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Coalescence in concentrated Pickering emulsions under shear† Published on 27 May 2014. Downloaded by Brown University on 27/10/2014 01:29:58.
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Catherine P. Whitby* and Melinda Krebsz We have investigated the rheology of concentrated oil-in-water emulsions stabilised by silanised silica nanoparticles. The emulsions behave like highly elastic solids in response to small, uniform strains. They become unstable and begin to break down, however, on yielding. We show that the emulsion elasticity is correlated with the salt concentration in the water and hence the particle aggregation in emulsions at a Received 4th March 2014 Accepted 2nd May 2014
given drop volume fraction. A supporting observation is that destabilisation is favoured by minimising the attractive interactions between the particles. Microscopic observations revealed that coalesced drops
DOI: 10.1039/c4sm00491d
have anisotropic shapes and wrinkled surfaces, direct evidence of the interfacial particle layer acting like
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a mechanical barrier to bulk emulsion destabilisation.
1. Introduction Concentrated emulsions resist owing and only give way under pressure.1 Mayonnaise and moisturiser cream, for example, can behave like elastic solids and hold their shape. They consist of drops packed closely together so they touch all their neighbours.2 The drop volume fraction is so high that they can support their own weight and resist mechanical shear. Attractive interactions between the drops can cause them to stick together as they come into close contact.3 The drops remain separated by thin lms of continuous phase. Emulsions stabilised by solid particles (Pickering emulsions4–9) are remarkably robust structures. Particle-coated drops can be squeezed together and the liquid evaporated to form a porous solid.10–13 The drops may coalesce, however, under shear stress. The mechanisms that cause the thin lms to rupture are not well understood. In this paper we investigate the shear response of aqueous Pickering emulsions as the drop volume fraction increases. Droplet adhesion was varied by controlling the salt concentration and hence the particle aggregation. The conditions leading to destabilisation under shear stress are of particular interest. The shear elasticity of concentrated emulsions is due to the drops being jammed together.14 Emulsion drops are geometrically conned at drop volume fractions (f) larger than about 64 vol% (fm). Princen15 argued that the drops are compressed by an osmotic pressure greater than their Laplace pressure (which is given by the ratio of the oil–water interface tension to the average drop diameter, 2gow/d). The drops deform into
Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia. E-mail: [email protected] † Electronic supplementary 10.1039/c4sm00491d
information
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(ESI)
available.
See
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polyhedral shapes with attened areas of contact. Small perturbations stretch the thin liquid lms between the drops. The additional surface area created determines the elastic response of the emulsion, which is characterised by the elastic storage modulus, G0 . Mason et al.16 demonstrated that the elastic shear moduli of surfactant-stabilised silicone oil-inwater emulsions (at a given f) scale with the drop Laplace pressure. They showed that G0 depends on the increase in the drop volume fraction above the maximum packing fraction16 G0 z ðf fm Þ ðgow =dÞ
(1)
Flow only occurs in concentrated emulsions above a critical value of applied stress, the yield stress (sy). At the solid–liquid transition, the yield stress is given by sy ¼ Ggy, where G is the emulsion elasticity and gy is the yield strain. Princen17 proposed that emulsions yield by irreversible changes in drop neighbours (rearrangements). Arditty et al.18 found that the elastic moduli of concentrated emulsions of micrometre-sized oil drops stabilised by silanised silica nanoparticles (of radius, rp ¼ 25 nm) were signicantly higher than the values measured for surfactant-stabilised emulsions at a given f. They proposed18 that the particles coating the drops aggregate and behave collectively like an elastic network (or membrane). Assuming that the network was a two dimensional solid layer, Arditty et al.18 estimated that its tension (the elastic energy required to increase the layer area) was an order of magnitude larger than the oil–water interfacial tension. This makes it the dominant contribution to the interfacial energy of the particle-coated drops and their response to deformation. Hermes and Clegg19 investigated the yielding behaviour of oil drops (d ¼ 15 mm) in water stabilised by silica nanoparticles
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(rp ¼ 330 nm). Adding high concentrations of salt (44.8 wt%) caused the drops to occulate and they saw evidence19 for this being due to aggregation between particles attached to neighbouring drops. The occulated emulsion (f ¼ 0.5 < fm) owed once sufficient strain was applied to break the drop clusters apart. Compressing the emulsion to f 0.95 ([fm) increased its elasticity by two orders of magnitude. The drops in the compressed emulsion coalesced instead of owing when the emulsion yielded. Hermes and Clegg19 argued that this was due to the particle stabilisation of the interface failing at high strains. We investigated the role of particle aggregation in the stability of Pickering emulsions deformed by shear stress. The emulsions were stabilised by silanised fumed silica particles which have been shown to aggregate and form weak particle gels at high concentrations (0.5 M NaCl) of salt.20 Reducing the salt concentration in the emulsions increased the repulsive interactions between the interfacial particles. We found that this minimises the contribution of the particle layer tension to the interfacial energy of the drops. Applying a stress of the order of the Laplace pressure destabilises the emulsions. We show that droplet coalescence is arrested at intermediate stages. The particle layers coating the merged drops buckle (wrinkle) due to the reduction in the total interfacial area.
