
helical
liquid
crystalphase.
The
resu
lting
fibe
rsex
hibitphy
sicalp
rope
rtiesthat
canbe
quiteunique,in
particu
larin
term
sof
theirmag
netoresistance.T
hishas
trigge
redmuch
interest
inarea
sve
ryfarfrom
liquid
crystalresea
rch[47].T
hech
iral
liquid
crystalrea
ctionfieldis
critical
forthesu
ccessin
thepreparationof
this
new
material.
An
interesting
recent
extension
ofliq
uid
crystalstructure
form
ation,related
tolyotropic
liquid
crystals
inman
yresp
ects
but
not
invo
lving
asolven
t,is
the
class
ofpolyp
hilic
mesog
ens
[8,48e
55].W
iththesemolecules,that
aream
phiphilicwithat
least
Fig.
12.Ca
rtoo
nillus
tratingthetw
omainap
proa
ches
toprod
ucena
nopo
rous
silic
aus
inghe
xago
nallyo
trop
icliq
uidcrystaltem
plates.R
eprinted
withpe
rmission
(from
referenc
e[37].C
opyright
(200
7)American
Chem
ical
Society).
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1396
three,
rather
than
two,
sectionshav
ingdifferentpreferred
envi-
ronmen
ts(for
instan
ceon
eperfluorinated
,onehy
drophilican
don
earom
aticsection)awholerange
ofnew
structureshav
ebe
enfound
toform
viaself-assem
bly.
Atthemom
enttheresearch
onthese
new
materialsispurely
acad
emic,b
uttheco
mplexity
ofthesenew
phases
isbe
ginningto
beelucidated
.Thetimemay
thusbe
ripefor
exploring
them
for
different
types
ofap
plic
ations,
where
anob
viou
sroute
toex
plore
isthat
oftemplating
from
these
new
nan
ostructures.
Com
pared
tolyotropic
liquid
crystals
ahuge
adva
ntage
with
the
polyp
hilic
thermotropics
isthat
they
can
generally
bequ
iteea
sily
aligned
over
macroscop
icarea
s.
3.2.2.
Synthe
sizing
nano
particlesin
liquidcrystalline
templates
Seve
ralg
roup
shav
eusedtheself-organ
ized
structuresof
liquid
crystals
as‘nan
oreactors’for
synthesizing
very
well-defi
ned
nan
oparticles
with
desirab
lesh
apes.
Inparticu
lar
lyotropic
columnar
phases
areusefulforsynthesizing
e.g.
semicon
ductor
nan
orod
s[56e
62]bu
tother
liquid
crystalphases
canbe
usedfor
synthesizingnan
oparticles
withother
shap
es[63e
67].
Amon
gtheke
yad
vantage
swith
this
tech
niqueco
mpared
tostan
dardmethod
sforsynthesizingnon
-spherical
nan
oparticles
are
low
dispersity,a
relative
ease
oftunab
ility
ofthe
geom
etrical
features,
andthefact
that
very
highaspectratioparticles
canbe
achieve
dby
carryingou
tthesynthesis
within
theve
rylongch
an-
nelsof
themicellesin
ahex
agon
alco
lumnar
phase.
Bytuningthe
composition
ofthe
phase,
often
including
anoilco
mpon
entin
additionto
theam
phiphile
san
dwater
that
form
thestarterkit,the
inner
diameter
ofthe
chan
nelscan
beva
ried
over
anorder
ofmag
nitude(1e10
nm)while
theirlengthcanbe
inthecentimeter
range
orev
enlonge
r.Another
benefi
cial
aspectis
that
itis
easy
toremov
etheco
mpon
ents
ofthelyotropic
phasetemplate.T
his
isin
contrastto
method
swheresolid
templatesareused,whichtypi-
cally
requ
ireetch
ingby
strongan
d/orharmfula
cids.
Obv
iouslyon
enee
dsalso
precu
rsor
materials
inad
ditionto
the
basic
constituen
tsof
the
liquid
crystalphase.
These
are
often
orga
nom
etallic
saltsthat
actas
precu
rsorsforthe
nan
oparticle
synthesis.It
iscrucial
toreco
gnizethat
thesesaltswill
normally
beco
meaco
mpon
entof
theliq
uid
crystalphaseas
wellan
dtheir
presence
will
almostalway
shav
esome
influen
ceon
the
self-
assemblyprocesses
lead
ingto
theliq
uid
crystalphaseform
ation.
Onethusge
nerally
nee
dsto
mod
ifytherecipes
forform
ingpure
liquid
crystalp
hases,g
ivingtheprecu
rsor
materials
active
rolesin
thestructure
form
ation.Insomecasestheinfluen
cefrom
thesalts
wou
ldsimply
betoo
strong
atthe
concentrationsrequ
ired
for
form
inginterestingparticles
andon
emust
then
findsp
ecialsolu-
tion
s.Asm
artap
proachem
ploye
dby
dos
Santoset
al.isto
inco
r-porateco
lloidsam
ong
theprecu
rsor
materials
[62].Th
isallows
arelative
lyhigh
deg
reeof
load
ingwith
inorga
nic
materialwith
minim
alinfluen
ceon
thelyotropic
phase.
3.2.3.
Collo
idal
particlesorga
nizedby
liquidcrystals
andliq
uid
crystalph
ases
from
collo
ids
Adifferentway
ofex
ploitingtheliq
uid
crystallineorder,that
doe
snot
necessarily
invo
lvefixa
tingthestructure
into
asolid
,isto
introd
uceco
lloidal
particles
intheliq
uid
crystal.Th
elatter
then
functionsas
ave
rysp
ecialc
ontinuou
sphase,actingin
uniqueway
son
thedispersedparticles
whichcanbe
inthenan
o-to
micrometer
size
range
andof
ava
rietyof
shap
es.T
herehas
been
aco
ntinuou
sly
grow
inginterest
inco
mbiningparticles
with
liquid
crystals
over
thelast
15ye
ars,
and
anumbe
rof
interestingphen
omen
ahav
ebe
endem
onstrated,invo
lvingpolym
eror
silic
amicrobe
ads,carbon
nan
otube
s,or
semicon
ductor
ormetal
nan
orod
sor
-spheres.
Theliq
uid
crystalcan
orga
nizethedispersedparticles
inva
riou
sway
san
dem
ploysomew
hat
differentmechan
isms,dep
endingon
theparticlesize
,sh
apean
dhow
theirsu
rfaces
interact
with
the
liquid
crystal.W
hat
allstudieshav
ein
common
,how
ever,isthat
theparticles
insomeway
disturb
theliq
uid
crystaldirectorfield,
whichthereforeex
erts
forces
ortorques
ontheparticles
inorde
rto
promoteaco
nfigu
ration
ofthem
wherethedistortionis
minim
al.
This
canlead
toan
alignmen
tof
anisom
etricparticles
aswella
sto
theorga
nizationof
particles
into
regu
larclusters.
Amon
gthemoststraight-forw
ardap
proa
ches
ofthiskind
isthe
alignm
entof
rod-sh
aped
particles
alon
gthenem
atic
directorfield.
The
firstparticle
type
tobe
studied
inthis
way
was
mag
netic
nan
orod
s,initially
cons
idered
theo
retically
byPierre-G
illes
deGen
-nes
and
Fran
çoiseBroch
ard
[68].In
recentye
arsthefirstnan
o-particleto
receiveco
nsiderab
leattention
asdop
anttoliq
uid
crystals
was
the
carbon
nan
otube
[69e
81].
Sing
le-as
wellas
multiw
all
carbon
nan
otub
eshavebe
ensu
ccessfully
dispersedin
thermotropic
aswellaslyotropicliq
uid
crystals,p
rimarily
ofnem
atictype
,and
the
alignm
entof
thetube
salon
gthenem
aticdirectorfieldwas
verified
byva
riou
stech
niques.Initially
only
indirect
proof
ofthealignm
ent
couldbe
provide
d[69,70
],bu
tsomew
hatlaterdirectin-situve
rifi-
cation
ofthenan
otub
eorientation
bymea
nsof
polarized
Ram
ansp
ectrosco
pyco
nfirm
edwithou
tdou
btthealignm
ente
ffect[82,83
].Th
eex
perimen
talworkwas
also
complemen
ted
byanum
berof
rigo
rous
theo
reticaltreatmen
ts[84e
87].
Arather
seve
reproblem
with
the
carbon
nan
otub
ee
liquid
crystalcom
posites
isto
maintain
asu
fficien
tlygo
oddispersion
ofthe
nan
otub
es.C
arbo
nnan
otube
sarenotorious
lydifficu
ltto
disperse
inan
ymed
ium
and
inliq
uid
crystals
long
-term
stab
ility
ofthe
dispersion
isex
trem
elydifficu
ltto
achieve
.Allinitialw
orkwas
don
ewiththermotropicliq
uid
crystalhosts
andon
lytraceam
ounts
ofcarbon
nan
otub
esweredispersedin
order
toav
oidseve
renan
otub
eag
greg
ation.
The
firstsu
ccessfulway
ofdrastically
raising
the
nan
otub
eco
ncen
tration
was
tosw
itch
tolyotropicliq
uid
crystal
hosts,w
here
theop
tion
touse
surfactantsforstab
ilizingthenan
o-tube
dispersion
allowed
amuch
grea
terco
ncen
trationof
thenan
o-particlegu
ests
[78,83
,88,89
]Particu
larlyhighnan
otube
load
ingwas
possibleby
usingtw
odifferentionicsu
rfactantsof
oppositech
arge
,on
efordispersing
thenan
otub
esan
dtheother
forbu
ildingthe
liquid
crystalpha
se.Inthis
way
Scalia
etal.succee
ded
inreaching
such
high
nan
otub
eload
ing
atsu
chgo
odalignmen
tthat
the
compositebe
have
dlik
ealiq
uid
linea
rpolarizer
[90,91
],cf.Fig.13:
asthe
nan
otub
esab
sorb
lightpolarized
alon
gthe
tube
axis,the
compositebe
camedarke
rforpolarizationalon
gthedirectorthan
perpen
dicularto
it.T
hiswas
thefirsttimethat
nan
oparticlealign-
men
tby
aliq
uid
crystalco
uld
beve
rified
bythenak
edey
e.Th
eco
mposites
also
acqu
ireve
ryinterestingviscoe
lastic
prope
rties,in
particu
lartheab
ility
toform
very
long
and
slen
derfilamen
tsin
Fig.
13.Acapilla
ryco
ntaining
alyotropicliq
uidcrystalph
asealigne
dwithn
alon
gthecapilla
ryax
isan
dco
ntaining
0.2wt.%
well-disp
ersedsing
le-w
allcarbon
nano
tube
s.Th
eun
iform
alignm
entof
thena
notube
s,presen
tat
such
high
conc
entration,
turn
stheco
mpo
site
fluidinto
apo
larize
r,as
demon
stratedby
view
ingthecapilla
rythroug
hapo
larize
rfilm
withitstran
smission
directionalon
g(a)an
dpe
rpen
dicu
lar(b)to
thecapilla
ry.F
rom
referenc
e[90].R
eprodu
cedby
perm
ission
ofTh
eRo
yalSo
cietyof
Chem
istry.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1397
which
thenan
otub
esarehighlyordered
[92].A
num
bero
fstudieson
dispersion
ofnan
otub
esin
lyotropicliq
uid
crystals
hav
efollo
wed
[93e
98],co
nfirm
ingtheve
rsatility
andusefulnessof
theap
proach.
Anim
portan
trecentachieve
men
tis
thepolym
erizationof
thelyo-
trop
ichostpha
sein
order
toturn
thenan
otub
ee
liquid
crystal
compositeinto
asolid
,withmaintained
uniform
alignm
entof
the
nan
otub
es[96].
