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Cite this: DOI: 10.1039/c2sm26700d

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Stabilization mechanism of double emulsions made by microfluidics†

Jonathan S. Sander,a Lucio Isa,b Patrick A. R€uhs,c Peter Fischerc and Andr�e R. Studart*a

Received 23rd July 2012, Accepted 6th September 2012

DOI: 10.1039/c2sm26700d

The stability of double emulsions is crucial for their application as delivery systems and microcapsule

templates. However, this stability is often challenged by many molecular species present in customized

formulations and by the fast dynamics when microfluidic emulsification processes are used. With the

help of designed single emulsion experiments, particle contact angle measurements and interfacial

rheology, we investigate the stabilization mechanisms of typical double emulsion formulations

containing colloidal particles in the middle oil phase and surfactants in the continuous aqueous phase.

In contrast to the inefficient stabilization with conventional surfactants, we find that colloidal particles

and surface active polymers are able to quickly form a strong elastic film at the oil–water interface that

prevents rupture of the thin fluid separating adjacent droplets, thus providing an efficient means to

stabilize double emulsions within the short timescales of microfluidic processes.

Introduction

Double emulsions can potentially be used as nanoliter reactors

and delivery systems for controlled chemical synthesis and release

in pharmaceutics, agriculture, food, high throughput analytics

and materials science applications.1–9 The recent development of

microfluidic techniques that create single and multiple droplets

under well-defined flow conditions has further increased the

interest in double emulsions due to the high encapsulation effi-

ciency and tight dimensional control achieved.10–12 The resulting

monodisperse double emulsions have not only been considered as

nanoliter reactors and delivery systems in the fluid state, but have

also been used as templates for the formation of functional col-

loidosomes and microcapsules upon consolidation of the middle

fluid phase. By enabling the incorporation of a wide variety of

chemicals in the middle and inner phases, numerous types of

microcapsules and functional nanoreactors have been developed

from double emulsions made by microfluidics.13–16

While the easy incorporation of chemicals in the middle and

inner fluid compartments allows for great versatility and flexi-

bility, the interference of such chemicals on the efficiency of

surface active species makes the stabilization of double emulsions

a recurring challenge. Given the many relevant parameters and

our poor understanding of the stabilization mechanisms involved

at the short timescales encountered in microfluidic

aComplex Materials, Department of Materials, ETH Zurich, 8093 Zurich,Switzerland. E-mail: andre.studart@mat.ethz.chbLaboratory for Surface Science and Technology, Department ofMaterials, ETH Zurich, 8093 Zurich, SwitzerlandcLaboratory of Food Process Engineering, Institute of Food, Nutrition andHealth, Department of Health Sciences and Technology, ETH Zurich,8092 Zurich, Switzerland

† Electronic supplementary information (ESI) available. See DOI:10.1039/c2sm26700d

This journal is ª The Royal Society of Chemistry 2012

emulsification, successful formulations are often obtained only

after tedious trial-and-error experiments. Among the numerous

surfactant systems available, polymers of amphiphilic nature

such as partially hydrolyzed poly(vinyl alcohol) have been shown

to be particularly effective in stabilizing single and double

emulsions made by microfluidics.10,11,14,15,17–20 Recent work on

the formation of functional colloidosomes from double emul-

sions has also suggested that colloidal particles dispersed in the

middle fluid might also contribute to emulsion stabilization.14

To shed light on the mechanisms required for the stabilization

of double emulsions made by microfluidics, we investigate typical

effective and ineffective formulations containing both amphi-

philic molecules and colloidal particles as surface active agents.

The effect of surfactant molecules and colloidal particles on the

stability of the inner and outer droplets is first investigated

through designed single emulsion experiments. The surface

activity of modified silica nanoparticles is then evaluated by

measuring their individual contact angle in situ at the oil–water

interface in the presence of different surface active species using

freeze-fracture, shadow-casting (FreSCa) cryo-SEM.21 Next, we

use interfacial rheology and pendant drop tensiometry to study

the possible formation and rheological behavior of particle–

polymer (surfactant) films at the surface of double emulsion

droplets. This systematic investigation showed that the fast

formation of a strong viscoelastic particle–polymer film at the

liquid interface is key for the effective stabilization of double

emulsions produced in microfluidic devices.