2.
The particle-coated oil drops in a concentrated Pickering emulsion (f ¼ 0.75) are closely packed together as shown in (a) scanning electron microscope and (b) confocal fluorescence images. (c) Schematic of a concentrated emulsion contained between two parallel plates (separated by a distance of z) as a shearing stress (s) is applied. When the upper plate is moved relative to the lower one over a small distance (Dx/z # 10%), the emulsion is strained uniformly throughout its mass. When the stress is released, the drops relax back into (roughly) spherical shapes. Applying larger stresses causes adjacent drops to coalesce when uncoated drop surfaces are exposed by the deformation. (d) Non-spherical structures form as drops are trapped in an intermediate stage of coalescence in sheared emulsions. Fig. 1
Experimental section
2.1. Emulsion preparation and characterisation Silica nanoparticles modied by reaction with hexadecylsilane were supplied by Evonik (Aerosil R816). A sample of the powder was compressed into a disc-shaped pellet to form a planar substrate for wetting measurements. The contact angle of water drops under oil on the powder surface (qow, measured through aqueous phase) is about 60 . Dispersions of nanoparticles in solutions of NaCl (Chem Supply, 99%) in water (resistivity 18.2 MU cm, pH 5.3–5.8) were sonicated in an ultrasound bath (Soniclean 160T, 70 W power, 44 kHz operating frequency) for 20 minutes. TEM images revealed that the approximately spherical nanoparticles had radii between 10 and 30 nm. The particles had a mean radius (rp) of 12 nm and a polydispersity of 30%. Emulsions were prepared by homogenising equal volumes of isopropyl myristate (Sigma Aldrich, 98%, passed through chromatographic alumina twice to remove polar impurities) with the aqueous silica dispersions (1.5 wt%) using a X1000D homogenizer with a 19 mm diameter sha (Ingenieurbuero CAT, M. Zipper GmbH) operated at 10 400 rpm for 2 min. The oil volume fraction in the emulsion (fo) was then increased in small steps (10 vol%), rehomogenising for 30 s aer each addition of oil. Scanning electron micrograph images of emulsion surfaces produced by fracturing samples of frozen emulsions were obtained using a Philips XL30 Field Emission Scanning Electron Microscope tted with a CT1500 HR Low Temperature Cryo system. A typical image of an emulsion surface is shown in Fig. 1a. SEM images showed that the drops were coated with layers consisting of particle aggregates about 50 nm in size (ESI
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Fig. S1†). The emulsion drop size distributions were determined by static light scattering using a Malvern Mastersizer 2000 (assuming a relative refractive index of 1.1 and a particle absorption coefficient of 0). The emulsions were stored in screw-cap vials at 25 C.
2.2. Confocal microscopy Confocal uorescence microscopy (CFM, Leica SP5 spectra scanning confocal microscope) was used to visualise droplet adhesion in the emulsions. An o-ring xed on a glass cover slip was lled with emulsion. A second cover slip placed on top of the o-ring sealed the cell. The particles were stained with Nile Blue (1 mM in 0.1 M NaCl) and excited at 633 nm. Nile Red (1 mM in isopropyl myristate) was used for staining the oil and the sample was excited at 496 nm. Preliminary studies conrmed that the presence of the dyes did not alter emulsion structure or stability. Fluorescence intensity data for Nile Blue and Nile Red were collected in two separate channels corresponding to 520– 570 and 653–750 nm, respectively. Each line of pixels in an image was scanned sequentially for Nile Red and Nile Blue uorescence to avoid interference due to cross-uorescence. Fig. 1b shows a typical image slice obtained at a depth of 260 mm in an emulsion sample. The two-dimensional confocal images were analysed using the ImageJ soware. Each image contained 1024 1024 pixels, with a grey value ranging from 0 to 255. The images were processed by converting them to a binary form and then segmented by ‘thresholding’ using an iterative selection procedure based on the composite average of the averages of the background and
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object pixels. The perimeters of pairs of drops in close contact in the Nile Blue images were used to measure the contact angles formed at the junction between adhesive interfaces (attened areas of contact) and isolated ones.