Thereis
how
ever
considerab
lepoten
tial
inusingalso
thermo-
trop
icliq
uid
crystalsas
thehostfornan
otube
dispersion
[99e
102],
inparticu
lardueto
theea
seof
aligningan
dreorientingthermo-
trop
icnem
atics,
andthereis
thusamotivationforim
provingthe
dispersion
stab
ility
also
inthermotropic
hosts.In
fact,althou
ghlong-term
stab
ility
isaseriou
sproblem
,thefirstsystematicstudyof
carbon
nan
otube
dispersion
inthermotropic
liquid
crystals
[103
]reve
aled
that
thermotropic
hosts
are
actually
amon
gthe
best
orga
nicsolven
tsforcarbon
nan
otube
sin
term
sof
aqu
ickreduction
ofinitialag
greg
atesize
.Th
eproblem
isto
ensu
reex
folia
tion
ofbu
ndlesto
singletube
leve
lan
dto
maintain
long-term
dispersion
stab
ility,v
erydifficu
ltsince
nosu
rfactants
canbe
added
forstab
i-liz
ingthedispersion
.An
importantresu
ltof
this
studywas
that
thereareco
nsiderab
ledifferencesbe
twee
ndifferentnem
aticsin
term
sof
their
suitab
ility
ashosts
for
carbon
nan
otube
s,the
common
lyusedqu
artenarymixture
E7an
dtheneg
ativedielectric
anisotropy
nem
atic
MBBA
comingou
tas
thebe
sthosts
ofthose
inve
stigated
[103
].A
promisingrecentdev
elop
men
tis
theintro-
duction
oftailo
r-synthesized
hybrid
molecules,
actingsimila
rto
surfactants
inaq
ueo
ussolution
,butherewithon
een
dop
timized
forcarbon
nan
otube
adhesionwherea
stheother
ismad
eforther-
motropic
liquid
crystalco
mpatibility.Th
isis
ave
rynew
research
thrust
butsomepromisingresu
ltshav
ebe
enpresented[104
e10
6].
Therehav
ealso
been
observationsof
positiveeffectsof
carbon
nan
otube
son
the
liquid
crystal
[71e
74,80,10
1,10
7,10
8],
even
including
the
function
ofcarbon
nan
otube
sas
chiral
dop
ants
[109
,110
],bu
tas
theliq
uid
crystalinthesecasesplays
apassive
role
wedonot
further
discu
sstheseissu
esin
this
review
.Alm
ostall
work
has
been
don
ewith
nem
atic
hosts
ofrod-lik
emesog
ens,
primarily
dueto
thedifficu
ltiesin
dispersingnan
otube
sin
more
orde
redan
dthusmuch
moreviscou
sliq
uid
crystalphases.A
rare
exam
ple
ofastudyof
nan
otube
sdispersedin
ach
iral
smecticliq
uid
crystalw
aspublished
byLage
rwalle
tal.,thesu
ccessin
dispersion
beingdueto
atw
o-step
process
wherethenan
otube
swerefirst
dispersedin
anem
atic
phasethat
was
then
mixed
withastrong
smectoge
nic
mesog
en[100].Th
enan
otube
sturn
edou
tto
hav
eadrastic
influen
ceon
thephasesequ
ence
ofthesm
ectichostbu
tthe
dispersion
was
still
not
sufficien
tly
good
toinve
stigate
ifsw
itch
ingof
thesm
ecticliq
uid
crystald
irectormighth
avean
effect
onthenan
otube
s.Kumar
andco
-worke
rssu
ccee
ded
indispersing
chem
ically
functionalized
nan
otub
esin
thermotropicdisco
tichosts
[3,111,112
],bu
talso
herelittleeffortsweredon
eto
inve
stigatethe
effectsof
thehoston
theorga
nizationof
thenan
otube
s.Rathe
rthan
dop
ingaliq
uid
crystalh
ostwiththena
notube
son
ecan
infact
also
getthenan
otub
esthem
selves,su
spen
ded
inan
isotropicliq
uid
host,to
form
alyotropicliq
uid
crystalp
hase,provide
dsu
fficien
tco
ncen
tration
and
qualityof
dispe
rsion
[16,18
,113
e12
4].
Thishas
thead
vantag
ethat
thenan
otub
eco
ncen
trationob
viou
slyis
high,
muc
hhigh
erthan
intheworkrelyingon
thermotropichosts,
butthe
rearetw
omajor
diffi
culties.First,ev
enin
isotropicsolven
tsit
isfarfrom
trivialtoachieve
sufficien
tlygo
oddispe
rsionat
such
high
conc
entrationof
nan
otub
esrequ
ired
toge
tlyo
trop
icliq
uid
crystalline
pha
ses,an
dthegrou
psdrivingthisap
proachhavethereforeresorted
toox
idationof
thenan
otub
esforsolubilityin
water
[18,118e
120],
using
super
acidsas
thesolven
t[12
1e12
3]or
take
ngrea
tcarein
using
efficien
twrapp
ingof
pristinenan
otub
esby
molecules
such
asDNA
that
stab
ilize
the
susp
ension
sev
enat
high
conc
entration
[16,113e
117].A
nex
ample
ofthelatter
type
issh
ownin
Fig.14
.The
seco
ndmajor
diffi
cultyis
toen
sure
macroscop
icorientationof
the
directorin
thesepha
sesov
eralongen
ough
timescalethat
the
aligned
nan
otub
escanbe
put
touse.G
enerally,she
aringistheon
lyway
toalignlyotropicpha
sesof
thistypebu
tassoon
asthesh
earfl
owisremov
edthealignmen
tisoftenlost.R
ecen
tlythePo
ulin
team
has
howev
ersh
ownsomeprogressin
thetask
ofretainingthealignmen
tev
enafterdryingalyotropicliq
uid
crystallinenan
otub
esu
spen
sion
[116
]an
dthisroutethusholds
promiseforthefuture.
Withtheve
rystrongcu
rren
tfocu
son
grap
hen
eit
was
hardly
unex
pected
when
thefirstreports
onliq
uid
crystalphases
with
grap
hen
ecameou
t.Like
withcarbon
nan
otube
ssomegrou
psuse
liquid
crystallinephases
foraligningthegrap
hen
esh
eets
[125
,126
]while
othersen
sure
sufficien
tco
ncentrationan
ddispersion
quality
ofox
idized
grap
hen
eflak
esthat
they
form
liquid
crystalp
hases
ontheirow
n[19e
21,127
].Carbo
nparticles
were
the
firstbu
tof
course
not
the
only
nan
oparticles
that
hav
ebe
enstudiedin
combinationwith
liquid
crystals.S
everal
studieshav
ebe
endev
oted
totheinve
stigationof
liquid
crystalline
phases
containing
gold
[128
e14
2],
semi-
conducting[143
e15
4]or
silic
anan
oparticles
[155
e15
9]as
wellas
mag
netic
[160
e16
7]or
ferroe
lectric[168
e17
2]particles.W
hile
the
extended
arom
atic
structure
ofcarbon
nan
otube
sprovides
for
benefi
cial
interactionswithtypical
mesog
ens,
promotingparticle
solubilityin
athermotropic
host[103
],theprimarily
hydrophilic
character
ofman
ynon
-carbo
n-based
inorga
nic
nan
oparticles
mak
esan
efficien
tcappingag
entpromotinggu
est-hostco
mpati-
bilityab
solutely
essential
ifmetal
orsemicon
ductor
nan
oparticles
shou
ldbe
efficien
tly
dispersed
inathermotropic
liquid
crystal.
Com
mon
choicesforsolving
thedispersion
problem
arealka
ne
thiols
[129
,136
,140
]whichbindco
valentlyto
ametal
surfacewith
the
thiolhea
dgrou
p,ex
tending
analky
lch
ain
more
orless
perpen
dicularto
theparticlesu
rface.
This
improve
stheco
mpati-
bility
with
athermotropic
hostco
nsiderab
ly,bu
tit
promotes
hom
eotrop
icalignmen
tof
themesog
enssu
rrou
ndingtheparticle,
hen
ceon
emay
expectthe
opposite
alignmen
tof
alka
ne
thiol-
functionalized
nan
orod
sco
mpared
tocarbon
nan
otube
s,i.e
.perpen
dicularrather
than
alon
gthe
liquid
crystaldirector.
The
much
lower
aspectratioof
mostinorga
nic
nan
orod
s(typ
ically
two
ordersof
mag
nitudeless
than
forcarbon
nan
otub
es)mak
esthe
alignmen
taslightlyless
seriou
sissu
e.In
fact,man
ystudiesdea
lwith
spherical
gold
orsemicon
ducting
quan
tum
dot
particles.
Another
altern
ativeis
touse
anon
ionic
polym
ersu
chas
poly-
viny
lpyrrolid
one(PVP)
foraidingthedispersion
[133
].Muc
hof
theworkdev
oted
toinorga
nicnan
oparticles
inliq
uid
crystalsisdev
oted
tomod
ification
oftheliq
uid
crystalp
rope
rtiesin
its
classic
electroo
ptic
dev
ice
(e.g.display
)ap
plication,e.g.
for
Fig.
14.Po
larizing
micrograp
hof
ane
matic
liquid
crystalph
ase
from
sing
le-w
all
carbon
nano
tube
sstab
ilize
dby
DNA
inwater
at4
wt.%
conc
entration
(scale
bar:
50mm
).From
referenc
e[114
].Co
pyrigh
tW
iley-VCH
Verlag
GmbH
&Co
.KG
aA.
Reprod
uced
withpe
rmission
.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1398
mod
ifyingsw
itch
ingtimeor
alignm
enttyp
e[137
,150
,153
,173
],an
dis
thusou
tsidethescop
eof
this
review
.From
thestan
dpo
intof
this
articletherearetw
olin
esof
research
inthisfieldthat
areof
particu
lar
interest.First,goldnan
oparticles
areattractive
due
totheirp
lasm
onic
resp
onse
butthis
dep
endsalso
onthedielectricpermittivity
ofthe
med
ium
surrou
ndingthenan
oparticles.The
refore,iftheco
ntinuo
uspha
seof
thego
ldnan
oparticleco
lloid
isathermotropicliq
uid
crystal,
theplasm
onicresp
onse
canea
sily
andrapidly
betuned
bych
anging
thedielectricco
nstan
tcom
ponen
texp
erienc
edby
thenan
oparticles
bysw
itch
ingtheliq
uid
crystalw
ithan
external
electricor
mag
netic
field
[132
,174
,175
].A
seco
ndthrust
that
may
turn
outto
beve
ryinterestingistheeffortto
dev
elop
metam
aterialsusingmetal
nan
o-particles
inliq
uid
crystals,b
utthisisco
nsiderab
lymorech
allenging
since
complexnan
oparticlesh
apes
arerequ
ired
,littlefluctua
tion
scanbe
accepted
,andhighlyregu
lara
rran
gemen
tsof
theparticles
are
sough
tfor[176
,177
].Wewill
return
tothisissu
ein
amom
ent.
Like
with
surfactant-stab
ilize
dcarbon
nan
otube
slyotropic
liquid
crystalhosts
canbe
quitead
vantage
ousin
ensu
ringahigh
load
ing
ofmetallic
nan
oparticles.Fo
rinstan
ce,using
the
same
cation
icsu
rfactant-ba
sedlyotropic
liquid
crystalhostas
was
used
very
successfully
forcarbon
nan
otube
alignmen
tby
Scalia
andco
-worke
rs,a
sdescribed
abov
e,Liuet
al.d
ispersedan
daligned
gold
nan
orod
sin
nem
atic
andhex
agon
alhostphases
[134
].Since
the
directorcan
bemacroscop
ically
aligned
and
laterrealigned
bymag
netic
fieldsin
thenem
atic
phaseor
shea
rflow
intheco
lumnar
phase,
theplasm
onic
resp
onse
could
betuned
bych
anging
the
nan
orod
orientation
.So
meof
themostb
eautiful
workon
particles
inliq
uid
crystalsha
sdea
ltwithsp
here-sh
aped
particles
ordropletsin
themicro-rather
than
nan
ometer
rang
e.W
hendispe
rsinge.g.
spherical
silic
abe
ads
functiona
lized
withcapping
agen
tsthat
induc
ehom
eotrop
icdirector
alignmen
tof
thesu
rrou
nding
liquid
crystal,or
glycerol
droplets,
gene
rally
induc
ingplana
rdirectoralignmen
t,ea
chinclusion
effec-
tive
lybe
comes
aunits
tren
gthtopo
logicald
efectinthedirectorfield
due
tothe
spherical
symmetry
ofthe
directoran
choring
atits
bound
ary.