Experimental section

Materials

SiO2 (d50 ¼ 100 nm, r ¼ 1.8 g cm�3, and surface area ¼ 2–6 m2

g�1) was purchased from Fiber Optic Center Inc. (New Bedford,

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MA, USA). Toluene (99.7%), 2-propanol (99.8%), PEG(20)sor-

bitan monolaurate (Tween 20), sodium dodecyl sulfate (SDS),

poly(vinyl alcohol) (PVA, 87–89% hydrolised, Mw ¼ 31 000–

51 000 g mol�1), PEO–PPO copolymer (poly(ethylene oxide)–

poly(propylene oxide) copolymer, Pluronic F-127), and trime-

thoxy(octadecyl)silane (90%) were purchased from Sigma

Aldrich Chemie GmbH (Germany).

To hydrophobize their surface, 15–20 wt% 100 nm silica

particles were dispersed in 2-propanol and mixed with 20 wt%

trimethoxy(octadecyl)silane and 2 wt% butylamine with respect

to the SiO2 content. The suspension was sonicated for 10 min

with an ultrasonic horn (Vibra cell VCX 130, Sonics, USA) and

stirred for 24 hours. To remove unreacted silane and 2-propanol,

the suspensions were washed with toluene at least four times by

repeated centrifugation and resuspension cycles.

Microfluidic double emulsification

Double and single emulsion droplets were prepared in micro-

capillary devices consisting of two round tapered capillaries

inserted into a square outer capillary as described else-

where.1,11,13,14,22 Oil-in-water single emulsions were produced by

flowing a suspension of modified silica particles in toluene

through one of the round capillaries, while a surfactant (poly-

mer) aqueous solution was flowed through the square capillary in

the same direction. Water-in-oil single emulsions were prepared

using the same configuration but with the fluids inverted

accordingly. Flow rates in the range of 500–2000 ml h�1 and

5000–30 000 ml h�1 were used for the inner and outer co-flowing

fluids, respectively. The preparation of water-in-oil-in-water (W/

O/W) double emulsions required pumping of a third fluid, in this

case another aqueous solution, from the other end of the square

capillary. Flowing in the opposite direction, this third fluid

encompasses the co-flowing aqueous and oily fluids to generate

well-defined double emulsions. Flow rates for double emulsifi-

cation were varied within the ranges 1500–4000 ml h�1, 4000–

6000 ml h�1 and 25 000–60 000 ml h�1 for the inner, middle and

outer phases, respectively. Single and double emulsions were

collected in two different ways. In a typical configuration, the

emulsions were collected by connecting a polyethylene tube to

the collector capillary and inserting it directly into a container

filled with double distilled water. In a second configuration, the

tube was immersed into a glass-sealed container fully filled with

water to prevent contact of the emulsion droplets with air.

Fig. 1 Schematics of the setup used to measure the rheology of

toluene–water interfaces in the presence of particles in the oil phase and

surfactant/polymer in the aqueous phase.

Particle contact angle measurements via FreSCa cryo-SEM

For FreSCa cryo-SEM imaging, 0.5 ml of 0.12 wt% hydrophobic

silica particle suspensions in toluene were placed inside a custom-

made copper holder with a 200 mm deep central cavity and

carefully covered by a 3.0 ml droplet of MilliQ water with or

without the different surfactants (polymers). The holder was then

closed with a flat copper plate and the ‘‘sandwich’’ was frozen in a

liquid propane jet freezer (Bal-Tec/Leica JFD 030, Balzers/

Vienna) with a cooling rate of 30 000 K s�1 to avoid water

crystallization. After freezing, the samples were mounted under

liquid nitrogen onto a double fracture cryo-stage and transferred

under inert gas in a cryo-high vacuum airlock (<5 � 10�7 mbar

Bal-Tec/Leica VCT010) to a pre-cooled freeze-fracture device at

Soft Matter

�140 �C (Bal-Tec/Leica BAF060 device). After partial freeze-

drying at �110 �C for 3 min, samples were coated with tungsten

at an angle a ¼ 30� to a total thickness d ¼ 2 nm at �120 �C and

by an additional 2 nm with a continuously varying angle between

90� and 30�. The second deposition is needed in order to avoid

charging of the shadow, produced by the first coating, during

imaging, which may compromise image stability at high magni-

fications. Freeze-fractured and metal-coated samples were then

transferred under high vacuum (<5 � 10�7 mbar) at �120 �C to

a pre-cooled (�120 �C) cryo-SEM (Zeiss Gemini 1530, Ober-

kochen) for imaging.