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2.3. Rheology Rheological measurements were made using a Rheometric Scientic Dynamic Stress Rheometer SR2000 at a xed temperature of (25 0.1) C and parallel plate (25 mm diameter) or a cone and plate geometry (25 mm diameter, 2 angle). The ow behaviour obtained using at plates with either smooth hydrophobic or sand blasted steel at large separations (2 mm) was similar to that measured using a cone and plate geometry, indicating an absence of wall slip. A solvent trap was used to prevent evaporation. For oscillatory measurements, the amplitude was increased in a series of logarithmic steps (30 s in duration) at a constant frequency (typically 0.1 Hz). The moduli in the linear viscoelastic regions were determined from scans of the oscillation frequency at a constant stress. For shearing experiments, a xed shear stress or rate was applied for a xed time (typically 300 s in duration) as illustrated schematically in Fig. 1c. Aer shearing, the rotating plate was slowly removed and samples of the sheared emulsion were collected. Optical microscopy (Olympus BX51 microscope) was used to visualise the microstructure of sheared emulsions.
3.
Results
Mechanically mixing aqueous dispersions of the fumed silica nanoparticles with oil drives the particles to assemble into layers at the freshly created oil–water interface. Over the range of oil volume fractions used (f # 0.8), monomodal distributions of spherical oil drops in water are formed. The mean drop diameter (d) and hence the total interfacial area is determined by the mass of particles present. At the low particle concentration used (Cp ¼ 1.5 wt% in the aqueous phase), coalescence dominates emulsion formation. During emulsication, drops merge together until they are completely coated and protected by a dense layer of particles. Assuming that all the particles attach to the oil–water interface, a simple mass-balance model predicts that the nal drop surface area will be proportional to the drop volume fraction (f). In the special case of a xed particle concentration in the aqueous phase, the mass of particles present is proportional to the water volume fraction (1 f). Thus the mean drop diameter is given by21 d ¼ (8f/1 f)(rprpfCP/Cp)
(2)
where rp is the particle density. fCP is the fraction of the surface area covered by attached particles. Consistent with this equation, the average emulsion drop size is a linear function of the ratio f/1 f – ESI Fig. S2.† By geometrical arguments, it can be shown that the fraction of a planar surface coated by hexagopffiffiffiffiffi nally closely packed spheres is given by p= 12 z 0:907. Assuming that fCP 0.9, it was estimated that the drops are coated with sufficient particles to form a bilayer of fumed silica particles using the value of the tted slope in Fig. S2.† This
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accounts for the drop stability until shear is applied (shown later). It should be noted that the nanoparticles occulate in water and thus attach to the oil drop surfaces in the form of aggregates. Careful inspection of the emulsions showed no evidence for there being excess particles in the water, supporting the assumption that all the particles attach to the drops. The attached particles are not easily displaced under quiescent conditions. Particle attachment changes the free energy of the system by reducing the total oil–water interfacial area. The energy required to detach the particles22,23 (E) is given by E ¼ pgowrp2(1 + cos qow)2, where gow is the interfacial tension. Assuming that particle attachment does not alter the interfacial tension, it can be calculated that the capillary energy trapping the particles is about two orders of magnitude larger than their thermal energy. The particle-coated drops pack together into structures that resist ow. Fig. 2 summarises the response of emulsions at different CNaCl to oscillatory strain. The drop volume fraction and hence the size of the drops was xed in these emulsions. At the lowest salt concentration studied (0.001 M, black squares in Fig. 2), the emulsions show a linear viscoelastic response at strain amplitudes (the relative deformation) of g < 10%. The component of the stress that is in phase with the applied strain (represented by the modulus G0 ) is at least an order of magnitude larger than the component out of phase by 90 (G00 ). This means that signicantly more energy is stored elastically by the emulsions (per unit volume) than is lost by viscous dissipation during each cyclic period of strain. G0 decreases as the strain amplitude increases above 10%. G00 increases with increasing strain up to a maximum at which G00 ¼ G0 . Both moduli decrease as g increases at strains larger than the critical (crossover) strain (gc 100%) and the emulsions yield. The emulsions become stiffer as the salt concentration increases (0.01 M and 0.1 M, green circles and blue triangles, respectively, in Fig. 2). They show a linear viscoelastic response only at g < 1% at higher salt concentrations. The strain at which
Fig. 2 Strain (g) dependence of the emulsion elastic storage moduli (G0 , filled symbols) and viscous loss moduli (G00 , open symbols) of 60 vol% emulsions at NaCl concentrations of 0.001 ( ), 0.01 ( ) and 0.1 ( ) M.