Becau
sethetotaltopo
logicalch
arge
must
beco
nserved
,be
ingzero
inaflat
surface,ea
chpositiveunitdefectin
aflat
sample
cellintrod
ucedby
aninclusion
mus
tbe
acco
mpan
iedby
aneg
ative
unitdefector
adisclinationringge
neratednex
tto
it,turningthe
particlean
ditssu
rrou
ndingeffectivelyinto
atopo
logicaldipoleor
quad
rupo
le[178
].W
henseve
ralsuc
hmultipoles
arebrou
ghtc
lose
toea
chothe
r,e.g.
bymov
ingthem
withop
ticaltw
eezers,theinterac-
tion
sbe
twee
nthem
,inman
yresp
ects
analog
ousto
theinteractions
ofelectrostaticdipoles,resultin
chainsor
2Dpatternsbe
ingge
ner-
ated
andstab
ilized.A
ltho
ughbe
autifule
xperim
ents
ontheinterac-
tion
ofglycerol
droplets
with
the
nem
atic
director
field
were
condu
cted
byPa
wel
Pieran
skian
dco
-worke
rsalread
yin
the19
70s
[179
]the
mod
erninterestin
thisnew
classof
resp
onsive
collo
idtook
offafteran
eleg
antstudy
pub
lished
in19
97of
chainsof
surfactant-
stab
ilizedwater
dropletsin
liquid
crystals
byPo
ulin
etal.[18
0].A
num
bero
fgroup
shavesince
achieve
dfascinatinglarge-scaleordered
arrang
emen
tsof
collo
idal
inclusion
sin
nem
atic
hosts
basedon
the
sameprinc
iples,
suppo
rted
also
byanum
berof
theo
reticaltrea
t-men
ts[158
,181
e20
6],and
somegrou
psha
vedem
onstratedthesame
typeof
phe
nomen
ain
free
-stand
ingSm
Cfilm
s[207
e21
2].
Theresearch
onco
lloidal
inclusion
sin
liquid
crystals
has
sofar
been
mainly
offundam
entala
cadem
ictypebu
tafew
proposalsfor
future
applic
ationhav
ebe
enpresented.F
orinstan
ce,M
usevican
dco
-worke
rsdem
onstratedtheusage
ofsu
chasystem
asatunab
leop
ticalgrating[213
] .Th
epoten
tial
applic
ation
that
has
proba
bly
attractedthemostattention
,how
ever,isthat
ofproducingmeta-
materials
byhav
ing
the
liquid
crystalorga
nize
tailo
r-designed
nan
oparticles,s
uch
that
composites
withnew
anduniqueop
tical
properties
can
berealized
.Th
emaingo
alis
toprodu
ceneg
ative
refractive
index
materials
functioningin
theop
ticalwav
elen
gth
regime,
which
could
beused
for
cloa
king
orperfect
lensing
[176
,177
,214
e21
7].T
his
typically
requ
ires
thepositioningof
nan
o-scaleco
nductiverodsan
dco
nductiveinco
mplete
rings
(split
ring
resonators)
inregu
lararrays
over
largedistances.
Obv
iously,
theresearch
onliq
uid
crystal-drive
npositioningof
particles
isstill
very
farfrom
reachingthis
goal.Th
eparticlesize
must
bein
thenan
ometer
range
,hen
cetheworkon
micronscale
particles
must
bescaled
dow
n(this
may
not
bepossible
for
spherical
particles
ason
emay
reach
thelim
itof
theco
ntinuum
description
oftheliq
uid
crystal,be
low
whichtheelasticity
theo
ryforthedistorted
directorfieldbrea
ksdow
n),an
dthepositioningof
spherical
particles
isge
nerally
not
ofmuch
use
perse
inthis
context.Th
esu
ccessesin
aligningmetallic
nan
orod
san
dcarbon
nan
otube
s(w
hichcanbe
metallic
orsemicon
ductingdep
endingon
the
geom
etry
ofthe
grap
hen
enetwork
form
ing
the
tube
)is
promisingbu
tit
isstill
not
sufficien
t.Con
trol
ofposition
isalso
requ
ired
,som
ethingwhichhas
not
yetbe
enachieve
d,a
ndge
ner-
ally
also
thesp
litringresonatorsmust
bearrange
din
therequ
ired
configu
ration
.Finally,on
emust
also
ask
ifthe
generally
rather
strongfluctuationsin
liquid
crystals
areacceptab
leforthereali-
zation
ofmetam
aterials
withacceptableloss
leve
ls.
Anoriginal
approachto
realizingliq
uid
crystal-orga
nized
split
ring
resonators
was
presented
bythe
Ljubljana
team
arou
nd
Musevican
dZu
mer.They
trap
ped
gold
nan
oparticles
indisclination
lines
surrou
ndingmicronsize
dsilic
abe
adsthat
areorga
nized
into
anarrayby
thenem
atic
continuum
phase[193
],cf.F
ig.15.
Inthis
case
thenan
oparticles
(100nm
indiameter
inthis
study)
migrate
into
theve
ryco
reof
thedefectrings
that
form
arou
ndthelarger
collo
idalparticles,asthisreducesthevo
lumeof
liquid
crystalw
ithin
whichtheorientation
alorde
rbrea
ksdow
ndueto
thedisclination.
This
typeof
nan
oparticleordering,
somew
hat
differentfrom
the
previou
slydescribed
approaches,is
also
beingex
ploredforother
purposes,in
particu
larwith
theaim
toex
pan
dthetemperature
range
ofliq
uid
crystalbluephases.T
heseareex
otic
liquid
crystal
phases
characterize
dby
acu
biclatticeof
disclinationsthat
may
form
incase
ofve
rystrongch
iral
interactions[218
].Th
ephaseis
infact
isotropicdueto
thecu
bicsymmetry
ofthelattice,bu
tbirefringe
nce
canbe
inducedby
applyingan
electricfield,andthedisplayindustry
curren
tlyhas
ake
eninterest
inthepossibilitiesof
exploitingthis
effect
indisplays
that
could
beve
ryfast
andrequ
irenoalignmen
tco
ntrol
[219
].How
ever,thetemperature
range
ofbluephases
isnormally
very
small,
typically
abou
ta
deg
ree,
and
the
hop
eis
thereforeto
mak
ethedisclinationlin
elatticemoreperman
entby
fillingitwithnan
oparticles.S
omepositiverepo
rtshav
eap
pea
red,
both
ofex
perim
ental[14
1,22
0]an
dtheo
retical[20
3,22
1]nature,but
sofartheachieve
men
tsarenot
comparab
leto
themoredev
elop
edap
proachof
expan
dingthebluephasetemperature
range
bypoly-
merizingareactive
mon
omer
additivewithin
thebluephase[222
].
Fig.
15.(a)Co
lloidal
gold
particles(100
nmdiam
eter)trap
pedinto
theSa
turn
ring
dis-
clinationlin
esu
rrou
ndingalargesp
hericalsilica
particle
(1mm
diam
eter)su
spen
dedin
athermotropicne
matic
liquidcrystalh
ost.(b)Th
ecalculated
stab
leco
lloidal
ring
supe
r-structureforthisco
mbina
tion
ofco
lloids(ins
etsh
owsasche
matic
oftheco
rrespo
nding
electricalcircuit).R
eprinted
figu
rewithpe
rmission
fhttp://pre.ap
s.org/ab
stract/PRE
/v77
/i6/e06
1706
gffrom
[193
]g.Co
pyrigh
t(20
08)b
ytheAmerican
Physical
Society.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1399
Aninterestingva
riationof
liquid
crystal-ba
sedco
lloid
ordering
has
been
dev
elop
edby
theteam
ofHee
-Tae
Jung
atKAIST,Korea
,and
Oleg
Lavren
tovich
,Ken
tState
University
[223
,224
].Rather
than
nem
atic
hosts
they
workwithsm
ectics
andthey
confinethephase
inside
narrow
chan
nels
microfabricated
ina
silic
onwafer.Th
ech
annelsaretrea
tedwithafluorinated
polym
erin
order
toen
sure
planar
directoran
choringalon
gtheside
walls
aswellas
atthe
chan
nel
bottom
.Th
ech
annelsare
open
atthe
top
and
the
air
interfaceindu
cesstrong
hom
eotrop
icalignm
entthere.
Theco
nflict
inalignm
entbe
twee
nthetopan
dbo
ttom
forces
thesm
ecticphase
todev
elop
aregu
lara
rray
oftoricfocalcon
icdefects,inea
chof
which
thelaye
rscu
rvearou
ndacentraldefectlin
e.Asin
thecasesdis-
cussed
abov
e,co
lloidalparticles
introd
ucedto
theliq
uid
crystalscan
betrap
ped
inthesedefectlines.Inthisway
theteam
coulddistribute
fluorescent
particles
alon
ghex
agon
alpatternswithve
rylong
-ran
gefidelity
solely
byliq
uid
crystalself-assem
bly
[223
].Th
eregu
lar
smecticliq
uid
crystald
efecta
rray
canalso
beusedin
gene
ralasasoft
lithog
raphic
template
[225
]or
asa
basisforfabricating
supe
r-hy
drop
hob
icsu
rfaces
[226
].Byva
ryingthecrosssectionge
ometry
ofthe
micro
chan
nels
and
byreplic
amolding
the
structureson
toapho
topo
lymerizab
leprecu
rsor
materialthegrou
precentlysu
c-ceed
edin
broa
den
ingtheco
ncep
t[227
].Th
eyco
uld
now
crea
teava
rietyof
defectp
atternsan
din
contrastto
theinitialw
orks,w
hich
requ
ired
rather
specialsemi-fluorinated
mesog
ens,
they
can
now
workalso
withthemostco
mmon
lyem
ploye
dSm
Amaterial8
CB.
3.3.
Compo
site
materialscontaining
thermotropicliq
uidcrystalsas
func
tion
alad
ditive
3.3.1.
Emulsion
sof
cholestericdrop
lets
fortuna
bleom
nidirectiona
llasing
applications
Thereis
also
astronginterest
inco
lloidsof
thereve
rsetype,
where
droplets
ofliq
uid
crystalare
dispersed
inan
isotropic
continuou
sphase.
Thesystem
sin
focu
srange
from
dispersednan
odroplets
oflyotropic
liquid
crystalfordrugor
nutrientdelivery,
somethingwewill
comeba
ckto
inSe
ction3.5,
todropletsof
short-
pitch
cholesteric
liquid
crystalfor
lasing
applic
ations.
Inthe
follo
wingwewill
describethis
dev
elop
men
tin
somedetail,also
takingtheop
portunityto
brieflydiscu
ssthepoten
tial
ofch
olesteric
liquid
crystals
forlasingin
general.
Aswithan
yem
ulsion,lon
g-term
stab
ility
canon
lybe
ensu
red
byad
ding
anem
ulsifier
(e.g.a
surfactantor
polym
er)to
the
continuou
sphase,
that
adsorbsat
theinterfacebe
twee
ntheliq
uid
crystalan
dthesu
rrou
ndingmed
ium.Th
isreducestheinterfacial
tension
betw
eenthetw
ophases
anditco
unteractsco
alescence
ofthedroplets.
Thetypical
choice
fortheco
ntinuou
sphaseof
anem
ulsion
iswater,
with
the
added
emulsifier
being
either
asu
rfactant,oftenof
ionic
type,e.g.
SDSor
CTA
B(see
Fig.
2dee),o
rawater
solublepolym
ersu
chas
polyv
inylalco
hol
(PVA)or
poly-
viny
lpyrrolid
one(PVP).In
case
ofliq
uid
crystallinedroplets
the
choice
ofem
ulsifier
isqu
itecritical
because
itwill
influen
cethe
alignmen
tof
thedirectorat
theinterface,
andthereb
ythrough
out
thedropletifitisnot
toolargein
diameter.A
surfactantad
sorbsat
theinterfacewithitsnon
-polar
alky
lchainex
tendingradially
into
the
liquid
crystalphase,
imposing
aradialorientation
ofthe
director.
This
mea
ns
that
asu
rfactant-stab
ilize
dliq
uid
crystal
dropletwill
generally
hav
ehom
eotrop
icalignmen
t.PV
Aor
PVP,on
theother
han
d,adsorb
attheinterfacein
amoreor
less
random
coil
conform
ation,w
ithou
tactually
enteringtheliq
uid
crystal,an
dthis
leav
estheliq
uid
crystalwith
planar
alignmen
t,since
this
isthe
mostfavo
rableat
aninterfacewithapolar
phasesu
chas
water.