Interfacial rheology

The rheological behavior of toluene–water interfaces was eval-

uated in a Physica MCR 300 rheometer (Anton Paar, Ostfildern,

Germany) using the biconal disc geometry schematically shown

in Fig. 1. After accurately positioning the bicone at the surface of

the relevant aqueous solution, about 18 ml of the particle-loaded

toluene suspension was carefully pipetted on top. The particle

suspension was sonicated for 10 min before each run. Amplitude

sweeps were performed from 0.01 to 10% strain with an angular

frequency of 1 rad s�1 after an equilibration time of 1 hour. Time

sweep experiments were performed with a constant strain

amplitude of 0.1% and a frequency of 1 rad s�1. For further

details on the bicone rheometer technique please see ref. 23.

Pendant drop measurements

Toluene drops loaded with hydrophobized silica particles were

subjected to expansion/retraction cycles in aqueous surfactant

(polymer) solutions using a pendant drop apparatus (PAT-1

Tensiometer, Sinterface Technologies, Berlin, Germany). Drops

were tested in an inverted configuration due to the lower specific

gravity of the toluene suspension as compared to water. In each

cycle the initial drop volume of 6 mm3 was changed sinusoidally

by plus or minus 6% within a timeframe of 50 seconds. The drops

were equilibrated for 20 min before each measurement and

pictures were taken every ten seconds during the analysis.

This journal is ª The Royal Society of Chemistry 2012

Fig. 3 (a and b) Schematics of the single emulsions investigated. (c

and d) Representative images of the (c) toluene-in-water emulsions with

2 wt% PEO–PPO copolymer and the (d) water-in-toluene emulsions

containing particles in the oil phase.

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Results and discussion

Double emulsion stability

Formulations comprising hydrophobized silica nanoparticles

and different surfactants (polymers) were investigated with

respect to their effectiveness in stabilizing double emulsions

produced in glass microcapillary devices. The silica nanoparticles

were initially dispersed in the middle toluene phase, whereas the

surfactants (polymers) were dissolved in the outer aqueous

phase, as schematically shown in Fig. 2a. To probe a wide range

of chemistries, two surfactants and two polymers of very

different nature were evaluated. Partially hydrolyzed poly(vinyl

alcohol) (PVA) and a PPO–PEO copolymer (Pluronic) were

selected as examples of surface-active polymers, whereas

PEG(20)-sorbitan monolaurate (Tween 20) and sodium dodecyl

sulfate (SDS) were chosen as typical non-ionic and anionic

surfactants, respectively.

In line with previous reports, the formulation with PVA

resulted in the most stable double emulsions, followed by the

PPO-PEO copolymer and the surfactants Tween 20 and SDS

(Fig. 2b–f). Emulsions with the PPO-PEO copolymer could be

successfully stabilized with nearly 100% yield, but only if

collected in a special chamber that avoided direct contact with air

(Fig. 2d). The conventional surfactants Tween 20 and SDS led to

emulsions whose inner water droplets occasionally coalesced

with the aqueous continuous phase, leaving behind a small

satellite droplet inside the larger oil droplet (Fig. 2e and f). This

effect was more pronounced in emulsions containing SDS.

Inner and outer droplet stability

To investigate separately the stability of the inner and outer

droplets, we prepared single emulsions with surface active species

in their respective locations as expected in the double emulsions

(Fig. 3a and b).

All the evaluated surfactants (polymers) were able to stabilize

toluene-in-water emulsions, indicating that they provide elec-

trostatic/steric forces that impede thinning of the aqueous film

and thus the coalescence between toluene droplets. Fig. 3c shows

a representative example where a 2 wt% PEO–PPO polymer was

added to the aqueous phase. Instead, the inverted system

comprising water-in-toluene emulsions was only stable if the

surface active polymers PVA and PEO–PPO were present in the

Fig. 2 (a) Schematics of the investigated double emulsions showing the initia

(b–f) Double emulsions made by microfluidics from formulations containin

Samples in (b and c) were collected in an open container, whereas specimens

This journal is ª The Royal Society of Chemistry 2012

water droplets (see an additional experiment andMovie S1 in the

ESI†).

