Nano-mechanical properties of clay-armoured emulsion droplets

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Nano-mechanical properties of clay-armoured emulsion droplets

Sin-Ying Tan,a Rico F. Tabor,ab Lydia Ong,ac Geoffrey W. Stevensa and Raymond R. Dagastine*ad

Received 14th December 2011, Accepted 23rd January 2012

DOI: 10.1039/c2sm07370f

There has been a growing interest in understanding the stabilization mechanism of particle-armoured

emulsion droplets over the last few decades due to their importance in many everyday products. Here,

the mechanical properties of clay-armoured emulsion droplets were investigated using laser scanning

confocal microscopy with an in situ atomic force microscopy measurement. This combination allows

the visualization of droplet shape as a function of applied force. The emulsion droplets were found to be

mechanically robust, stable against coalescence during drop collisions and able to recover from large

deformations without disintegration. A Hookean constitutive law was used to extract the surface

Young’s modulus of the clay-armoured droplets as a function of a range of solution conditions. The

clay-armoured droplets were relatively insensitive to changes in solution ionic strength and pH.

However, in the presence of cationic surfactants, the surface Young’s modulus decreases and shows

significant reduction well above the critical micelle concentration. These changes are most likely due to

desorption of clay platelets from the oil–water interface after charge neutralisation and the eventual

solubilisation of the oil droplet. The elasticity measurements in this study should help illuminate the

impact of the clay-armoured droplets on macroscopic properties of emulsions including rheological

properties and emulsion stability.

Introduction

Solid-stabilized emulsions (or Pickering emulsions)1,2 have

received increasing attention over the last two decades due to

their superior stability when compared to traditional emulsions

stabilized by surfactant molecules.3–6 These emulsions can be

found in a wide range of industries such as food, pharmaceutical,

paint and petrochemical. In most applications, the stability of

emulsion droplets is very important; however, destabilization of

these droplets is sometimes necessary. One example is the use

of these emulsion systems in topical medications, where stability

of emulsion droplets is necessary for storage, but destabilization

is required for spreading on the skin. Thus, the stabilization

mechanisms of solid-stabilized emulsions have been studied

intensively using various techniques.7–10 Well-defined spherical

particles were generally used in the studies. However, it has also

been shown that it is possible to attain stable emulsions using

non-spherical particles such as plate-like or needle-like11–14 or

alternatively non-homogenous particles such as amphiphilic

particles have been used.15,16 Interestingly, a number of studies

aParticulate Fluid Processing Centre, Department of Chemical andBiomolecular Engineering, The University of Melbourne, Victoria, 3010,AustraliabSchool of Chemistry, The University of Melbourne, Victoria, 3010,AustraliacThe Bio21 Molecular Science and Biotechnology Institute, The Universityof Melbourne, Victoria, 3010, AustraliadMelbourne Centre for Nanofabrication, 151, Wellington Road, Clayton,Victoria, 3168, Australia. E-mail: [email protected]

3112 | Soft Matter, 2012, 8, 3112–3121

have shown that non-spherical particles can stabilize emulsions

at a much lower solids volume fraction than spherical parti-

cles.17–19 Non-spherical particles, such as clay platelets, have been

used as stabilizers in Pickering emulsions.20,21 Although clay can

be found in everyday products such as medicines, papers and

ceramics, it is also a contaminant in many mining industrial

applications. Clay-stabilized systems show very robust behav-

iour, where recent work by Subramaniam et al.22 has shown that

bubbles stabilized by clay platelets, or armoured bubbles are

stable in water and other liquids, and show evidence of size

selective permeability, i.e. they are semi-porous.

The mechanical properties, such as the elasticity, of these clay-

armoured emulsion droplets are an important determinant of

their stability. Commonly, rheology has been used to determine

the elastic and deformation properties of bulk emulsion drop-

lets.23–26 An alternative technique for elasticity measurements,

which can be targeted at individual droplets, is Atomic Force

Microscopy (AFM). AFM has been widely used to examine the

mechanical properties of deformable interfaces, for example

polymer capsules and biological cells.27–30 Ferri et al.27 has

successfully used AFM to study the deformation properties of

single emulsion droplets stabilized by spherical biological

(cowpea mosaic virus) nanoparticles. Additionally, reflection

interference contrast microscopy (RICM) was used to monitor

droplet shape simultaneously during deformation. In this study,

AFMwas used to probe the mechanical properties (i.e. elasticity)