Asan
altern
ativeto
usingaq
ueo
ussolution
sof
surfactants
orpolym
erssomeresearch
ersch
oose
touse
silic
oneoil[228
,229
]or
glycerol
[230
,231
] astheco
ntinuou
sphase.Th
iscannot
give
along-
term
stab
leco
lloid,asnothingpreve
nts
theco
alescence
ofdroplets
andas
theinterfacialten
sion
isstill
non
-neg
ligible
(althou
ghnot
ashigh
aswith
water)bu
ton
atimescaleof
hou
rsto
day
ssu
chasystem
canbe
sufficien
tlystab
leforex
perim
ents.T
hebe
auty
ofthis
approachis
thesimplic
ityin
preparation.F
orinstan
ce,H
umar
andMusevicusedsimple
han
dstirringof
ach
olestericliq
uid
crystal
inglycerol
topreparean
emulsionwithdroplets
ofdiameter
inthe
15e50
mm
range
[230
].Th
eyused
cholesteric
mixtures
with
differentco
mpositionto
tunethehelical
pitch
over
alargerange
,themostinterestinghav
ingapitch
of30
0e40
0nm.W
ith
such
short-pitch
the
droplets
show
selective
Bragg
reflection
inthe
visiblerange
alon
gthedropletradius.Th
isisattractive
because
the
helix
ofach
olestericor
other
chiral
liquid
crystalsh
owingvisible
reflection
canactas
aresonan
tcavity
formirrorlesslasingifon
ead
dsafluorescentdye
with
emission
wav
elen
gth
tuned
tothe
reflection
band
ofthech
olesteric[232
e23
5].Indee
d,by
adding
Fig.
16.(a)Sche
matic
ofthech
olestericdrop
lets
func
tion
ingas
amed
ium
formirrorlesslasing
.(b)
Polarizing
microscop
yim
ageof
ash
ort-pitchch
olestericdrop
letreflecting
red
light.(c)
Lasing
from
thedrop
letwhe
npu
mpe
dwithan
extern
alpu
lsed
lasersource.A
dapted
withpe
rmission
(Cop
yright
2010
Optical
Societyof
America)
from
referenc
e[230
].(For
interpretation
ofthereferenc
esto
colorin
this
figu
relege
nd,the
read
eris
referred
totheweb
versionof
this
article.)
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1400
asm
allam
ountof
thelaserdye
4-dicya
nom
ethy
lene-2-methy
l-6-
(p-dim
ethy
laminostyryl)-4H-pyran
(DCM)
and
pumping
the
collo
idwithapulsed
freq
uen
cy-dou
bled
Nd:YAG
laser,lasingwas
achieve
dfrom
thedroplets
atab
out6
00nm,cf.Fig.16
.What
mak
esthis
dem
onstration
particu
larly
interesting
isthat
the
spherical
geom
etry
ofthe
cholesteric
droplets
lead
sto
omnidirection
allasing,
i.e.thelaserlig
htisem
ittedradially
inallthreedim
ension
s.Atthesametime,
theea
sein
tuningthech
olestericpitch
,e.g.b
ych
angingthetempe
rature,allowsforsometuningof
thelasing
wav
elen
gth,althou
ghtherange
islim
ited
bytherequ
irem
entto
match
theem
ission
wav
elen
gthof
thelaserdye
.Th
epreparationprocedure
isattractive
initssimplic
itybu
tthe
method
suffersfrom
lack
oflong-term
stab
ility
anditdoe
snot
offer
very
good
control
ofdroplet
diameter.A
fairly
largedispersity
inthe
dropletsize
isalso
tobe
expected.
Forim
prove
dco
ntrol
ofthe
collo
idch
aracteristicsitmay
thusbe
preferableto
use
morestan
-dard
emulsion
preparation
tech
niques,typically
dispersing
the
liquid
crystalin
anaq
ueo
us
continuou
sphase,
stab
ilizing
the
emulsion
with
added
polym
er(surfactants
wou
ldnot
beuseful
heresince
theradialorientation
ofthech
olesterichelix
requ
ires
planar
alignmen
t).S
till,
thestab
ility
ofsu
chasystem
may
not
besatisfactory,a
situationthat
motivated
Landfester
andco
-worke
rsin
theirworkon
‘miniem
ulsions’
ofoilphases
inwater
prepa
red
byultrasonication
todev
elop
asm
arttrick
[236
].Th
eyad
ded
asm
allam
ountof
hex
adecan
eto
thenon
-polar
phase,
actingas
a“super
hydrophob
e”.Its
very
low
solubility
inthe
aqueo
us
continuou
sphasecrea
tesan
osmotic
pressuredirected
into
the
droplet
interior
that
counteracts
the
Laplace
pressure,which
promotes
dropletco
alescence.T
hey
reportedadramatic
improve
-men
toftheem
ulsionstab
ility
inthisway,andthey
even
tested
the
concept
successfully
onliq
uid
crystald
roplets
[237
].How
ever,their
studydea
ltwithnon
-chiral
nem
atic
droplets
ofsu
bmicrondiam-
eter.If
one
wishes
toap
ply
theirmethod
olog
yto
the
case
ofch
olestericdroplets
forlasing,
whichmust
besometw
oordersof
mag
nitude
grea
terin
size
,on
ewou
ldthusnee
dto
reduce
the
quan
tity
ofstab
ilize
rin
order
tomatch
themuch
smallersu
rface-
to-volumeratio
ofthelarger
droplets.It
isalso
proba
blyad
vis-
able
touse
aless
intense
dispersion
method
than
ultrasonication,
whichtypically
yieldsve
rysm
alldroplet
size
.Anicealtern
ativeif
theeq
uipmen
tis
availableis
touse
asp
ecially
designed
micro-
fluidic
system
toproduce
dropletsof
controlledsize
,anap
proach
that
has
enab
led
quite
beau
tifulstudiesof
non
-chiral
nem
atic
liquid
crystaldroplets
under
varyingco
nditions[238
,239
].A
general
problem
with
lasing
from
dye
-dop
edch
iral
liquid
crystals
isthefact
that
continuou
swav
e(CW
)lasinghas
not
yet
been
achieve
d.Th
ismay
hav
eseve
ralcauses,
thetw
omostprob-
able
beinghea
tingof
theliq
uid
crystalsu
chthat
itspitch
chan
ges
toomuch
orev
ento
theex
tentthat
aphasetran
sition
into
isotropic
liquid
occu
rs,an
dthelim
ited
stab
ility
oftheorga
nic
dye
s(inor-
ganic
dye
sge
nerally
hav
etoopoo
rsolubilityin
theliq
uid
crystal).
Inthis
resp
ectthe
droplet-ba
sed
liquid
crystallasermay
offer
asolution
,provided
that
they
are
susp
ended
ina
low-visco
us
continuou
sphase
that
can
easily
bepumped
through
narrow
chan
nels.
By
inco
rporating
the
cholesteric
droplet
collo
idin
amicrofluidic
system
itsh
ould
bepossibleto
continuou
slypump
theco
lloid
pastthearea
exposed
bythepumpinglaser,su
chthat
no
singlech
olestericdropletis
exposed
totheCW
pumpingradiation
longe
rthan
itcansu
pport.Asnew
droplets
continuou
slyen
terthe
active
area
ofthe
system
aqu
asi-CW
laser
outputmightbe
achieve
d.
Thefuture
dev
elop
men
tof
this
very
new
concept
ofap
plying
aliq
uid
crystalem
ulsionwill
behighly
interestingto
follo
w.Th
einitial
resu
lts
hav
ealread
yattracted
much
attention
inthe
photon
icsco
mmunity[240
],hen
cethereis
astrongdrivingforce
fortaking
theco
nceptto
astag
ewherepractical
applic
ation
is
viab
le.If
the
remaining
obstaclescan
beov
erco
me,
this
wou
ldallow
ave
rych
eapfabricationof
tunab
le3D
lasers
ofan
entirely
new
type.
3.3.2.
Liqu
idcrystalline
shells
Colloids
with
liquid
crystaldroplets
can
beproduced
with
relative
lysimple
method
s.Arecentdev
elop
men
tmad
epossibleby
theintrod
uctionof
somew
hat
moread
vancedeq
uipmen
t,aco
axial
nestedcapillarymicrofluidic
system
forpreparingmultiple
emul-
sion
swithex
trem
elylow
dispersity
[241
],is
thepreparationan
dstudyof
collo
idswheretheparticles
arethin
shellsof
liquid
crystal
rather
than
droplets.E
achsh
ellco
ntainsadropletof
anisotropic
phase,
generally
anaq
ueo
us
surfactant
orpolym
ersolution
althou
ghglycerol
orsilic
onoilcan
also
beused,an
dthese
‘com
positedroplets’aresu
spen
ded
inaco
ntinuou
sphasethat
can
hav
ethesameor
differentco
mpositionas
theinner
dropletphase.
Mostworkhas
been
don
ewith
aqueo
usinner
andou
terphases
since
this
allowsforea
sytuningof
thedirectorge
ometry
atthe
shell’s
inner
and
outerbo
undaries.
Asdiscu
ssed
inthepreviou
ssu
bsection
,anionic
surfactantyieldshom
eotrop
icalignmen
tan
dapolym
erlik
ePV
Ainducesplanar
alignmen
t.Th
ework
inthis
new
field
has
largely
been
motivated
byaseminal
theo
reticalpap
erby
Dav
idNelson,published
inNan
oLettersin
2002
[242
].Hepointedou
tthat
aplanar-alig
ned
spherical
liquid
crystalsu
rfacecan
beex
trem
elyinterestingas
aba
sisfor
adva
ncedco
lloids.Th
etopo
logy
ofsu
chasystem
dictatesthat
the
liquid
crystaldirectorfield
must
hav
eatotaldefectstrength
ofs¼
þ2ov
erthesu
rface.
Wearealla
wareof
thissituationfrom
the
orientation
alfieldsp
anned
bythemeridianson
theea
rth,which
cometoge
ther
into
twos¼
þ1topolog
ical
defects
atthenorth
and
south
pole.
Thesamedefectarrange
men
tis
possiblein
aplanar-
aligned
spherical
liquid
crystalsu
rface,
butNelson
pointed
out
that
this
isnot
theen
ergy
minim
izingco
nfigu
ration
.Bysp
litting
each
unitstrengthdefect,or
disclination,into
twos¼
þ1/2
defects
theen
ergy
isreduced,a
stheen
ergy
den
sity
ofadefectis
propor-
tion
alto
thesquareof
itsstrength.(Se
eFig.7foran
overview
ofthe
mostco
mmon
nem
atic
topolog
ical
defects.)Moreo
ver,be
cause
the
directorfield
inthe
vicinity
ofa
disclination
isdistorted
,with
aco
nsequ
ent
elastic
energy
pen
alty,the
totalen
ergy
ofthe
spherical
system
isminim
ized
when
thefourhalf-va
lued
defects
arrange
asto
minim
izethetotald
istortionof
thedirectorfield.T
his
hap
pen
sforatetrah
edralco
nfigu
ration
ofthedisclinations.
Thereason
that
thisisparticu
larlyinterestingisthat
thedefects
arehigh
energy
points
wheretheliq
uid
crystallineorde
rbrea
ksdow
n,an
difaforeignnon
-liquid
crystallinesp
eciesis
added
,its
moleculesmay
beex
pectedto
aggreg
ateat
thedefects.T
hiswou
ldminim
izetheen
ergy
astheforeignsp
ecieswou
ldtake
upthesp
ace
where
otherwise
mesog
ens
must
be,putting
them
ina
non
-favo
rabledisordered
isotropic
statewith
higher
energy
than
the
orde
redbu
lk.If
theforeign
speciesareinco
mpatible
with
liquid
crystallineorder
yetmisciblewiththeliq
uid
crystallinematerialin
itsisotropic
state,
this
wou
ldfurther
promotemigration
ofthat
species
toward
the
defects.Th
istype
oflocaliz
ation
ofgu
est
particles
ormoleculesto
disordered
region
sof
aliq
uid
crystalh
ost
has
been
used
tocrea
tesoft
solid
swith
unusu
alproperties
[181
,182
]an
ditisthou
ghtto
betheorigin
ofthedramatic
polym
erstab
ilization
ofliq
uid
crystalbluephases
[222
],cu
rren
tlystrongly
explored
asa
poten
tial
basis
for
nex
tge
neration
high
spee
ddisplays.R
ecen
tlyitwas
also
usedforman
ipulatingan
dtran
slating
polym
ermoleculeswithin
anem
atic
liquid
crystalwithlocaliz
edreductionof
orientation
alorde
r[243
].Nelson’sco
njecture
was
that
thisphen
omen
onco
uld
beusedfor
anch
oring
ligan
dmolecules
tothe
topo
logicaldefects
inthe
sphericalliq
uid
crystalsurface,wheretheex
tern
alen
dof
thelig
and
moleculeswou
ldhav
ethecapacityto
bindto
other
ligan
ds.