Surprisingly, the water-in-toluene single emulsion containing

particles in the oil fluid was found to be very unstable, indicating

that the silica nanoparticles present in the middle toluene phase

of the double emulsions are not able to prevent rupture of the oil

film between the outer and inner droplets within the short

timescales of the microfluidic process. This contrasts with high

stability typically achieved in Pickering emulsions and foams

processed by mechanical agitation.24–31

These experiments suggest that stabilization of the investigated

double emulsions must rely on the properties of the outer droplet

surface, with a major role not only of the colloidal particles but

also of the surfactant (polymer) present in the outer aqueous

phase. Synergetic effects of particles and surfactants at the

interface have been reported earlier for single emulsions and

foams and are usually connected to flocculation of oppositely

charged surfactant–particle combinations.32,33

Particle contact angles

To evaluate the possible effect of the surfactants on the

adsorption behavior of the silica nanoparticles at the surface of

l location of the silica nanoparticles and surfactant (polymer) molecules.

g (b) PVA, (c and d) PPO-PEO copolymer, (e) Tween 20 and (f) SDS.

in (d–f) were collected in a closed chamber.

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the toluene outer droplets, we measured the contact angle of

these particles at the toluene–water interface in the presence of

different surfactants (polymers) in the aqueous phase via FreSCa

cryo-SEM imaging. With this method mm sized planar particle-

laden oil–water interfaces are exposed upon fracture and imaged

with a cryo-SEM after ultrafast freezing. 3D information on the

particle position relative to the interface is obtained by coating

the interface with a thin tungsten layer at a 30� angle relative tothe interface so that particles trapped at and protruding from the

interface leave a shadow behind them. By measuring directly the

particle size at the interface and the shadow length, one can

calculate the vertical position of individual nanoparticles at the

interface with sub-nanometer accuracy and thus their individual

contact angles q ¼ cos�1(|h � r|/r), where h is the protrusion

height of the particle from the interface into the oil and r is its

radius. More details on the method can be found in the Experi-

mental section and in ref. 21.

The obtained data reveal that the nanoparticles adsorb at the

interface at average contact angles in the range 130–150�

(measured through the aqueous phase, Fig. 4). However, the

contact angle is not significantly affected by the type of surfac-

tant (polymer).

Although the presence of interfacially adsorbed particles

positioned primarily in the oil phase (high contact angles) should

in principle ensure stabilization of the oil film, the shorter time-

scales involved in the microfluidic emulsification process as

compared to the FreSCa measurements may explain the ineffi-

ciency of the silica particles in preventing the rupture of the

middle oil film in single water-in-toluene emulsions (Fig. 3d). The

short emulsification timescale and the long time required for

particles to diffuse to and adsorb at the interface probably result

in a low density of particles at the interface at early stages. Such a

hypothesis is supported by previous studies in which particle-

stabilized single emulsions could only be formed in microfluidic

devices for very high concentrations of surface-active particles in

the continuous phase (>20 vol%).17,34 These results show that,

contrary to our previous expectations,14 neither the adsorbed nor

the non-adsorbed particles alone are effective in preventing

thinning down and rupture of the oil film between the inner and

Fig. 4 FreSCa cryo-SEM images of the silica nanoparticles immobilized

at the toluene–water interface (a) in the absence of surfactants and (b–d)

in the presence of (b) PVA, (c) PPO-PEO copolymer, and (d) Tween 20.

(e) Particle contact angle distribution for the water + PVA–toluene

system. (f) Average particle contact angles in the presence of different

surfactants (polymers).

Soft Matter

outer droplets. Thus, the question remains as to how PVA and

the PPO-PEO copolymer prevent coalescence in double emul-

sions, despite the poor stabilizing effect of the silica nanoparticles

alone.

Formation of interfacial film

Despite their similar contact angle at the toluene–water interface,

the nanoparticles initially dispersed in the middle oil phase were

found to interact more strongly with the polymers than with the

surfactants present in the continuous aqueous phase. The silica

particles and the polymers might interact favorably at the

interface because of hydrophobic interactions or hydrogen

bonding between their polar groups. In fact, we found that such

favorable interactions lead to the formation of a particle–poly-

mer film at the interface, which is readily visible upon shrinkage

of representative toluene-in-water single emulsions containing

PVA (Fig. 5 and Movie S2 in the ESI†). Shrinkage in this case is

driven by the slow removal of the toluene molecules through the

continuous aqueous phase.