of emulsion droplets stabilized by a natural occurring plate-like

clay, kaolinite, to elucidate structure-function correlations with

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solution conditions. This was achieved by compressing an

emulsion droplet, or armoured droplet, with a colloidal probe,

under various solution conditions such as solution ionic strength,

pH, and surfactant concentration. As AFM does not provide

a direct measure of drop deformation, in this work, laser scan-

ning confocal microscopy (LSCM) was integrated with the AFM

measurement to provide simultaneous visualization of droplet

shape as a function of applied force. Previous work combining

LSCM with AFM has been focused on complex biological

systems,31–36 whilst only a few studies,37–41 including this one,

have focused on either liquid droplets or capsules to make

a quantitative link between the simultaneous visualization of

geometry and forces measured using AFM.

Materials and methodology

Materials

Tetradecane (C14H30, $99%, Aldrich), poly(allylamine hydro-

chloride) (PAH, C3H8ClN, avg. Mw�15 kDa, Aldrich), hex-

adecyltrimethylammonium bromide (CTAB, C19H42BrN,

$99%, Sigma), sodium chloride (NaCl, AR, Chem Supply),

sodium hydroxide (NaOH, pellet, AR, Chem Supply) and

calcium chloride (dihydrate, CaCl2$2H2O, AnalaR, B.D.H) were

used as received. The fluorescent dyes, nile red (C20H18N2O2) and

acid red 88 (C20H13N2NaO4S) were purchased from Sigma-

Aldrich. Water with a resistivity of 18.2 MU$cm used in this

work was purified by a Milli-Q system (Millipore, USA). The

clay sample, kaolinite (K15GM), was obtained from APS

Chemicals and was used without further purification or modifi-

cation. The AFM image in Fig. 1 shows that the clay sample has

a plate-like structure and that these plates are hexagonal in

shape.

Emulsion preparation and characterization

An aqueous dispersion of 0.2 wt% clay sample in 0.01 M sodium

chloride was first dispersed using sonication until all particles

Fig. 1 A tapping mode AFM height image of kaolinite platelets

adsorbed on a mica substrate in air. The platelets show some aggregation

and stacking due to the drying process during deposition from solution

onto the substrate.


were dispersed based on visible inspection. The size distribution

of the suspension was determined by laser light diffraction using

a Malvern Mastersizer 2000 (Malvern Instrument Ltd., Wor-

cestershire, UK). In this technique, the particle size distribution

is determined by converting the angle of scattered light to volume

of a sphere and then back-calculating the diameter. For a thin

platelet, only the basal plane will scatter light as the edge is too

thin to be detected; thus, the resultant volume weighted particle

size will be biased to the basal plane length, not the plate thick-

ness. The approximated size distribution of clay platelets is

showed in Fig. 2. The platelets distribution was bimodal on

a logarithmic scale with an average volume diameter of 80 mm. It

is important to note that the size measurements by laser light

diffraction are volume based, however, measurements from

AFM images are based on surface area, which results in the

differences in platelets dimensions between the two techniques.

Furthermore, the AFM sample was prepared by deposition of

the clay platelets on a surface where agglomeration of small and

large platelets is expected. The AFM data was used to provide

a description of the platelets shape, not necessarily a compre-

hensive measure of particle size.

Clay-stabilized emulsions were prepared by mixing an

approximate volume fraction (1 : 4) of tetradecane (labelled with

nile red in the case of confocal experiments) to clay dispersion

and then shearing it with a vortex mixer for 30 s at a speed of

2800 rpm (IKA� lab dancer, IKA� Werke, Germany). The

morphology of the droplets was observed with an optical

microscope (Nikon TE-2000, Nikon, Japan) and a field emission

gun scanning electron microscope (Quanta, Fei Ccompany,

Hillsboro, Oregon, USA) fitted with an Alto 2500 cryo system

(Gatan, Abingdon, Oxon, U.K.). Cryo-scanning electron

microscopy (Cryo-SEM) sample preparations were carried out in

the following steps: a small volume of emulsion was mounted

on a copper rivet and then plunged into liquid nitrogen slush

(�210 �C). The frozen sample was then transferred immediately

to the cryo chamber attached to the SEM, fractured with a chil-

led scalpel blade at�140 �C under high vacuum condition (>10�4

Pa), etched for 30 min at �95 �C and then sputter coated with

a gold/palladium alloy (60/40) using a cold magnetron sputter

Fig. 2 Volume weighted size distribution of kaolinite dispersion

measured by laser diffraction. The platelets have an average volume

diameter of 80 mm.