The
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1401
water
soluble
endsof
the
ligan
dmoleculesco
uld
forinstan
ceco
mprise
complemen
tary
single
DNA
strands.
Becau
sethe
dis-
clinationsarearrange
don
atetrah
edronthelig
andswou
ldgive
the
liquid
crystalspheres
thepossibilityto
bondin
thesameway
assp
3 -hy
bridized
carbon
doe
s,bu
ton
amuch
larger
collo
idal,rather
than
atom
ic,scale.
Inthis
way
the
shells
could
self-assem
ble
into
collo
idal
diamon
d-like
crystals,as
illustrated
schem
atically
inFig.
17,s
omethingwhichis
exception
ally
difficu
ltto
achieve
with
regu
larco
lloids
which
typically
crystallize
inhex
agon
alclose-
packe
darrange
men
ts.A
collo
idal
crystalwith
adiamon
dlattice
structure
isex
trem
elyinterestingforthephoton
icsco
mmunity,
asthis
wou
ldallow
thege
nerationof
metam
aterials
withco
mplete
polarization-indep
enden
t3D
photon
icba
ndga
p.
Onecanim
agineafew
differentway
sof
achievingtherequ
ired
planar-alig
ned
liquid
crystalsu
rfacebu
tso
fartheon
lyroute
that
has
been
pursued
isviathemicrofluidics-producedliq
uid
crystal-
line
shells.Th
eteam
arou
nd
Dav
idWeitz,Alberto
Fern
andez
-Nieve
san
dTe
resa
Lopez
-Leo
nwerethefirstto
explore
this
line
ofresearch
andtoge
ther
they
hav
epublished
aseries
ofbe
autiful
experim
entalstudiesof
nem
atic
shells
[244
e24
7],c
omplemen
ted
bytheo
reticalan
alyses
from
other
grou
ps[248
e25
0].Itturn
sou
tthat
thedesired
tetrah
edralco
nfigu
ration
isnot
quiteas
easy
toachieve
inpracticeas
expected,thereason
being
theex
tension
within
thesh
ello
fthes¼
þ1/2
disclinationlin
es.W
hile
theen
ergy
den
sity
isproportion
alto
thesquareof
thedefectstrength,the
actual
energy
isalso
proportion
alto
thelengthof
thedisclination
line.
Thus,
the
liquid
crystaltypically
minim
izes
the
energy
bydistortingthesh
ellsomew
hat,renderingit
asym
metricwiththe
thinnestpointat
thetopor
bottom
(incase
ofim
perfect
den
sity
match
ingof
liquid
crystala
ndinner
fluid,w
hichis
theusu
alcase),
and
bringingallfourdefects
closeto
this
regime.
Althou
ghthis
increa
sesthedirectorfielddistortionin
thevicinityof
thedefects,
thereis
atotalen
ergy
gain
dueto
thereduced
extension
ofthe
disclinationlin
eswithin
thesh
ell[247
].In
aneleg
antrecentstudy
Lopez
-Leo
nan
dco
-worke
rssu
c-ceed
edin
thinningthesh
ellin
aco
ntrolledway
byex
pan
dingthe
intern
aldropletvo
lumethrough
osmotically
drive
nflow
from
the
continuou
sphase[245
].Ev
entually
thesh
ellge
tsso
thin
that
the
shellasym
metry
isneg
ligible
and
then
thetetrah
edralarrange
-men
toffou
rdefects
canindee
dbe
stab
ilize
d.H
owev
er,thissu
ccess
comes
attheco
stof
expan
dingthesh
elldiameter
quitedramati-
cally
,arather
subs
tantial
problem
consideringthat
thetarget
size
isthesu
bmicronrange
ofco
lloidswithattractive
opticalp
rope
rties.
Infact,thelargesize
ofthesh
ells(they
aretypically
intherange
ofab
out10
0mm)is
oneof
thetw
oke
yob
stacleson
theroad
toward
realizingNelson’svision
.Eve
nwithou
tthinningthesh
ellitis
very
difficu
ltto
produc
esh
ells
that
aresm
allerthan
some50
e60
mm
usingthemicrofluidic
tech
nique,
hen
ceamethod
toreduce
the
shellsizeby
abou
ttw
oordersof
mag
nitudeisrequ
ired
.Thismight
bepossibleby
produc
ingthesh
ells
withalargeam
ountof
volatile
solven
tsu
chas
chloroform
mixed
with
theliq
uid
crystal,which
could
beev
aporated
aftersh
ellproductionin
order
toreduc
ethe
shellsize
,bu
tcarryingou
ttheev
aporationwithou
tbrea
kingthe
shellis
adelicatetask
.Itis
not
unlik
elythat
themicrofluidic
shell
productiontech
niqueev
entually
has
tobe
replacedby
adifferent
tech
niquethat
issp
ecifically
adap
tedto
thetask
ofprodu
cingsu
bmicronsh
ells.T
heseco
ndissu
eto
address
istherealizationof
the
requ
ired
ligan
dmolecules.
Todateno
grou
phas
presented
any
effortson
designingan
dsynthesizingthesemoleculesin
practice.
While
thereis
proba
blynofundam
entalproblem
preve
ntingtheir
phy
sicalrea
lization,itisach
allenge
that
will
certainly
requ
iresome
ded
icated
effortsfrom
skilled
synthetic
team
s,an
dthech
emistry
invo
lved
islik
elyto
neither
besimple
nor
low
cost.
Theap
plie
dgo
alof
realizinginterestingpho
toniccrystalmeta-
materialsusing
liquid
crystalshe
llsisthusstill
farah
ead,w
ithsome
keych
allenge
sremainingto
bead
dressed
,bu
tthesh
ells
that
are
curren
tlyavailablearenev
erthelessex
trem
elyattractive
asthey
offer
afantastic
playg
roun
dfor
fund
amen
talscientific
research
.Fo
racad
emic
research
thesize
rang
eof
abou
t10
0mm
diameter
isqu
ite
perfect
andseve
ralgrou
pshavepub
lished
beau
tifulstudies
ofthe
texturaldev
elop
men
tsan
ddefectco
nfigu
ration
sin
thesh
ells.T
heab
ovemen
tion
edstudies
havegive
nusago
odund
erstan
dingof
how
thedirectorfieldan
dthedefects
arrang
ein
nem
atic
shells
ofva
ryingthickn
ess,foremostw
ithplana
ralignmen
tatb
othinterfaces
totheaq
ueo
uspha
ses.Astudy
whe
rethealignmen
tato
neinterface
was
chan
gedbe
twee
nplana
ran
dhom
eotrop
icwhile
theothe
ron
e
Fig.
17.Prod
uction
ofliq
uidcrystals
hells
inane
sted
capilla
rymicrofluidicde
vice,tog
ethe
rwithacartoo
nillus
tratingthepo
stulated
self-
assemblyof
ligan
d-eq
uipp
edtetrav
alen
tliq
uidcrystalsh
ells
into
adiam
ond-lik
eco
lloidal
crystal.Microfluidics
photog
raph
courtesy
ofHsin-
Ling
Lian
g.
Fig.18
.Tran
sition
from
thene
matic
(a,b
)totheSm
Aph
ase(c,d
)inaplan
ar-alig
nedsh
ello
f8CB
.The
shelld
iameter
isab
out1
00mm
.Pho
tograp
hsco
urtesy
ofHsin-
Ling
Lian
g,from
referenc
e[251
].Co
pyrigh
t(201
1)by
theAmerican
Physical
Society.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1402
was
kept
plana
rga
veinsigh
tintothedyn
amicsas
wella
sthephy
sics
preva
iling
inanem
aticsh
ellw
ithhy
brid
bound
aryco
ndition
s[246
].Th
efirststud
ieson
smecticsh
ells,an
dthetran
sition
betw
een
nem
atic
andsm
ecticstates,w
ererecentlypresentedby
Lian
get
al.
[251,252
],an
dLo
pez-Le
onet
al.[25
3].T
head
ded
complexity
of1D
positiona
lorder
heregive
srise
tonew
defects
andtexturalfea
tures
incase
ofplanar-alig
ned
shells,cf.theex
ample
inFig.18
.The
frus-
trationof
acrosssectionsu
rfacearea
that
dim
inishe
son
goingfrom
thesh
ellexteriortotheinterior,andthus
alaye
rnum
berthat
shou
ldge
tsm
allerin
case
ofa
cons
tantdirectororientation
,lead
sto
bucklin
ginstab
ilities
whe
rethedirectortw
ists
from
thein-to
the
outside,
with
aco
nseq
uen
tincrea
singund
ulation
ofthesm
ectic
laye
rs[251
].In
arecent
study
Lian
get
al.also
studied
the
nem
aticesm
ectictran
sition
insh
ells
withhy
brid
bound
aryco
ndi-
tion
s,reve
alingave
ryrich
seto
ftex
turalfea
tures,withbe
autifuland
intriguingtran
sitory
texturesarisingduringthetran
sition
[254
].A
strongdep
ende
nceon
shellsizean
dthickn
essan
don
thean
choring
strengthof
thedifferentstab
ilize
rsusedwas
also
found
,providing
seve
ralh
andlesfortuningthedefectco
nfigu
ration
inthesh
ell.
Thestudyof
liquid
crystallinesh
ells
isstill
very
much
inits
infancy,with
nostudiesex
istingye
ton
awholerange
ofliq
uid
crystalp
hases.T
heinfluen
ceof
chiralityhas
forinstan
cenot
atall
been
inve
stigated
sofar.Th
efuture
will
beex
citingforthis
field
whichmay
holdman
ysu
rprisesof
fundam
entalscien
tificnature,as
wellas
trueprogresson
theap
plie
dtrack,
even
tually
lead
ingto
highly
attractive
nov
elmetam
aterials.
3.3.3.
Liqu
idcrystal-fille
delectrospu
nmicro-/na
nofibe
rsA
very
new
dev
elop
men
tin
liquid
crystalresearch
that
holds
considerab
leap
plic
ationpoten
tial
istheproductionof
textile
fibe
rsfunctionalized
with
liquid
crystalin
the
core.W
ith
the
aim
tofabricateakindof
wov
enelectron
icpap
er,N
akataet
al.e
mploye
damod
ified
meltsp
inningtech
niqueto
produce
core-shea
thpoly-
olefi
nfibe
rsthat
contained
either
cholestericliq
uid
crystalor
anelectrop
horetic
dispersion
intheco
re[255
].In
case
ofsh
ort-pitch
cholestericliq
uid
crystalco
resthe
fibe
rswerereflecting
inthe
visiblerange
andtheco
lorco
uld
besw
itch
edoffby
applyingan
electric
field.T
his
how
ever
requ
ired
ave
ryhighdrivingvo
ltag
eof
400Vov
erthefibe
rs,w
ithou
terdiameter
inthe80
e10
0mm
range
.W
hile
not
atrad
itional
textile
,theweava
bility
oftheproduced
fibe
rsop
ensfortaking
the
concept
into
the
direction
ofsm
art
textile
s.Adifferent
approachto
produ
cing
liquid
crystal-co
ree
polym
ersh
eath
fibe
rs,with
conside
rably
smallerdiameter,was
recently
introd
ucedby
Lage
rwallan
dco
-worke
rs[256
].Using
coax
ialelec-
trospinning
they
could
introd
uceava
rietyof
liquid
crystalsinto
the
core
ofthesp
inning
jet,whe
rethesh
eath
fluid
was
asolution
ofwater
solublepolym
erslik
epolyv
inylpy
rrolidon
e(PVP)
orPV
A,som
etim
eswithinorga
nicsol-ge
lprecu
rsor
forprodu
cing
aco
mpo
site
shea
th,
e.g.
withaco
mpo
nent
ofTiO2.Electrospinningisaprocess
forsp
in-
ningpolym
erfibe
rsthat
israpidlyga
iningin
attentionacross
the
international
nan
oscien
cean
dtech
nolog
yan
dmaterials
scienc
earen
a.In
contrast
toco
nven
tion
altech
niques
thesp
inningis
not
drive
nby
highpressurebu
tthepolym
erfibe
ris
essentially
‘pulled’
outby
astrongelectric
field
from
adroplet
protruding
from
the
spinneret.T
heproduc
tisanon
-wov
enmat
offibe
rswithdiameters
typically
intherang
efrom
some10
0nm
toafew
microns
.Withthe
simplest
colle
ctor
geom
etries
thematsco
nsist
ofrand
omly
criss-
crossing
fibe
rsbu
tit
isrelative
lyea
syto
achiev
equ
ite
uniform
alignmen
t,e.g.
byusing
split
electrod
esfortheco
llector
[257
e25
9].