The formation of a particle–polymer interfacial film is evi-

denced by the strong buckling and wrinkling of the droplet

surface upon a minor decrease in the toluene droplet volume

(Fig. 5a–c). The shrinkage-induced wrinkling occurs within

timeframes that can range between several minutes to up to one

hour, depending on the droplet size and drying conditions

(Movie S2†). Similar wrinkling effects have been reported for

interfacially adsorbed particles35 and polymers.36 In contrast,

droplets made in the presence of the surfactant Tween 20

undergo pronounced shrinkage without surface wrinkling at

early stages. In this case, texture develops on the droplet surface

only when the volume reduction is high enough to cause jamming

of interfacially adsorbed particles (Fig. 5d–f). This observation

confirms the low initial density of modified silica particles at the

toluene–water interface.

Additional shrinking experiments on single toluene-in-water

droplets without silica particles in the oil phase revealed no

interfacial film formation even in the presence of PVA in the

aqueous continuous phase. This is in agreement with the poor

stability of particle-free double emulsions containing PVA alone,

Fig. 5 Shrinkage of toluene-in-water single emulsions containing 5 wt%

silica nanoparticles in the oil phase and 2 wt% of (a–c) PVA or (d–f)

Tween 20 in the continuous aqueous phase.

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confirming the importance of the interactions between particles

and polymers at the interface for the stabilization of double

emulsions.

Rheology of interfacial film

To obtain further insights into the formation dynamics and the

mechanical properties of the interfacial film we measured the

interfacial rheology of toluene–water interfaces in the presence of

different polymers and surfactants. The dynamics of film

formation was studied by measuring the time evolution of the

storage and loss moduli of the interface (G0i and G

0 0i , respectively)

under oscillatory mode. In accordance with the droplet shrinking

experiments, the interfaces exposed to particles from the toluene

phase and PVA from the aqueous phase showed strong shear

elasticity directly after starting the experiment, which typically

corresponds to less than 5 minutes (Fig. 6a). A similar but much

less pronounced effect is also visible for the PEO–PPO system

(Fig. 6b) for which intermediate double emulsion stability was

observed (Fig. 2c and d). No stable interfacial elasticity was

detected in the presence of particles and surfactants, as well as

with the polymers alone (Fig. 6c, d and f). This confirms that the

formation of elastic interfacial films requires the combination of

both particles and polymers that interact favorably at the

interface.

Interestingly, a viscoelastic film also gradually develops in the

absence of surfactants and polymers (Fig. 6e), suggesting that the

interfacially adsorbed particles form an elastic interconnected

Fig. 6 Rheological properties of the toluene–water interfaces measured

as a function of time. In (a)–(e) the toluene phase contains 5 wt% of

modified silica nanoparticles and the aqueous phase contains 2 wt% of (a)

PVA, (b) PPO-PEO copolymer, (c) Tween 20, (d) SDS and (e) pure water.

(f) shows a measurement for a system comprising 2 wt% PVA in the water

phase and no silica particles in the toluene phase.

This journal is ª The Royal Society of Chemistry 2012

network over time driven by interfacial aggregation. However,

the rheological data show that it takes at least 30 minutes for the

complete formation of such an elastic network. These results

further support the hypothesis that the interfacial adsorption of

colloidal particles is not sufficiently fast to form a protective layer

against the coalescence of droplets within the short timescales of

microfluidic processes. The fact that no or unstable interfacial

elasticity develops in the presence of SDS and Tween 20 (Fig. 6c

and d) indicates that these surfactants prevent the formation of a

strong network of particles at the interface, probably due to the

lack of attractive interactions between the particles and the layer

of surfactants that quickly adsorb at the interface. In fact, the

silica particles and SDS molecules probably repel each other due

to their negative charges at the pH used in these experiments (pH

¼ 6–7). Although most of their surface OH groups are covered

during the silanization process, the silica particles were found to

have a zeta potential of around�10 mV within the pH range 5–9.

It is important to note that the interfacial rheology data dis-

cussed above should be taken just in a comparative basis, since

the experimental methods used here cannot assess the dynamics

of film formation at the short timescales of the microfluidic

process.

The viscoelastic properties of the interface were also investi-

gated by performing amplitude sweep oscillatory experiments, in

which G0i and G

0 0i are measured for increasing strains at a constant

frequency of 1 rad s�1. Comparative measurements were per-

formed after an equilibration time of 1 hour to ensure the

formation of stable interfaces.

