Looking through a liquid crystal ball.€¦ · Overview • A brief introduction to liquid crystals – the simplest systems. • Adding complexity to tune bulk properties: – Layered

New directions in liquid crystals. Looking through a liquid crystal ball.

Prof. Helen Gleeson OBE FInstP Cavendish Professor of Physics

School of Physics and Astronomy The University of Leeds

Leeds LS2 9JT , UK

Overview

•  A brief introduction to liquid crystals – the simplest systems.

•  Adding complexity to tune bulk properties: –  Layered structures;

–  Bending molecules.

•  New directions and possible applications. –  Sensors, colloidal systems, artificial muscle, lenses.

•  Concluding remarks

•  Solids, liquids and gases are well-known states of matter. •  Ordered fluids can form from geometrically anisotropic

molecules or aggregates of molecules – usually rod-like.

Nematic liquid crystals exhibit orientational order only.

Molecules have dipoles, and the phase is birefringent, allowing a simple electro-optic switch used in LCDs

Introduction to liquid crystals

•  Extremely successful technology: more than 50 million LCDs are shipped monthly.

•  Mostly use nematic state & employ carefully engineered mixtures.

•  Benefits include:

•  Still too slow (~1ms) for some applications – important driver for research.

•  Low power (battery driven - mobile phones) •  High complexity (good pictures - camera viewers, laptops) •  Lightweight (portable, suitable for aircraft) •  Fast (can show DVDs, TV signal etc)

Liquid crystal devices LCDs

•  Molecular chirality is common in nature. •  It often results in bulk chirality and/or frustrated phases. •  Chiral nematic (cholesteric) liquid crystals…

Adding a twist!

Chirality is ‘handedness’

Chirality and the Great British Bake Off!

•  Temperature-dependent helicoidal structure – thermometry.

•  ‘Bragg’ reflection of circularly polarized light. •  Optical bandwidth related to birefringence. •  Common in nature.

Applications & examples

•  Smectic liquid crystals - positional and orientational order.

•  The molecules can tilt - subtly different order translates into very different bulk properties.

Adding layers…

•  Tilted, chiral smectic liquid crystals can exhibit ferroelectricity (predicted in 1986)

•  Curie temperature is the orthogonal (SmA) to tilted (SmC) transition.

Chiral Smectic C phase

Smectic C phase

Rotation axis(y-axis)

Mirror Plane(x-z plane)

Symmetry equivalence of polarisation vector and the

phase... Rotation 180˚:

(Px, Py, Pz) = (-Px, Py, -Pz)

Reflection (SmC only): (Px, Py, Pz) = (Px, -Py, Pz)

A bit of physics…

•  Fluid antiferroelectric, ferrielectric and ferroelectric systems are known.

•  Interesting for electro-optic devices (fast, multistate switching).

•  Phases readily observed via polarizing microscopy.

•  Structures best determined via Resonant X-ray Scattering.

The ferroelectric family.

Synthesised by Goodby (Hull/York University)

K 46.3 SmC*A 82.6 SmC*FI1 83.6 SmC*FI2 86.3 SmC* 84.3 SmA 93.7 I

K 67.7 SmC*A 97.8 SmC*FI1 99.0 SmC* 109.4 SmA 116.6 I(SmI* 33.3 SmI*A 42.2)

C12H25O C-O

O

C-O-*CH

C6H13

CH 3Se

O

C12 H25 O C-O

O

C-O-*CH

C6H13

CH3Se

OF F

Isotropic

Orthogonal

FE

4-layer

3-layer

AFE

Fascinating molecules.

Position in Q-space gives layer repeat unit: •  3-layer phase: 0.66, 1.66… •  4-layer phase: 0.75, 1.75…

Splitting due to helical structure (separation gives pitch)

Different peak heights due to 3-D structure

(ratio gives distortion angle)

Stations X14A and X6B of NSLS

Qz Q0 ___ = l + m _ 1 n

d p _ + ( )

X-rays for structure. Experiments carried out at synchrotrons – UK, USA, France

•  E-fields can transform structures e.g. 4-layer (antiferroelectric) into 3-layer (ferrielectric)

•  5-state switching possible (4 - 3 - FE - 3 – 4).

Applying a voltage.

•  Nematic phase could be biaxial – no clear evidence.

•  Fast switching (

Intriguing new self-assembled soft-matter structures include:

•  Highly directional fibres •  Spontaneous separation of chiral

domains in isotropic and other very fluid structures.

•  Spontaneous formation of a soft solid (gel?).

Bending the molecules.

•  ‘Dark conglomerate’ sponge-like phase.

•  Chiral domains grow on application of E-field.

•  Remarkable change in refractive index with applied field – no induced birefringence.

•  Possible uses of this dramatic, isotropic refractive index change include lenses.

0 3 6 9 12 15 18 21 24

1.61

1.62

1.63

1.64

1.65

1.66

1.67

Eth2

n avg

E (V/µm)

Eth1

A new electro-optic mode?

Sensor for thermal tracking in the perishable goods chain.

Battery free thermal recorder includes:

•  Ferroelectric liquid crystal – temp. dependent capacitance

•  Controllable chemical crosslinking system for immobilising liquid crystal phase – clocking mechanism

•  Circuit measurement system with RFID antenna

Sensor module: Array of liquid crystal temperature sensitive capacitors & time sensitive

crosslinkable polymer networks

Measuring module with RFID communication capability

Sensors. THROUGH THE LIQUID CRYSTAL BALL

SmA

SmC*

Sensors. THROUGH THE LIQUID CRYSTAL BALL

•  Capacitance of a ferroelectric LC shows a smooth variation from 50°C - 20°C.

•  Capacitance is ‘locked in’ by polymer stabilization depending on time/temperature.