Part
ofthebe
auty
ofthemetho
dis
that
itrequ
ires
very
simple
equipmen
tthat
easily
fits
onan
ordinarych
emistryor
phy
sics
lab
bench
andit
isve
rydyn
amic
inthesense
that
thedetails
ofthe
spinningset-upcanea
sily
andqu
icklybe
varied
,allo
wingformuc
hex
perimen
tation
andrapidtestingof
new
idea
s.Wedonot
gointo
the
details
behindtheelectrospinningprocess
herebu
trefertheinter-
estedread
erto
oneof
seve
ralexcellent
curren
trev
iew
articles
onthe
topic[260
e26
5]as
wellas
abo
okch
aptersp
ecifically
dev
oted
toliq
uid
crystale
lectrosp
inning[266
].Th
eco
axialelectrospinningtech
niqueis
particu
larlyusefulfor
liquid
crystalresea
rchan
dap
plic
ationsas
itallowsessentially
any
fluid
tobe
spunin
thefibe
rco
re,since
thestronglynon
-New
tonian
polym
ersh
eath
solution
stab
ilize
stheliq
uid
jetan
dpreve
nts
the
Ray
leighinstab
ility
tobrea
kit
upinto
droplets.
Inprinciple,an
yliq
uid
crystalp
hasewithviscositylow
enou
ghto
bepumped
tothe
spinneret
can
thus
beinco
rporated
inside
electrospun
fibe
rs.
Lage
rwalla
ndco
-worke
rsdem
onstratedthisso
faron
regu
larnon
-ch
iral
nem
atics[256
],non
-chiral
smectic-A[267
]as
wellas
short-
pitch
chiral
nem
atics[268
].In
particu
larthelatter
isinteresting
from
anap
plic
ational
pointof
view
,since
theselectivereflection
oftheliq
uid
crystalisvisiblethrough
thesh
eath
forsu
fficien
tlythick
fibe
rs,thusgivingthewholemat
theco
lorof
theliq
uid
crystal,
easily
visibleby
thenak
edey
e.On
theother
han
d,if
themat
ishea
tedab
ovetheclea
ringpointof
theliq
uid
crystal,thisturn
sinto
anisotropic
liquid
within
thefibe
rco
re,remov
ingtheco
loran
dleav
ingthemat
withawhiteap
pea
rance
from
lightscatteringoff
thepolym
ersh
eath,cf.Fig.19
.Thissimple
experim
entprovided
anim
portant
dem
onstration
ofthe
applic
ation
poten
tial
ofthe
producedliq
uid
crystal-functionalized
matsin
sensingdev
ices.T
he
mat
was
ave
ryba
sicnon
-wov
entextile
basedtemperature
sensor,
butthe
same
conceptcan
bedev
elop
edforinstan
ceinto
gas
sensors
[269
e27
2],w
heretheprimead
vantage
swou
ldbe
low
cost
andsimple
production,rapid
andsensitive
resp
onse
dueto
very
largesu
rface-to-volumeratio,
directop
ticalrea
d-outby
thenak
edey
eby
untrained
users,fullau
tono
myan
dindep
enden
ceof
pow
ersu
pply,a
ndea
seof
integrationinto
clothing.
While
theco
axiale
lecrospinningap
proachis
very
efficien
tan
dprovides
good
control,on
ecanactually
achieve
theliq
uid
crystal
core-polym
ersh
eath
fibe
rmatsusingco
nve
ntion
alelectrospinning
aswell.Buyu
ktan
iret
al.m
ixed
stan
dardnem
atic
5CBwithpoly-
lactic
acid
usingamixture
ofch
loroform
andaceton
eas
common
solven
tan
dsp
unthis
usingsingle-fluid
electrospinning[273
].As
the
solven
tev
aporated
the
liquid
crystalan
dpolym
erphase
separated
,luck
ilywiththeliq
uid
crystalp
rimarily
intheco
re,thus
contained
bythepolym
er.T
hesimplic
ityof
this
approachis
very
attractive
butthedeg
reeof
control
doe
snot
quitereachtheleve
ls
Fig.
19.An
electrospu
nno
n-wov
enmat
ofpo
lyviny
lpyrrolid
one
fibe
rsco
ntaining
ash
ort-pitch
cholestericliq
uid
crystalmixture
inthe
core.(a)Re
flection
atroom
tempe
rature.(b)
Tran
smission
atroom
tempe
rature,sho
wingtheco
mplem
entary
red-
brow
nco
lorto
thereflectedblue
color.(c)Re
flection
afterhe
atingab
ovetheclea
ring
point.Onlythewhite
scattering
from
thepo
lymer
shea
this
visible,
theco
lorha
ving
disapp
earedwiththedisapp
earanc
eof
theorde
redstructureat
thech
olesterice
isotropic
tran
sition
.Ph
otog
raph
sco
urtesy
ofEv
aEn
z,from
referenc
e[268
].Re
prod
uced
bype
rmission
ofTh
eRo
yalSo
cietyof
Chem
istry.
(For
interpretation
ofthereferenc
esto
colorin
thisfigu
relege
nd,the
read
erisreferred
totheweb
versionof
thisarticle.)
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1403
oftheco
axialelectrospinningtech
nique,
hen
ceitsslightlyhigher
deg
reof
complexity
canin
man
ycasesbe
clea
rlyworthwhile
.In
term
sof
theprodu
ced
fibe
rsthedistingu
ishingfeature
ofelectrospinning
compared
toco
nve
ntion
alelectrospinning
isprimarily
theve
rythin
diameter.W
hile
itis
challengingto
reach
fibe
rdiametersbe
low
seve
raltensof
micronswith
conve
ntion
alsp
inningmethod
s,dueto
thedye
swella
sthepolym
ersolution
ormeltex
itsthesp
inneret,e
lectrosp
inningfreq
uen
tlyyieldsfibe
rsin
therange
oftensof
nan
ometersupto
afew
microns.Th
isgive
sthe
produced
matsve
ryhigh
surface-to-volumeratio,
mak
ingthem
very
attractive
forinstan
ceforsensingap
plic
ations.Itmay
also
bead
vantage
ousin
term
sof
bringingdow
nthevo
ltag
esrequ
ired
for
switch
ing
the
liquid
crystal,
althou
ghthe
stronge
rsu
rface
anch
oringupon
reducedinner
diameter
may
raisethethresh
old
voltag
e.Th
eelectrod
esrequ
ired
forap
plyingan
electric
fieldco
uld
berealized
indifferentw
ays.Th
ewholefibe
rmat
may
belaminated
betw
eenco
nductivesh
eets,rea
lized
forinstan
ceby
coatingregu
lar
textile
fibe
rswithcarbon
nan
otube
s[274
,275
],or
oneco
uld
add
athird
capillarytube
totheelectrospinningset-up
toallow
for
tripple
fluid
coax
ialsp
inning.
Inthis
way
fibe
rsco
nsistingof
aninterm
ediate
phasebe
twee
nash
eath
andacentral
core
material
hav
ebe
enrealized
[276
]an
difco
nductiveparticles
such
ascarbon
nan
otube
saread
ded
totheinner
andou
terpolym
ersolution
s,at
aco
ncentrationab
ovetheperco
lation
thresh
old,a
coax
iale
lectrode
geom
etry
wou
ldbe
realized
directlyin
each
fibe
r.Th
ecritical
challenge
forelectrospinningifthetarget
issm
art
textile
s,or
wea
rabletech
nolog
yas
itis
also
calle
d,isthedifficu
lty
inproducing
continuou
sya
rns
ofthe
fibe
rs.Electrospinning
generally
produc
esnon
-wov
enmatswherea
sattempts
topickup
thefibe
rson
asp
ool,individually
orsp
uninto
aya
rn,gen
erally
fail.
This
mak
esit
impossible
toweave
orkn
itfabricsusing
fibe
rsproduced
byelectrospinningan
dtheprosp
ects
forbringingthe
liquid
crystalco
refibe
rsto
truetextile
garm
ents
arethereb
yseri-
ouslylim
ited
.W
hat
issometim
esreferred
toas
‘pseudo-wov
en’
matscanbe
realized
bysp
ecialtech
niques
providingparticu
larly
good
control
ofalignmen
t,su
chthat
fibe
rscanbe
spunin
subs
e-qu
entlaye
rsof
parallelfi
bers,w
herethefibe
rdirection
ofon
elaye
ris
perpen
dicularto
that
ofitsneigh
borlaye
rs[277
e27
9].T
his
may
yield
matswith
better
mechan
ical
properties
than
random
non
-wov
ens,
althou
ghthey
wou
ldhav
edifficu
ltiesto
competewith
wov
enor
knittedtextile
s.Oneap
proach
toinco
rporatean
electrospu
nnon
-wov
eninto
atextile
fabricisto
laminateitbe
twee
nsh
eets
offabricwov
enfrom
trad
itional
textile
fibe
rs,an
approach
that
has
been
used
for
inco
rporatingprotectivefiltersmad
eou
tof
electrospunfibe
rmats
into
garm
ents
[280
].Th
ismay
beaway
forw
ardalso
forthefibe
rmatsfunctionalized
byliq
uid
crystal.Alternativelyon
emightsee
the
liquid
crystalelectrospinning
activity
primarily
asa
useful
exploratory
phasetech
nique,
itsve
rsatility
beingof
grea
tva
lueas
longas
thequ
estion
sto
bean
swered
dea
lwiththesp
innab
ility
ofcertainphases,theirresp
onse
tothestrongcy
lindrical
encapsu
la-
tion
within
thefibe
rs(quitestrongeffectson
liquid
crystalphase
sequ
ence
werefoundin
someof
thematsstudiedso
far[256
,268
]),
the
resp
onse
toch
ange
sin
the
environmen
talco
nditionsor
toex
tern
alfieldsetc.
Once
aco
ncept
iden
tified
asusefulsh
ould
bebrou
ghtto
astag
ewheretruetextile
scanbe
produc
ed,this
could
then
betran
sferredto
conve
ntion
alsp
inningtech
niques,a
task
that
shou
ldbe
feasible
provided
that
theve
rythin
diameter
ofelec-
trospun
fibe
rsis
not
requ
ired
.Fo
rfabricsthefibe
rdiametersare
usu
ally
much
larger,h
ence
this
shou
ldge
nerally
beacceptable.
3.4.
Actua
tors
mad
efrom
liquidcrystalelastomers
Theco
mbinationof
liquid
crystals
andpolym
ersis
attheco
realso
ofan
other
subfieldof
mod
ernliq
uid
crystalresea
rch,that
has
dem
onstratedgrea
tpoten
tial
andattractedman
yplaye
rsfrom
all
over
theworld.Th
isis
thefield
ofliq
uid
crystallineelastomers,
produced
bywea
klycross-lin
king
reactive
mesog
enspresentin
athermotropic
liquid
crystalphase,
afteren
suringamacroscop
i-cally
uniform
directoralignmen
t.Th
eprodu
ctis
anem
atic
orsm
ectic
side-
ormain-chain
polym
errubb
erwith
actuating
capacity.
Due
tothe
long-range
order
ofthe
liquid
crystalthe
polym
erba
ckbo
ne
issu
bstantially
stretched
outin
the
liquid
crystallinestate,bu
tifthesystem
istake
nto
theisotropic
statethe
back
boneisfree
toad
optan
entrop
ically
favo
redrandom
coilstate
instea
d[281
e28
3].A
saresu
ltthewholesystem
shrinks
alon
gthe
initialdirectororientation
,while
itex
pan
dsin
theperpen
dicular
dim
ension
s.Th
etran
sition
betw
een
the
liquid
crystalline
and
isotropicphases
canbe
trigge
redeither
bytempe
rature
chan
geor
bylig
htirradiation
,thelatter
option
mad
epossibleby
thead
dition
ofphotoisomerizab
legrou
pssu
chas
azob
enze
ne.