In all systems, except SDS, the interface shows a well-defined

dynamic yield strain (Fig. 7), in agreement with previous reports

on the rheology of interfaces loaded with adsorbed particles or

globular-shaped protein–polysaccharide hybrids.37–39

Taking the cross-over between G0i and G

0 0i as a measure of

yielding, we find that the dynamic yield strain gy of the visco-

elastic films increases in the following order: Tween 20 (0.4%) <

PPO-PEO copolymer (3%) < no surfactant (3.8%) < PVA (4.8%).

Fig. 7 Oscillatory rheological measurements of the toluene–water

interfaces for increasing amplitude strains. The toluene phase is initially

loaded with 5 wt% of modified silica nanoparticles, whereas the aqueous

phase contains 2 wt% of (a) PVA, (b) PPO-PEO copolymer, (c) Tween 20

and (d) pure water.

Soft Matter

Fig. 8 Snapshots of buoyant toluene drops containing 5 wt% SiO2

particles surrounded by a continuous aqueous phase with 2 wt% (a–c)

SDS and (e–g) PVA. The images show the drops (a and e) at the initial

state, (b and f) expanded by 6% in volume, and (c and g) retracted by 6%.

The contours of all three drops are overlapped in (d and h). The scale bar

is 2 mm.

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Overall, our comparative rheological evaluation suggests that

the fast film formation ability and the higher yield strains of the

viscoelastic interfacial films formed in the presence of silica

particles and surface active polymers make the outer droplet

sufficiently strong at short timescales to prevent the rupture of

the thin oil film, thus allowing for the stabilization of the inves-

tigated double emulsions. Indeed the presence of strong,

compliant films has recently been proven to have extraordinary

effects on emulsion stabilization.40

Besides the droplet shrinking and interfacial rheology experi-

ments, the elastic properties of the particle–polymer interfacial

film can also be easily probed by purposely expanding and

shrinking a representative single drop in a pendant drop appa-

ratus. Contrary to the reversible deformation observed for a

reference drop in a SDS aqueous solution (Fig. 8a–d and Movie

S3 in the ESI†), drops coated with the particle–PVA interfacial

film do not recover to their original shape after an initial increase

and subsequent decrease of the drop nominal volume (Fig. 8e–h

and Movie S4 in the ESI†), again indicating a high shear elas-

ticity of the interface.36 The high elasticity of the particle–PVA

film is also evident in the videos available in the ESI.† This shows

that the pendant drop method provides a straightforward means

to assess the ability of surface active species to quickly form a

strong viscoelastic film at the oil–water interface and thus to be

possible candidates for the stabilization of tailored double

emulsions in microfluidic devices.

Conclusions

The fast formation of a strong viscoelastic film at the oil–water

interface enables the stabilization of double emulsions made in

microfluidic devices. For the water-in-oil-in-water double emul-

sions investigated in this study, such film can be formed in less

than 5 minutes through favorable interactions between hydro-

phobic silica nanoparticles initially dispersed in the toluene

middle phase and surface active polymers initially present in

the continuous aqueous phase. Because of its high yield strain,

the resulting viscoelastic film elastically resists thinning of the

toluene middle film, preventing coalescence between the inner

Soft Matter

and outer droplets of the double emulsion. This mechanism

circumvents the poor stability of the inner water droplet in the

presence of silica nanoparticles alone. Despite forming a favor-

able high contact angle at the toluene–water interface, these

particles cannot build a sufficiently densely packed protective

layer on the droplet surface within the short timescales of the

microfluidic emulsification process. These findings provide useful

guidelines for the selection of surface active species for the effi-

cient stabilization of customized double emulsions produced

with microfluidic approaches.

Acknowledgements

We thank the Swiss National Science Foundation (grant number

200021_126646) and ETH Zurich for the financial support. LI

acknowledges financial support from MC-IEF-2009-252926.

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8 S. van der Graaf, C. Schroen and R. M. Boom, J. Membr. Sci., 2005,251, 7–15.