Soft Matter, 2012, 8, 3112–3121 | 3113


Fig. 3 A schematic of the compression of a clay-armoured droplet using

a colloidal probe AFM combined with laser scanning confocal micros-

copy. The radius of the colloid probe, a, is much larger than the radius of

the droplet, R, minimizing the effect of the colloidal probe curvature on

the geometry of the compression. The deformation of the droplet, Dz, is

in inferred from the changes in the piezo motion, Dl, and deflection, Dd,

or measured directly using laser scanning confocal microscopy from

below the droplet.

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coater at 300 V and 10 mA for 120 s (�6 nm thick). SEM imaging

of sample was carried out in a nitrogen gas cooled module

(maintained at�140 �C) with a solid state backscattered electron

detector.

Immobilization of clay-armoured droplets for force

measurements

Clay-armoured droplets were immobilized on a PAH modified

glass substrate. This step imparts a positive charge to the

substrate to facilitate binding of the negatively charged clay

platelets. First, the substrate was immersed into an aqueous

solution which consisted of 5 mg mL�1 PAH in 0.5 M NaCl for

10 min at room temperature. After the substrate was removed

from the solution, it was rinsed with copious amount of milliQ

water, to remove non-adsorbed polymer, before drying in the

oven at 100 �C for about 20 min. A drop of emulsion solution

was deposited on the modified substrate after it was cooled down

in order to immobilize the clay-armoured droplets on its surface.

The solution around the droplets was exchanged several times to

remove any free droplets prior to force measurements.

Atomic force microscopy

Two commercial AFMs: MFP3D (Asylum Research, Santa

Barbara, U.S.A) and Nanowizard 2 Bioscience (JPK, Germany)

were used to determine the stiffness of clay-armoured droplets

using a colloid probe that was significantly larger than the

droplet. Spherical silica beads of 30–50 mm in diameter (Thermo

Fisher Scientific Inc., Waltham, U.S.A) were glued to the apex of

a rectangular silicon AFM cantilever (Multi75, Budget Sensors,

Bulgaria).42,43 The spring constant of the cantilevers was deter-

mined using the thermal method44 ranging from 3 N m�1 to 10 N

m�1. All force measurements were carried out in aqueous solu-

tion at room temperature.

During measurements, the colloidal probe was positioned

above an immobilized droplet which was much smaller than the

dimension of the bead. This was to ensure that plate-plate

geometry can be assumed for deformation analysis. A schematic

of the experiment setup is shown in Fig. 3. The AFM records the

cantilever detector photodiode voltage as a function of the

change in the piezo distance, Dl, using a linear variable differ-

ential transformer (LVDT). The raw photodiode voltage was

converted to cantilever deflection, Dd, by scaling the photodiode

detector sensitivity. The photodiode detector sensitivity was

determined from the slope of the force curve which was obtained

by pressing the cantilever against the substrate surface. The

force, F, and deformation, Dz, were then calculated by multi-

plying the cantilever deflection with the cantilever spring

constant, k (F¼ kDd) and subtracting by the cantilever deflection

from the piezo distance (Dz ¼ Dl � Dd). For each droplet,

a series of force measurements were taken by varying the applied

force on the droplet.

Laser scanning confocal fluorescence microscopy

The Nanowizard 2 Bioscience AFM was mounted on a Nikon

Eclipse Ti–E inverted microscope that was equipped with

a Nikon A1 confocal imaging system (Nikon, Japan). This

combination allowed the visualization of droplet shape

3114 | Soft Matter, 2012, 8, 3112–3121

simultaneously during compression at a given applied force. For

this experiment, tetradecane was labelled with nile red (excitation

and emission wavelength of 488 nm and 525 nm, respectively)

prior to emulsification. The armoured droplet was compressed at

a constant force for five to ten minutes to capture a confocal

image. Image intensity was adjusted by the camera pinhole and

detector sensitivity in the Nikon elements Control software. A

porosity test of the armoured droplets was performed by adding

acid red 88 (excitation and emission wavelength of 561 nm and

595 nm, respectively), a dye that is soluble in both aqueous and

oil phases, to the aqueous phase of the AFM setup with emulsion

droplets.