Thesh
apech
ange
canbe
quitedramatic
andve
ryfast,a
ndthe
concept
has
thusbe
enproposed
foruse
inva
riou
stypes
ofactuator
dev
ices.F
ilmsor
ribb
onsof
liquid
crystalelastomer
wereusedas
lightdrive
nmotors[284
]or
artificial
muscles[285
],sh
eets
ordisks
can
resp
ond
tolig
htirradiation
bybe
ndingalon
gan
ydirection
,ch
osen
bysetting
the
lightpolarization
[286
],or
turn
edinto
swim
mersthat
mov
eaw
ayfrom
alig
htsource[287
],an
dfibe
rsof
liquid
crystalelastomer
show
ed‘hea
vy’weigh
tlifting
capacity
[288
].W
hen
adye
-dop
edsh
ort-pitch
chiral
nem
atic
was
turn
edinto
arubb
eritco
uld
beusedas
alaserwithem
ission
wav
elen
gth
that
could
betuned
bystretchingtherubb
er[289
].A
crucial
requ
irem
entforachieving
efficien
tactuation
from
liquid
crystale
lastom
ersisamon
odom
ainsample,i.e.thedirector
field
must
beuniformly
aligned
through
outtheru
bber
sample.
Seve
raltechniques
hav
ebe
endev
elop
edto
mee
tthisrequ
irem
ent
[290
],e.g.
starting
with
anon
-polym
erized
system
ofreactive
mesog
ensthat
can
bealigned
usingco
nve
ntion
alliqu
idcrystal
alignmen
tlaye
rsprior
tocross-linking,
orap
plyingatw
o-stag
ecross-linking
process
with
amom
entof
uniaxial
stretching
inbe
twee
n.R
ecen
tlyafew
interestingaltern
ativeap
proaches
were
added
tothetoolbo
xformak
ingliqu
idcrystale
lastom
eractuators,
includingmicro-or
nan
otem
plating[291
,292
]an
dinkjet
printing
[293
] .Another
highly
original
rece
nt
addition,allowing
the
productionof
quitepeculiar
liqu
idcrystalelastomer
objects,
was
introd
ucedby
theZe
ntelg
roup.T
hey
pumpdroplets
ofanem
atic
liquid
crystallinephaseform
edby
areactive
mesog
enic
mon
omer
through
amicrofludic
system
athigh
shea
rrate,su
chthat
the
director
field
within
each
droplet
iswell-defi
ned
[238
,294
].Duringtheflow
thedroplets
ewhichco
ntain
photoinitiatoran
dcross-linke
rin
additionto
thereactive
mesog
ense
areex
pos
edto
high-p
ower
ultraviolet
radiation
that
ensu
res
arapid
cros
s-lin
king,
thusturn
ingthedroplets
into
rubb
erparticles.Byhea
t-ing
orco
oling
pastthe
nem
aticeisotropic
tran
sition
ofthese
elastomeric
particles
their
shap
ecan
bereve
rsibly
chan
ged
betw
eensp
herical
andellipso
idal
orbe
twee
nob
late
andprolate
ellipso
id.
This
very
recentad
ditionto
thetoolbo
xof
liquid
crystalelas-
tomersmay
provideseve
ralnew
open
ings
forex
citingresearch
aswellas
applic
ations.
Althou
ghtheparticles
sofararerelative
lylarge,
the
dispersity
issm
allan
dthe
poten
tial
todyn
amically
switch
betw
een
differentsh
apes
isclea
rly
very
attractive
for
adva
ncedco
lloid
science.Iftheparticlesize
canbe
shrunkto
the
range
ofab
outamicronwecanex
pectve
ryinterestingresu
ltsin
term
sof
collo
idcrystalswhichcanbe
mad
eto
chan
gesymmetry
bymea
nsof
temperature
chan
geor
lightirradiation
.Moreo
ver,itcan
beve
ryinterestingto
disperse
theliq
uid
crystalrubb
erparticles
inanon
-polym
ericliq
uid
crystalh
ost,allowingthedyn
amictuningof
bych
angingthesh
apeof
theparticleby
inducingtheliq
uid
crystal-
isotropicphasetran
sition
.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1404
Another
compellin
grecentdev
elop
men
talon
gthis
trackis
the
combination
oftheliq
uid
crystalsh
ellco
nceptdescribed
abov
ewith
liquid
crystalelastomers,
explored
inajointeffort
ofthe
Zentelan
dLage
rwallteam
s[295
].Byprodu
cingthesh
ells
with
areactive
mon
omer
nem
atic,dop
edwithcross-lin
keran
dphoto
initiator,an
dirradiatingthem
withUVlig
htduringflow
,thefluid
shellscanbe
turn
edinto
stab
lesh
ellsof
rubb
er,thesh
apeof
which
can
bech
ange
dsu
bstantially
bytaking
the
system
past
the
nem
aticeisotropic
tran
sition
.Bypok
ingasm
allholein
thesh
ell
with
amicropipette
this
shell
dem
onstrated
rapid
pumping
capacityan
dtheco
nceptco
uld
thusbe
usedforrealizingon
e-piece
micro
pumpsthat
could
finduse
forinstan
cein
microfluidics.
3.5.
Liqu
idcrystals
inbiolog
y,biotechn
olog
y,food
scienc
ean
dph
armacolog
y
Itis
no
coinciden
cethat
the
mem
bran
esof
the
cells
inou
rbo
diesarein
alyotropiclamellarliq
uid
crystallinestate.Itprovides
just
theco
mbinationof
sturdinessan
dflex
ibility
that
liferequ
ires.
Asolid
phasewou
ldhav
ebe
enfartoostiff,rigidan
dim
permea
ble,
wherea
san
isotropic
liquid
could
not
separatethecell
from
its
surrou
ndings
orprovideascaffoldingformem
bran
eproteinsan
dinternal
compartm
ents
fortheorga
nelles.
Itis
thusperhap
snot
surprisingthat
liquid
crystalsplayave
ryim
portantrole
inbiolog
y.W
hat
ismoresu
rprising
isthat
this
role
has
long
been
largely
neg
lected
and
that
itis
only
recentlythat
such
issu
esas
phase
tran
sition
sin
lipid
mem
bran
esan
dtheeffect
ofch
olesterolo
nthe
lyotropic
phasesequ
ence
oflip
idshav
ebe
comeahot
topic.
Furthermore,theco
nnection
ofbiolog
yan
dliq
uid
crystalsisnot
limited
tocellmem
bran
es.Ithas
longbe
enkn
ownthat
DNAform
sliq
uid
crystalp
hases
[296
e29
8],b
utrecentlyitwas
even
proposed
that
this
very
fact
may
hav
ehad
ake
yrole
intheea
rliest
dev
el-
opmen
toflifein
theprebioticera[299
e30
1].Likew
ise,present-day
cryo
electron
microscop
ytech
niques
hav
eallowed
studiesof
virus
capsidsthat
reve
althat
theviralD
NAcanbe
stored
soco
mpactlyin
thecapsidbe
cause
itform
sahighly
ordered
liquid
crystallinephase
[302
e30
5].A
ninterestingob
servationis
that
man
yfeaturesfound
inliv
ingorga
nismsareve
ryreminiscentof
liquid
crystaltextures,
andcanoftenbe
mod
eled
usingdescription
sof
liquid
crystalline
order
[306
e30
8].
Theinterest
inliq
uid
crystals
inbiolog
yan
dtheim
plic
ationsof
liquid
crystallineorde
rforbiolog
ical
processes
istoday
increa
sing
strongly.
Ween
dthis
briefreview
ofgrou
ndbrea
kingactivities
inmod
ernliq
uid
crystalscien
ceby
highlig
htingafew
ofthethem
esin
thisfield.First
wewill
how
ever
discu
sssomerelatedtopics,where
liquid
crystalsareusedas
biod
etectors
oras
smartc
argo
carriers
for
delay
eddrugdelivery.
3.5.1.
Biod
etection
withliq
uidcrystals
Becau
sea
reorientation
ofthe
director
has
such
dramatic
consequ
enceson
the
opticalproperties
ofa
liquid
crystal,
and
because
thedirectoralignmen
tat
aninterfaceisso
sensitive
tothe
chem
ical
andphy
sicale
nvironmen
tat
theinterface,
liquid
crystals
hav
egrea
tpoten
tial
foruse
insensors.An
exam
ple
that
has
attracted
strong
attention
during
thelast
decad
esis
theuse
ofthermotropic
nem
aticsforam
plifying
biolog
ical
binding
even
ts,
aresearch
field
that
was
largelypionee
red
bytheteam
arou
nd
Nicholas
Abb
ott
atthe
University
ofW
isco
nsin-M
adison
[309
e31
5].Th
eke
yidea
isto
prepare
anem
atic
phase
with
aparticu
laralignmen
t,either
uniformly
planar
orhom
eotrop
ic,a
tan
interfacewith
anaq
ueo
usphase.
Dep
endingon
thedirector
alignmen
ttheliq
uid
crystalwill
exhibitach
aracteristic
texture
inpolarizingmicroscop
y.Th
ean
alyteis
then
added
totheaq
ueo
us
phasean
difabiom
olecularbindingev
entlik
ean
tige
nean
tibo
dy
reco
gnition
take
splace
atthe
interface,
the
liquid
crystalwill
experience
ach
ange
initsen
vironmen
tat
theinterface.
Asaresu
ltthedirectormay
reorient,mod
ifyingtheop
ticalprope
rtiesof
the
liquid
crystalan
dadistinct
chan
gein
thepolarizingmicroscop
ytexture
resu
lts.Th
eliq
uid
crystala
mplifi
esthebindingev
entsu
chthat
itbe
comes
easily
visibleon
amicroscop
icscale.Th
eco
ncept
ismostoftenap
plie
dto
planar
liquid
crystalsamplesbu
talso
liquid
crystaldroplets
weresu
ccessfully
usedfordetectingba
cteria
and
viruses[309
].W
hile
mostbiod
etectors
requ
irefluorescenttagg
ing
orlabe
lingof
keymolecules,
this
isunnecessary
with
theliq
uid
crystal-ba
sed
detection
tech
nique.
Thusit
isoften
referred
toas
labe
l-free
biod
etection
.Re
centlyother
research
ersex
tended
this
concept
todetectthe
presence
ofsingle-stranded
(ss)
DNAusingliq
uid
crystals[316
,317
].Pricean
dSchwarz[317
]prepa
red
athermotropic
nem
atic
atan
interfacewithan
aqueo
ussolution
ofacation
icsu
rfactant,inducing
hom
eotrop
icdirectoralignmen
tsu
chthat
thesample
appea
rsdark
inpolarizingmicroscop
y.IfssDNA(anionic)is
added
,electrostatic
complexa
tion
betw
eentheop
positelych
arge
dDNAan
dsu
rfactant
take
splace,ch
anging
theliq
uid
crystalalignmen
tan
dthusthe
texture.Ifthen
theco
mplemen
tary
DNA
strandis
added
thetw
ostrandsform
aduplex,
brea
kinguptheelectrostatically
complexe
dDNA-surfactantstructure
and
thusag
ain
chan
ging
the
nem
atic
directororientation
,an
dag
aintheop
ticalap
pea
rance
chan
ges.
Asimila
rap
proachwas
follo
wed
byChen
andYa
ng[316
]an
dthey
even
succee
ded
inqu
antitative
determinationof
theDNAco
ncen-
trationba
sedon
theliq
uid
crystalrea
lignmen
t.
3.5.2.
Drug/nu
trient
deliv
erywithliq
uidcrystalcu
bosomes
and
hexo
somes
orliq
uidcrystalline
crem
esByreplacingthethermotropic
cholestericliq
uid
crystalin
the
concept
ofliq
uid
crystal-co
ntaining
dropletsor
collo
idal
particles
discu
ssed
inSe
ction
3.3.1
bya
suitab
lelyotropicpha
se,an
dby
bringingtheparticlesize
dow
nto
thesu
bmicronrang
e,system
swith
strong
applic
ationpoten
tial
inthefieldof
pha
rmaceu
ticalsciences
andfunctiona
lfoo
dscanbe
realized
[318
e32
2].The
lyotropicpha
seis
form
edby
bioc
ompatibleam
phiph
ilesin
water,p
ossiblyalso
withan
oil(alsobioc
ompatible),an
dtheinterest
has
sofarbe
enfocu
sed
primarily
oncu
bicor
hex
agon
alpha
ses.Th
eliq
uid
crystalpha
seis
contained
inathin
shello
famphiph
ilicblockco
-polym
er.T
hese
soft
matternan
oparticles,g
enerally
prepared
viaultrasonication
giving
diametersin
the10
0nm
rang
e,arereferred
toas
cubo
somes
and
hexosom
es,respe
ctively,an
dthey
canbe
heav
ilyload
edwithdrugs
fororal
delivery.So
meex
amples
aresh
ownin
Fig.
20.