•  Well-formed polymer network observed in isotropic phase.

•  Refraction of laser light through the particle results in a momentum change.

•  This holds the particle in a trap near the focus. •  Forces are small, typically pN, ideal for micron-sized

objects. Similar to elastic forces in liquid crystals.

Self-assembling colloids THROUGH THE LIQUID CRYSTAL BALL

•  Particles disrupt the liquid crystal orientation field •  Liquid crystals ‘anchor’ on the particle surface:

Saturn-ring Satellite defect Boojums


Movies courtesy: I Muševič, Ljubljana, Slovenia

Build colloidal crystals (artificial opal) taking advantage of

interactions >> kT.


•  Particles attract or repel via the director field;

•  Melting the LC can change the defect position/type.

P A

n

All-optical switches THROUGH THE LIQUID CRYSTAL BALL

•  Circularly polarized laser tweezers carry angular momentum.

•  This can be transferred to a small droplet of liquid crystal as it is birefringent.

•  Mechanism is effectively droplet acting as a wave-plate.

•  Rotation speeds of up to 1kHz are observed.

•  Chiral nematic droplets have a complex droplet structure because of the helix - depends on pitch value (p) and droplet diameter (d)

•  Can make droplets with variable p and d, ensuring no dependence of p on temperature

•  Also possible to transfer angular momentum

Remarkable rotation in

LINEARLY POLARIZED light

(no angular momentum in the light!)


Rotation to stable orientation (wave-plate behaviour)

nonlinear light-induced unwinding (optical Freedericksz transition)

further reorientation to new stable state pitch ‘re-winds’

Linear polarization


•  Cholesteric microdroplet. •  Helical structure can cause ‘onion ring’ to form (b).

•  Periodic nature allows lasing with gain medium.

•  Microdroplets can be moved as well as pumped by lasers.

Movie courtesy: I Muševič, Ljubljana, Slovenia Optics Express, 18 26995 (2010)

Chapter 3 Liquid Crystal Droplets 37

p < r, see in Fig. 3.8. A very high chirality structure (p r, where a twisted bipolar structure

is formed.

Figure 3.8: The optical appearance of the chiral nematic droplet of (a) the twisted

bipolar and (b) the Frank-Pryce structure.

The Size Measurement

Diameter of the droplet can be determined by analyzing the digitized trans-

mission on the captured picture of the droplet by the digital camera. As

demonstrated in the Fig. 3.9, diameter of the droplet is identified as the dis-

tance between two low peaks of the digitized transmission of the selected path

on the picture. Borders which are the di�raction fringes in the rim of the

droplet are also measured and averaged as the error of the diameter.

Micron-sized lasers THROUGH THE LIQUID CRYSTAL BALL

Presbyopia – ageing eyes, affects 100% of population over 55.

Liquid crystal lenses - change focal length on application of a voltage via

field-induced change in refractive index. High n Low n

Switchable contact lenses THROUGH THE LIQUID CRYSTAL BALL

•  1st generation uses PMMA. •  Switches ±2 D on application

of < 3Vrms. •  Excellent optical quality. •  Remote switching possible.

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Applied Voltage (Vrms)

2.5 2.0 1.5 1.0 0.5 0.0

Add

Opt

ical

Pow

er (m

-1)


•  1st generation uses PMMA. •  Switches ±2 D on application

of < 3Vrms. •  Excellent optical quality. •  Remote switching possible.


•  High tensile fibres such as Kevlar and spiders’ web can be formed. •  Optical information can be stored (used in security applications). •  Some polymers are also elastic, so ‘elastic liquid crystal polymers’ (elastomers) can be made.

When liquid crystals are made into polymers, new things can happen…

Elastomers THROUGH THE LIQUID CRYSTAL BALL

H Finkelmann, Freiburg University


Elastomeric liquid crystals have remarkable properties ….

•  Soft elasticity. Almost no energy is expended if the material extends by rotating the director;

•  Light actuation. The director can be reoriented by light.


Elastomeric liquid crystals have remarkable properties ….

•  2-D to 3-D transformations. Perhaps one of the most interesting effects occurs when a 2-D elastomeric structure experiences different elastic moduli in different directions. Circle transforms to a cone:

c0 = 2πr0

r0

c0

c1 = λ1c0

r1 = λ2r0

c1 ≠ 2πr1 r1

c1


Taylor H. Ware et al. Science 2015;347:982-984


Iris‐Like Tunable Aperture Employing Liquid‐Crystal Elastomers

Advanced Materials Volume 26, Issue 42, pages 7247-7251, 10 SEP 2014 DOI: 10.1002/adma.201402878 http://onlinelibrary.wiley.com/doi/10.1002/adma.201402878/full#adma201402878-fig-0001

•  LCDs are a mature technology, but challenges still exist for new electro-optic phenomena.

•  Chirality and frustration allow remarkable bulk effects to be controlled and exploited.

•  New applications are continuously emerging including sensors, lasers, colloidal systems and, for the ageing amongst us, switchable lenses!

Liquid crystals are fascinating materials – far more than just components of display devices.

Concluding remarks

•  Support (funding, beamtime, materials, studentships)

•  Co-workers: –  Chemists from John Goodby FRS (York), Mike Hird (Hull),

Verena Görtz (Lancaster);

–  Theoreticians from Mikhail Osipov (Strathclyde University); –  Synchrotron co-workers Philippe Barois (CNRS Bordeaux),

Ron Pindak (NSLS USA), C C Huang (Minnesota (USA); Wim Bras ESRF)

•  Colleagues and co-workers…

Thanks!

Documents

Looking through a liquid crystal ball.€¦ · Overview • A brief introduction to liquid crystals – the simplest systems. • Adding complexity to tune bulk properties: – Layered