Results and discussion

Characterization of clay-armoured droplets

Clay-armoured droplets were successfully produced by emulsi-

fying tetradecane with kaolinite dispersions. This emulsification

method has also been successfully used by many others to

produce particle-stabilized emulsions using either hydrophobic

or hydrophilic particles.10–12 The resulting emulsion droplets can

be diluted in aqueous solution without destabilization, thus they

are water continuous emulsions (oil-in-water emulsion). As

shown in Fig. 4a, the clay-armoured droplets are spherical in

shape; with a ‘crusty’ appearance to the surface. This suggested

the presence of clay platelets at the oil–water interface. Hexag-

onal, plate-like structures can be seen clearly on the cryo-SEM

image of the droplets, Fig. 4b, which confirms the adsorption of

clay platelets on the interface. The image also clearly demon-

strates that the droplet is fully covered with clay platelets and the



Fig. 4 (a) An optical microscopy image and (b) a cryo-SEM image of

clay-armoured droplets produced from the emulsification of tetradecane

with a kaolinite dispersion.

Fig. 5 Size distribution of clay-armoured emulsion droplets measured

using laser diffraction. The shearing time for emulsion formation

exhibited little change when varied from 15 to 300 s indicating emulsion

formation was completed by 15 s.

Fig. 6 XY slice from the 3D confocal image of a clay-armoured droplet

(without fluorescent labelled) after exposed to dye molecules for twenty

minutes. Both aqueous and oil phases contained siginificant concentra-

tions of fluorescent dye molecules were illuminated by the fluorescent

dye.

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coating layer appears to be thick and multilayered in nature (see

Yan and Masliyah11). However, there are limitations as to what

can be learned from the image. For example, it is not possible to

ascertain the droplet porosity from the image. In addition, the

clay platelets are arranged randomly on the droplet surface, with

some lying flat while the others are sticking out (suggesting that

platelets are not highly flexible). It is not clear if the disorgani-

zation or protruding platelets are the results of the formation

process or arise from changes induced by the freezing process

prior to imaging.

The size distribution measurement shows that a large number

of the armoured droplets produced have an average volume

diameter of 150 mm, while a small population of the droplets

centred around 5 mm, Fig. 5. No change in drop size distribution

was observed when the oil and clay dispersion mixtures were

sheared for different times, indicating that the emulsification

process is complete. In addition, the smaller droplets dimension

(both size distribution measurements and cryo-SEM observa-

tions) compared to the platelets indicated that smaller platelets

were favoured for adsorption to droplets during the emulsifica-

tion process.

The cryo-SEM image (Fig. 4b) does not clearly indicate

whether the clay-armoured droplets were perforated with

submicron pores. The permeability of the droplets was tested by

adding a drop of concentrated acid red (88) solution to the


aqueous solution containing the clay-armoured droplets that

were not fluorescently labelled. A confocal image was obtained

after twenty minutes and it indicated that the armoured droplets

were permeable as both the aqueous and oil phases contained

fluorescent dye molecules, as shown in the confocal image in

Fig. 6. It is worth noting that acid red 88 partitions more strongly

in the oil phase, thus, the droplet illuminates compared to the

surrounding solution.

Force curves and confocal microscopy images

A typical force versus deformation curve for a clay-armoured

droplet compressed by a colloid probe is given in Fig. 7. At large

separation distances, i.e. the probe is far away from the droplet,

no force is acting on the droplet. However, as the two surfaces

come into contact with each other, the force increases with

increasing droplet deformation and is non-linear with deforma-

tion. A slight hysteresis was observed upon loading (solid curve)

and unloading (dashed curve). This is most likely due to the

interaction between the PAH coated substrate and droplet27 or

due to the rearrangement of clay-platelets.45,46 Multiple

measurements were done on the same droplets; however, no

Soft Matter, 2012, 8, 3112–3121 | 3115


Fig. 7 A typical force-vs-deformation curve for an immobilized clay-

armoured droplet, with a diameter of 5 mm compressed by a colloid probe

up to a set applied force. The approach and retract branches of the force

curve show little hysteresis and subsequent curves also exhibit this

reproducible deformation.

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change in the shape of the force curve was observed. This sug-

gested that the droplets do not collapse and deform in a repro-

ducible fashion.

The change in droplet shape at a given applied force can be

observed simultaneously by using the combination of AFM and

laser scanning confocal microscopy. Fig. 8 shows the force-vs-

deformation curve for a droplet with a diameter of 5 mm and its

corresponding confocal images. As the compression force

increases, the deformation of the droplet increases. Upon

unloading, the original shape of the droplet was recovered.