Theinterest
intheserather
adva
ncedve
sselsfordrugdelivery
has
seve
ralo
rigins[323
,324
].First,be
cause
theco
reof
theparticle
containsroug
hly
equal
amou
nts
ofpolar
andnon
-polar
regimes,
theparticles
can
carryhy
drophilicas
wellas
hydrophob
icdrug
cargo,
even
both
atthesametime.
Withsimplerclassiclip
osom
es(also
calle
dve
sicles,essentially
ash
ell
ofa
lamellar
phase
mem
bran
eof
lipidssu
rrou
ndingan
aqueo
usinterior)thecargocan
only
behy
drophilic.Aseco
ndim
portanta
dva
ntage
isthat
thedrugs
are
protected
inthe
particle
core
from
chem
ical
oren
zymatic
deg
radationduringthepassage
throug
hthedigestive
system
tothe
site
ofrelease.Directlyrelatedto
thisisthefact
that
thereleaserate
issu
bstantially
slow
erthan
forsimple
liposom
esdueto
themore
complexsoft
solid
build
upof
theco
re.A
conve
ntion
allip
osom
ereleases
itstarget
immed
iately
once
thelip
idmem
bran
ehas
been
brok
enthrough
.Th
eslow
release
and
high
capacity
forhy
dro-
phob
icload
mak
ethese
liquid
crystalline
collo
idal
particles
particu
larlyinterestingforcancertrea
tmen
t,as
thecy
totoxicity
of(frequ
entlyhy
drophob
ic)cancerdrugs
may
been
han
ced.
What
isso
farprimarily
discu
ssed
inthis
fieldin
term
sof
drug
deliverycanin
general
easily
betran
slated
tonutrientdeliveryvia
smartfood
sinco
rporatingfood
-gradeliq
uid
crystals.Apartfrom
thead
vantage
salread
ylis
ted,lyo
trop
icliq
uid
crystalphases
may
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1405
also
beusefulinfood
sby
controllin
gthetexture,v
isco
sity
etc.Th
isco
ncept
iscu
rren
tlybe
ingex
plored
byseve
ralmajor
compan
ies
active
inthetech
nical
food
ssector.
While
thisfieldisfascinatingan
dholdspromise,be
ingtheba
sis
even
forafew
compan
iesactive
inthefield,therearesomeke
ych
allenge
sto
address.P
rimarily
they
rega
rdrobu
stnessan
dstorag
estab
ility
oftheco
lloidal
particles
andthescalab
ility
issu
e.W
hen
tran
sition
ing
toa
commercial
produ
ctthe
production
method
sem
ploye
din
alabarege
nerally
not
applic
able.T
heco
stis
another
importantissu
e.En
capsu
latingdrugs
ornutrients
insideco
lloidal
liquid
crystallineparticles
isarather
expen
sive
venture
andthere
must
beasu
bstantial
gain
inefficien
cyto
motivatesu
chadiffer-
ence.So
far,
the
experim
entalev
iden
ceof
added
value
isnot
sufficien
tly
convincing.
The
requ
irem
ents
onbioc
ompatibility/
food
-gradematerials
arealso
quiterestrictivean
drule
outman
yop
tion
sthat
workwelld
uringin-vitro
labex
perim
ents
forcrea
ting
stab
leco
lloidal
particles
withdesired
prope
rties.
Butperhap
sthe
grea
test
challenge
istheim
men
seco
mplexity
ofthehuman
body.A
myriad
ofbiop
hysico
chem
ical
param
etersmust
beop
timized
inparallelan
dthepathway
from
themou
thto
thetarget
site
inthe
bodyis
soco
mplexan
dinvo
lves
soman
ystag
eswithdrastically
varyingen
vironmen
tsthat
itis
extrem
elydifficu
ltto
findadrug
deliveryve
ssel
designthat
withstan
dsthejourn
ey.
Aslightlyless
tech
nically
adva
nced
way
ofem
ploying
liquid
crystals
indrugdeliveryis
intopical
orother
directap
plic
ation
[325
,326
].Th
ehigh
viscosity
oftheliq
uid
crystalphaseaidsin
localiz
ationof
drugs
e.g.
attheskin
orsu
bcutaneo
usly(under
the
skin),
intram
uscularly,
orin
body
cavities
such
asthe
mou
th.
Moreo
ver,
tran
sition
sbe
twee
ndifferentliq
uid
crystallinephases
induced
bytemperature
orco
ncentration
chan
geallow
dyn
amic
tuning
ofproperties
like
consisten
cy,sh
ape
stab
ility
etc.
tothe
requ
irem
ents
ofea
chstag
e.Fo
rex
ample,skin
crea
mcan
bein
acu
bic/hex
agon
alphaseat
room
temperature
butmeltto
lamellar
byhea
tingfrom
rubb
ing,
allowingqu
ickan
dsimple
administration
butreduced
stickinessan
dsm
earinessafterw
ard
asthe
higher
orde
redphaseis
again
form
edupon
cooling.
This
strategy
holds
importance
also
inco
smetics.
Another
smartex
ample
iswhere
alow-visco
us
solution
containing
almostno
water
turn
sinto
aviscou
sliq
uid
crystallinephasewhen
mixed
with
water
atthe
target
location
,whichin
thiscase
wou
ldtypically
bethemou
th.In
this
way
thedrugcan
behan
dledan
dap
plie
dve
ryea
sily,ye
tit
stay
sin
place
once
administered.
3.5.3.
Lipido
micsan
dtheliq
uidcrystalph
ases
ofcellmem
bran
esAsmen
tion
edab
ovetheL a
lamellarliq
uid
crystallinephaseis
the
naturalstate
ofou
rcell
mem
bran
es.In
recentye
arsit
has
beco
me
increa
singly
clea
r,how
ever,that
one
actually
nee
dsto
distingu
ishtw
osu
bclasses
ofthis
phase.
Tothis
endtheco
ncepts
liquid-orde
red
(lo)
and
liquid-disordered
(ld)
states
were
recently
introd
uced[327
e33
1].T
heterm
“liquid”hererefers
tothelack
oflong-range
positional
order
(andthuslack
ofalattice)
ofthelip
ids
within
thebilaye
rplanean
dahighsp
eedof
lateraldiffusion
,in
contrastto
thege
lphases
likeL0 b
which
inthis
term
inolog
yare
considered
as“solid”(they
do
hav
epositional
order
within
the
mem
bran
ean
dlaterald
iffusion
isve
ryslow
).Th
ead
ditions“ordered
”or
“disordered
”referto
thestateof
the
acyl
chains.Th
eclassicL a
picture
fits
thel dphasebe
stbe
cause
there
areman
yga
uche
conform
ersam
ongthech
ainsan
dthey
areve
ryflex
ible
and
dyn
amic.Th
ech
ains
are
considered
molten,in
arando
mco
nform
ation.In
theen
dof
the19
80’s
itwas
realized
,how
ever,that
astatecan
dev
elop
wherethech
ainsaremainly
(albeitn
otsolely)inall-tran
sco
nform
ation,yet
withou
tdev
elop
ing
thein-planepositional
order
ofthege
lphases.S
uch
astatecanbe
described
as“liquid”as
wella
s“ordered
”an
ditisthusthel ostate.
Itsap
pea
rance
ispromoted
bythepresence
ofch
olesterol,thestiff
and
largely
hydrophob
icstructure
ofwhich
prefers
tobe
sur-
rounded
byordered
alky
lch
ainsan
dthusen
han
cesthe
chain
ordering.
Atthesametimeitsbu
lkysh
aperuinslong-range
in-
planepositional
orde
rof
thelip
idsan
ditthuspromotes
a“liquid”
stateov
era“gel”state.
Theco
nsequ
entfrustration
isthou
ghtto
betheorigin
ofthel ophase.
Itis
not
clea
rif
thel o
phasecan
existmacroscop
ically.It
istypically
found
asmicroscop
icdom
ainssu
spen
ded
inaseaof
l dphasein
mod
elmem
bran
es.Th
ephases
thusco
existlaterally
asaresu
ltof
phaseseparation.T
hissituationhas
give
nthel odom
ains
the
nam
e‘lipid
rafts’.A
dee
per
analysis
has
reve
aled
them
asdyn
amic
fluctuatingnan
oscale
regimes
enrich
edin
sphingo
lipids,
cholesterola
ndtran
smem
bran
eproteins.Th
esu
rrou
ndingl d‘sea
’is
instea
den
rich
edin
ordinary
glycerop
hospholipids.
One
cannot
draw
theco
nclusion
that
thel oan
dl dstates
necessarily
occu
rin
live
Fig.
20.Cryo
-TEM
imag
esof
softna
nopa
rticlesco
ntaining
reve
rsed
bico
ntinuo
us(aed),spo
nge(eef)an
dreve
rsed
hexa
gona
l(ge
h)lyotropicliq
uidcrystalp
hases.Re
printedwith
perm
ission
from
referenc
e[320
].Co
pyrigh
t(200
5)American
Chem
ical
Society.
J.P.F.L
agerwall,G.S
calia
/Cu
rren
tApp
liedPh
ysics12
(201
2)13
87e1412
1406
cell
mem
bran
es,since
living
system
sare
farfrom
equilibrium.
Nev
ertheless,
the
lipid
raft
parad
igm
with
localincrea
ses
insp
hingo
lipid
and
cholesterolco
ntent
has
now
been
generally
accepted,a
lsoformem
bran
esof
livingcells
.Thetenden
cyof
this
lipid
compositionto
promoteincrea
sedch
ainorde
rcertainly
plays
arole
intheprocess,m
ostlik
elyin
providingco
nnectivity,w
hichis
then
further
mod
ulatedby
thesp
ecificity
ofthemem
bran
eproteins
aswellas
theiran
tibo
dies[330
].Th
einve
stigationof
lipid
raftsan
dl o-ldphaseco
existence,a
nd
theco
nsequ
encesforcellsign
alingan
dtraffick
ing,
isave
ryactive
research
area
that
form
san
importantco
mpon
entof
theem
erging
fieldof
‘lipidom
ics’.W
ecanex
pectve
ryinterestingresu
ltsin
the
future
and
asthe
(still
relative
lynew
)liq
uid-ordered
phase
beco
mes
better
understoo
dthiswill
certainly
beago
odsu
bjectfor
studyfrom
aclassicliq
uid
crystalp
hysics
pointof
view
.Atpresent,
the
research
isco
nducted
primarily
follo
wing
abiop
hysical
chem
istryap
proach,h
ence
theinputfrom
research
erswithstrong
liquid
crystalba
ckgrou
ndmay
providenew
andinterestinginter-
disciplin
arylin
esof
research
.Enco
uragingex
amplesaregive
nby
theworkof
theHirst
grou
p,USA
[331
e33
3],foc
usingstronglyon
photo-induced
phaseseparation
effectsin
lipid
mem
bran
esan
dtubu
les,an
dof
theLe
egrou
p,K
orea
,whouse
surfacetopog
raph
yto
influen
cean
dev
enpattern
lipid
rafts[334
].
4.Co
nclusionsan
doutlook
Wehop
ethat
this
briefreview
oftheex
citingactivities
inthe
broa
dfieldof
liquid
crystalresearch
detached
from
displayap
pli-
cation
swill
hav
einsp
ired
theread
eras
much
astheau
thorsare
insp
ired
bytheresearch
discu
ssed
.Sev
eral
new
routeshav
ebe
eninitiated
during
thelast
few
yearsan
dsomemay
dev
elop
into
highly
successfulresea
rchfieldsan
dpossiblyap
plic
ations,wherea
sothersmay
turn
outnot
tobe
soprolifi
c.In
mostof
thefieldsdis-
cussed
wearestill
atarelative
lyea
rlyex
plorative
stag
e,an
dwe
hav
eto
waitafew
yearsto
assess
thepoten
tial
ofea
chthem
e.In
anycase,thisisave
ryex
citingdev
elop
men
t,ou
tliningabrightan
drich
future
fortheop
en-m
inded
liquid
crystalscien
tist
orother
soft
matternan
o-/m
icrotech
nolog
yresearch
erwithan
interest
inliq
uid
crystals.
Ack
now
ledgm
ents
Theau
thorsgratefully
ackn
owledge
finan
cial
supportfrom
the
National
Resea
rchFo
undationof
Korea
(NRF),theAdva
ncedInsti-
tutesof
Con
vergen
ceTe
chnolog
y(A
ICT),K
orea
,andSe
oulN
ational
University,K
orea
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