9 S. Neethirajan, I. Kobayashi, M. Nakajima, D. Wu, S. Nandagopaland F. Lin, Lab Chip, 2011, 11, 1574–1586.

10 L. Y. Chu, A. S. Utada, R. K. Shah, J. W. Kim and D. A. Weitz,Angew. Chem., Int. Ed., 2007, 46, 8970–8974.

11 A. S. Utada, E. Lorenceau, D. R. Link, P. D. Kaplan, H. A. Stoneand D. A. Weitz, Science, 2005, 308, 537–541.

12 R. M. Erb, D. Obrist, P. W. Chen, J. Studer and A. R. Studart, SoftMatter, 2011, 7, 8757–8761.

13 P. W. Chen, R. M. Erb and A. R. Studart, Langmuir, 2012, 28, 144–152.

14 J. S. Sander and A. R. Studart, Langmuir, 2011, 27, 3301–3307.15 D. Lee and D. A. Weitz, Adv. Mater., 2008, 20, 3498.16 O. Skurtys and J. M. Aguilera, Food Biophys., 2008, 3, 1–15.17 A. R. Studart, J. Studer, L. Xu, K. Yoon, H. C. Shum and

D. A. Weitz, Langmuir, 2011, 27, 955–964.18 H. C. Shum, Y. J. Zhao, S. H. Kim and D. A. Weitz, Angew. Chem.,

Int. Ed., 2011, 50, 1648–1651.19 W. J. Duncanson, A. Abbaspourrad, H. C. Shum, S. H. Kim,

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Mater., 2009, 21, 5548–5555.21 L. Isa,F.Lucas,R.Wepf andE.Reimhult,Nat.Commun., 2011,2, 438.22 R. K. Shah, H. C. Shum, A. C. Rowat, D. Lee, J. J. Agresti,

A. S. Utada, L. Y. Chu, J. W. Kim, A. Fernandez-Nieves,C. J. Martinez and D. A. Weitz, Mater. Today, 2008, 11, 18–27.

23 P. Erni, P. Fischer, E. J. Windhab, V. Kusnezov, H. Stettin andJ. Lauger, Rev. Sci. Instrum., 2003, 74, 4916–4924.

24 T. N. Hunter, R. J. Pugh, G. V. Franks and G. J. Jameson, Adv.Colloid Interface Sci., 2008, 137, 57–81.

25 B. P. Binks and T. S. Horozov, Angew. Chem., Int. Ed., 2005, 44,3722–3725.

26 B. P. Binks and S. O. Lumsdon, Phys. Chem. Chem. Phys., 1999, 1,3007–3016.

27 B. P. Binks and S. O. Lumsdon, Langmuir, 2000, 16, 8622–8631.28 I. Akartuna, A. R. Studart, E. Tervoort, U. T. Gonzenbach and

L. J. Gauckler, Langmuir, 2008, 24, 7161–7168.

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29 U. T. Gonzenbach, A. R. Studart, E. Tervoort and L. J. Gauckler,Langmuir, 2007, 23, 1025–1032.

30 U. T. Gonzenbach, A. R. Studart, E. Tervoort and L. J. Gauckler,Langmuir, 2006, 22, 10983–10988.

31 U. T. Gonzenbach, A. R. Studart, E. Tervoort and L. J. Gauckler,Angew. Chem., Int. Ed., 2006, 45, 3526–3530.

32 T. N. Hunter, E. J. Wanless, G. J. Jameson and R. J. Pugh, ColloidsSurf., A, 2009, 347, 81–89.

33 B. P. Binks, A. Desforges and D. G. Duff, Langmuir, 2007, 23, 1098–1106.

34 A. R. Studart, H. C. Shum and D. A. Weitz, J. Phys. Chem. B, 2009,113, 3914–3919.

This journal is ª The Royal Society of Chemistry 2012

35 S. S. Datta, H. C. Shum and D. A. Weitz, Langmuir, 2010, 26, 18612–18616.

36 P. Erni, H. A. Jerri, K. Wong and A. Parker, Soft Matter, 2012, 8,6958.

37 D. Y. Zang, E. Rio, G. Delon, D. Langevin, B. Wei and B. P. Binks,Mol. Phys., 2011, 109, 1057–1066.

38 D. Y. Zang, E. Rio, D. Langevin, B. Wei and B. P. Binks, Eur. Phys.J. E, 2010, 31, 125–134.

39 P. Erni, E. J. Windhab, R. Gunde, M. Graber, B. Pfister, A. Parkerand P. Fischer, Biomacromolecules, 2007, 8, 3458–3466.

40 T. T. Liu, S. Seiffert, J. Thiele, A. R. Abate, D. A. Weitz andW. Richtering, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 384–389.

Soft Matter