Hence, the armoured droplets were said to be robust and do not

disintegrate in agreement with the reproducibility of the force

data.

The geometry of clay-armoured droplets can be reconstructed

from confocal images.37 This was achieved by extracting the

Fig. 8 Result of a combined AFM and laser scanning confocal

microscopy measurement on a clay-armoured droplet. The droplet was

compressed to a set force marked by the arrows on the force-vs-defor-

mation curve and a 3D confocal image stack was acquired over 5 to 10

min to capture the shape of the deformed droplet. Unloading of the

droplet resulted in a return of the drop original shape, consistent with the

reproducibility observed in AFM compression measurements.

3116 | Soft Matter, 2012, 8, 3112–3121

droplet interface profiles from each image slice at regular step

heights. In this analysis, it was assumed that the interface was

located at the pixel point where there was a sudden change in the

intensity corresponded to the perimeter of the 2D circle in the X

(solid line) and Y (dashed line) directions, Fig. 9. By compiling

these XY data and the known height for each slice, the profile of

the droplet can be determined. When dealing with droplets in

confocal microscopy, a lensing effect from the curvature of the

drop is a common problem due to the differences in refractive

indices of the solvent (in this case, water) and tetradecane.47,48

Hence, only data obtained from the bottom hemisphere were

fitted to a circle in this procedure as they were not affected by the

lensing effect. The change in clay-armoured droplet geometry

upon compression reconstructed from the confocal images

(symbols) is shown in Fig. 10. Note that, this drop has two axes

of symmetry and the shape extracted was applied to the lower

half of the drop and inverted for convenience.

It is also possible to construct the geometry of the armoured

droplet based on the force curves obtained from the AFM by

assuming a symmetric compression and a constant volume

constraint, i.e. the volume of the droplet does not change during

the compression. Additionally, the radius of curvature of the

probe is much larger than the droplet, thus parallel plate

geometry and symmetric compression are assumed. The radius of

curvature of a deformed droplet, rc can be calculated if the

original radius of the droplet, R and the amount of deformation

Fig. 9 (a) 3D confocal image of clay-armoured droplet, XY slices from

3D confocal images at a height of (b) 8.85 mm, (c)5.7 mm and (d) 3.9 mm

and (e) normalised intensity profiles for the strip chosen in the X (solid)

and Y (dash) directions of (b) where one pixel corresponds to 150 nm.



Fig. 10 The changes in clay-armoured droplet shape for compression at

different applied forces acquired from confocal images (symbols) and

calculated based on force measurements (solid line). Only one quarter of

the drop is shown based on the axisymmetric nature of the drop. The

droplet shape extracted from the confocal images is based on the shape of

the lower half of the drop to avoid lensing effects.

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for a given applied force are known. The contact radius of the

deformed droplet, rcontact is calculated using the standard

approach for constant volumes of revolution. A detailed analysis

procedure can be found in Appendix 1.

Fig. 10 also compares the droplet shape calculated based on

the AFM force curve measurements (Fig. 7) and reconstructed

from the confocal images. There are slight variations in droplet

radius of curvature and contact radius between the two methods.

This discrepancy is within the error of both the image analysis

and the dependence of the calculated profile on the accuracy of

cantilever calibration. Furthermore, there is no evidence of oil

leaking out from the droplet or the formation of secondary

droplets near the particular armoured droplet in the confocal

images. Thus, these data support the assumption of constant

volume conservation.

Fig. 11 Comparison between experimental (symbol) and calculated

(line) force as a function of compression for solution at (a) natural pH, (b)

pH 8, (c) pH 9 and (d) pH 9.6. A fitting surface Young’s modulus of 0.04

N m�1 was used for natural pH and 0.1 N m�1 for higher pHs.

Stability of clay-armoured droplets under various solution

conditions

As detailed previously, the deformation of droplets can be

observed from confocal images or calculated from force curve

measurements for a known applied force. A constitutive relation

between the applied force and the droplet deformation is

required to compare the change in clay-armoured droplet

response to different solution conditions. There have been

a number of methods developed to analyse the compression of

a micro-capsule or shell. Many of these techniques, however,

were only suitable for systems where the thickness of the capsule

wall or shell is known.40,49,50 For Pickering emulsions (in this

case, clay-armoured droplets), the thickness of the coating is

poorly defined where it may vary in thickness as well as exhibit

regions where the oil–water interface is exposed due to the

porous nature of the clay-armoured droplet.

An alternative approach is to use the theory described by Ferri

et al.27 as it only incorporates the droplet contact radius; more

specifically the shell or wall thickness is not required at the cost of

limiting the analysis to a surface modulus, not a bulk modulus of

the shell. In this analysis, the relationship between applied force,

contact radius and mechanical properties of a clay-armoured

emulsion droplet is described by a combination of surface

tension and the deformation of a membrane. If the mechanical


properties of a clay-armoured droplet can be assumed to be

directionally independent, i.e. isotropic compression, the stresses

along the principal directions of the membrane are given by their

average, ½(T1 + T2), and can be related to the geometry of the

compression27:

F

pr2contact¼ ðT1 þ T2Þ

Rl(1)

where l ¼ (A/A0)0.5 is the stretch on the droplet surface; with A

and A0 are the surface areas of the deformed and original

droplets, respectively. A linear elastic constitutive law, i.e.Hooke

law, was used to relate the stresses experienced by the droplet

during compression and droplet stretch. It is given by:

1

2ðT1 þ T2Þ ¼ Es

2ð1� vÞ�l2 � 1

�(2)

where Es is the surface Young’s modulus and v is the Poisson

ratio. Eqn (2) has been used previously to investigate the elastic

constants of Pickering emulsion droplets stabilized by cowpea

mosaic virus particles with and without cross-linking. Further

details can be found in Ferri et al.27

The effects of solution pH, ionic strength and CTAB

concentration on the surface Young’s modulus of emulsion

droplets were examined. The solution pH was adjusted from

natural pH to approximately 10 by addition of NaOH, whilst the

NaCl and CTAB concentrations were raised from 10 to 1000 mM

and 0.01 to 100 mM, respectively. Shown in Fig. 11 is

a comparison of the experimental force as a function of

compression (1 � rcontact/R) at different solution pH conditions

and the calculated force by combining eqn (1) and (2) and using

a poisson ratio of 0.5. In the calculation, the surface Young’s

modulus was used as a fitting parameter and varied to minimize

the error between experimental and theoretical data. The force

increases with increasing compression and a reasonable agree-

ment was found between the experimental and calculated force

using a surface Young’s modulus of 0.04 � 0.0015 N m�1 for

natural pH and 0.1 � 0.005 N m�1 at higher pH. The increase in

surface Young’s modulus at higher solution pH is likely due to

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charge regulation of the clay platelets. It is well-known that clay

platelets have an unequal charge distribution.51 At natural pH or

lower, the faces of the platelet are negatively charged, while the

edges are positively charged. As the pH increases above 8 or

higher, the charge on the edges of the platelet changes from

positive to negative. Thus, by implication, the increase in droplet

stiffness is due to the increase in repulsion between the platelets.

This finding is in contradiction to the work of Yan and Mas-

liyah11,52 where the clays desorbed from the oil–water interface at

high solution pH. It is important, however, to note that there is

a difference in the surface chemistry of the clay platelets used in

the two studies. Clay platelets were not modified by any means in

the current work; however, they were made hydrophobic by

adsorption of asphaltenes in the work of Yan and Masliyah.11,52

This is also in contrast to the optical microscopy observation

where the clay coating maintains a ‘crusty’ appearance at high

solution pH, which is consistent with the observations of Sub-

ramaniam et al.22 for clay-armoured bubbles.

The theoretical model (eqn (2)) exhibits a small but systematic

error in describing the deformation of the clay-armoured drop-

lets. The deviation at high compression indicates that clay-arm-

oured droplets may not be well described by the Hooke

constitutive law over the entire compression region. The extent of

this non-Hookean behaviour can be visualized by calculating the

surface Young’s modulus at each point in the force-vs-defor-

mation curve. The resultant surface Young’s modulus can then

be correlated with the engineering strain of a droplet, s, which is

given by: s ¼ 0.5(l2 � 1). A Hookean elastic shell would have

a constant surface Young’s modulus, but as shown in Fig. 12, the

surface Young’s modulus decreases with increasing engineering

strain, for a given solution pH. For a given engineering strain, the

surface Young’s modulus was found to increase with increasing

pH. Little or no hysteresis was observed in the loading and

unloading force curves and the confocal studies demonstrated

that clay-armoured droplets were porous (Fig. 6). Therefore, the

non-Hookean elastic behaviour is likely to arise from the oil–

water interfacial tension and the formation of new interfaces

during droplet deformation.

Fig. 12 Surface Young’s modulus of clay armoured droplet as a func-

tion of engineering strain at various solution pHs. The surface Young’s

modulus decreases with increasing engineering strain for a given solution

pH and increases with a set engineering strain when the solution pH

changes from natural pH to pH 8.

3118 | Soft Matter, 2012, 8, 3112–3121

Droplet compression at various solution ionic strengths also

exhibits similar systematic deviation from a purely elastic

constitutive relationship with an average surface Young’s

modulus of 0.13 � 0.04 N m�1 (data not shown here). Hence, the

surface Young’s modulus was calculated as a function of engi-

neering strain, Fig. 13. The droplet surface Young’s modulus

does decrease with increasing engineering strain (similar to the

observation with changing solution pH), but does not vary

(within the limits of uncertainty) with increasing solution ionic

strength. The independence of surface Young’s modulus with

solution ionic strength suggests that electrostatic interaction do

not play a major role in the adsorption or any rearrangement of

clay platelets on the oil–water interface. The clay platelets were

provided with sufficient energy during shearing to overcome

repulsion and allow them to access to their primary van der

Waals minimum, which causes them to strongly adsorb on the

droplet surface (i.e. irreversibly adsorb).4,22,53 Alternatively, the

‘hole’ in the interface created by the platelets energetically

favours its adsorption to the oil–water interface regardless of

charge and hydrophobicities.54 At higher salt valencies (CaCl2was used in this study), the stiffness of the droplet is similar to

those that were exposed to monovalent salt. These results are in

agreement with the observations of Subramaniam et al.22 where

they did not observe any change in their clay vesicles after

exposure to salt of various valencies and concentrations.

An interesting result was observed when clay-armoured

droplets were exposed to surfactants, particularly at concentra-

tions above the critical micelle concentration (cmc). As seen in

Fig. 14, the surface Young’s modulus decreases with increasing

engineering strain at a constant CTAB concentration similar to

the trend observed for solution pH and ionic strength (see Fig. 12

and 13). Yet, the behaviour differs significantly with changes in

surfactant concentration. At a set engineering strain, the surface

Young’s modulus decreases with an increasing CTAB concen-

tration. This decrease in strength suggests a change in the

mechanical integrity of the clay-armoured droplet coating. The

positively charged surfactant molecules presumably adsorb on

the negatively charged basal plane of kaolinite, and at CTAB


tion of engineering strain at various solution ionic strengths. The surface

Young’s modulus decreases with increasing engineering strain for a given

solution ionic strength but does not vary (within the limits of uncertainty)

with changing electrolyte concentrations and valencies.




tion of engineering strain at various CTAB concentrations. The surface

Young’s modulus decreases with increasing engineering strain for a given

CTAB concentration and decreases with increasing CTAB concentration

for a given engineering strain.

Fig. 15 Confocal images of two clay armoured droplets compressed on

top of each other during (a) loading and (b) unloading. The compression

duration was five to ten minutes, where the droplets exhibited exceptional

stability against coalescence at large deformations and compressive

forces of several micro-Newtons.

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concentrations higher than the cmc (approximately 1 mM at

room temperature55,56), charge reversal may occur. It is expected

that this process will lead to the solubilisation of clay platelets.

The appearance of flocs in the bulk solution, at high surfactant

concentration, is an indication that clay platelets had desorbed

from the oil–water interface. This is consistent with the literature

where emulsion droplets tend to become unstable at high

surfactant concentrations.57–61 Additionally, a drastic decrease in

surface Young’s modulus was observed with increasing CTAB

concentrations up to cmc. The large change in surface Young’s

modulus at CTAB concentrations below the cmc is believed to be

associated with the interfacial tension. The interfacial tension of

CTAB-tetradecane decreases to 7.5 mN m�1 with increasing

CTAB concentration.62 Above the cmc, the interfacial tension

becomes constant and the magnitude of the change in surface

Young’s modulus decreases. These observations suggest that

interfacial tension plays a dominant role in droplet deformation

until the clay platelets desorb leading to the dramatic drop in

surface Young’s modulus at CTAB concentrations higher than

50 mM. The role of interfacial tension in the mechanical prop-

erties of Pickering emulsion droplets is also supported by the

analysis of Ferri et al.27 for the compression of nanoparticle-

stabilized droplets with and without cross-linking. Although, one

difference to this study was that the cross-linked nanoparticles

networks exhibited some evidence of breakdown from droplet

compression, whereas the overlapping clay platelets network

appears to be more robust and recover from deformation.

It is worth noting that at CTAB concentrations of 50 mM and

100 mM, the surface Young’s modulus values obtained from the

experimental data may be larger than expected as clay-armoured

droplets lose their sphericity due to oil solubilisation (i.e.

reduction in droplet volume). Thus, the surface Young’s

modulus at such high concentrations is low and the accuracy of

the measurements has decreased.

The above results support the perception of a superior stability

associated with clay-armoured droplets. The extent of this

stability was highlighted in the confocal images, shown in

Fig. 15, where two armoured droplets were compressed, with one

attached to the colloidal probe and the other immobilized on the

surface. The drops were held under compression for five to ten


minutes to acquire the confocal image (Fig. 15a) with no coa-

lescence observed upon unloading (Fig. 15b). These additional

data also indicated that the clay-armoured droplets were

mechanically stable.

Conclusions

The mechanical properties of clay-armoured emulsion droplets

were investigated using a combination of AFM and laser scan-

ning confocal microscopy. This allows for the visualization of

droplet shape in situ in a compression experiment. The con-

structed geometries of droplets, at a given applied force,

extracted from confocal images and AFM force curves were in

close agreement with each other.

Experimental data suggested that clay-armoured emulsion

droplets are mechanically robust and recover from large

deformations without disintegration. These emulsion droplets

remain stable under various solution ionic strength and pH, but

become unstable at high cationic surfactant concentrations.

The destabilization of these droplets at surfactant concentra-

tions above the cmc is due to both oil solubilisation and

desorption of clay-platelets from the oil–water interface. The

lack of hysteresis on the force-vs-deformation curves under all

solution conditions and compression strength, and the effect of

CTAB concentrations on the mechanical properties of clay-

armoured droplets imply that both interfacial tension and

strengths of the overlapping clay network play a role in droplet

elasticity.

As clay-armoured emulsion droplets can be found in a number

of everyday products, it is hoped that these results will provide

a new insights into the mechanical strength and response of these

droplets to external applied forces and also provide a better

roadmap for industry to deal with the problems arising from

these emulsion systems.

Soft Matter, 2012, 8, 3112–3121 | 3119


Fig. 16 Schematic of a (a) deformed droplet and (b) force-vs.-defor-

mation curve.

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Appendix 1

When a spherical drop with a radius, R is compressed with

a colloidal probe, it is deformed. The radius of curvature and

contact radius of the deformed drop are denoted as rc and rcontact,

respectively, Fig. 16a. The deformation of a drop at a given

applied force can be determined from the force curve, which is

denoted as z in the force-vs.-deformation curve in Fig. 16b. The

radius of curvature of the deformed droplet is then given by:

rc ¼ 2R� z

2(3)

The standard approach of constant volumes of revolution was

used to calculate the contact radius of the deformed droplet and

it is given by:

rcontact ¼p�R2 � r2c

�4rc

(4)

In order to calculate the stretch on a droplet, l during

compression, the surface area of both deformed and undeformed

droplet needs to be known. The surface area of undeformed

droplet is given by: S$Asphere ¼ 4pR2. The deformed droplet is

made up of two half spheres and a cylinder. The surface area

generated by a half sphere, S Ahalf sphere and cylinder, S Acylinder is

given by:

S:Ahalf sphere ¼ 8p

�4rc

3pþ rcontact

�4rc

3p(5)

S$Acylinder ¼ 4prcrcontact (6)

Acknowledgements

The financial support of the Australian Research Linkage

scheme, AMIRA International, and state governments of South

3120 | Soft Matter, 2012, 8, 3112–3121

Australia and Victoria are gratefully acknowledged. The ARC is

also thanked for their financial support, and the Particulate

Fluids Processing Centre, a special research centre of the ARC,

provided infrastructure support for the project. Part of this work

was carried out in the Melbourne Centre for Nanofabrication,

which is the Victorian node of the Australia National Fabrica-

tion facility, an initiative partly funded by the Commonwealth of

Australia and the Victorian government.

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Nano-mechanical properties of clay-armoured emulsion droplets