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the emulsions yield and move irreversibly decreases as CNaCl increases. The crossover strain is about 20% for emulsions at 0.01 M NaCl and about 3% for emulsions at 0.1 M salt. Over the range of drop volume fractions studied (0.4 < f < 0.8) the emulsions behave like solids at small strains (g # 1%). In the linear viscoelastic regime, the moduli increase with the drop volume fraction, as shown in Fig. 3 (only G0 is plotted, for clarity). This is unlike the volume fraction dependence of the elasticity of surfactant-stabilised emulsions,16 which shows a sharp increase at volume fractions corresponding to random close packing of the drops, f frcp. Fig. 3 shows that the elasticity of the particle-stabilised emulsions varies with the salt concentration in the aqueous phase, CNaCl. The moduli are normalised by the Laplace pressure of the drops (given by the ratio 2gow/d). At the highest salt concentration studied, CNaCl ¼ 0.1 M, the elastic storage moduli of the concentrated emulsions (f $ frcp ¼ 0.64) is up to two orders of magnitude larger than G0 of surfactant-stabilised emulsions. The layers of particles attached to the drop surfaces enhanced the emulsion elasticity. Emulsion elasticity at a given f decreases as the salt concentration and hence the ionic composition of the aqueous phase decreases. At the lowest salt concentration studied, CNaCl ¼ 0.001 M, the (normalised) elastic moduli of the concentrated emulsions (f $ frcp ¼ 0.64) are similar to those measured for surfactant-stabilised emulsions (Fig. 3a). The yielding behaviour of the emulsions is also sensitive to CNaCl. This was characterised by microscopic examination of the emulsions aer shearing at stress values above the yield stress. Emulsions formed at high salt concentrations (0.01 M and 0.1 M) resist deformation. The drop size distribution in the emulsions does not change, indicating that the emulsions remain stable to coalescence. Emulsions formed at at CNaCl ¼ 0.001 M do not withstand the disruption associated with yielding. Applying larger shear stresses (s > 100 Pa) induces drop coalescence in closely-packed emulsions.
(a) Drop volume fraction (f) dependence of the Pickering emulsion elastic storage moduli, G0 at NaCl concentrations of 0.001 ( ), 0.01 ( ) and 0.1 ( ) M. The elasticity is scaled by the Laplace pressure, the ratio of the oil–water interfacial tension (g) to the average drop diameter (d). The blue solid line shows the variation in elasticity of surfactant-stabilised emulsions obtained by Mason et al.16
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Coalesced drops tend to have shapes that resemble an intermediate stage of the coalescence process, as shown in Fig. 1d. This indicates that coalescence is arrested before the merging drops fused into a single spherical drop. The morphology of coalesced drops changes with the proportion of oil in the emulsions. As f increases, anisotropic shapes formed by clusters of more than two droplets are observed (Fig. 4a). Coalesced drops that relax into (relatively) spherical shapes have wrinkled (buckled) surfaces, as shown in Fig. 4b. The wrinkles are prominent on larger coalesced drops and tend to orientate in a xed direction. Their periodicity (l) was determined to be about 10 mm by estimating the wavelength of the undulations (Fig. 4c). Staining sheared emulsions with uorescent dyes that bind preferentially to the particles revealed that the coalesced drop surfaces are completely coated with particles – Fig. S3.† Wrinkling occurs because the area occupied by the attached particles remains constant although the droplet surface area has decreased. Lateral compression causes the coating to expand (wrinkle) in the perpendicular directions. The particles are acting collectively, like a thin, two-dimensional solid attached to a thicker foundation (the oil drop). The response of the solid can be characterised by the ratio of the perpendicular and parallel strains, the Poisson ratio (n). Assuming the particles attached to the oil–water interface are hexagonally close
Fig. 3
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Fig. 4 Optical images of (a) partially merged drop cluster and (b) the crumpled surface of a merged drop in a sheared concentrated emulsion (f ¼ 0.80). (c) Line profile of the intensity extracted along the solid path shown in (b).
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packed, n z 1/3.24 Following the general theory for wrinkling of elastic sheets, the periodicity (l) of the wrinkles will depend on the balance between the bending stiffness of the solid (B) and the elastic stiffness (K) of the oil drop, l z (B/K)1/4.25 The elasticity of a drop surface is given by its capillary ratio, rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi . ffi Lc ¼ gow rg , where r is the oil density. Thus for interfacial particle layers l can be estimated24 by Published on 27 May 2014. Downloaded by Brown University on 27/10/2014 01:29:58.
l¼
4
3ð1 fCP Þð1 þ nÞ
1
4 pffiffiffiffiffiffiffiffiffi
Lc rp
(3)
This gives a result of 33 mm, which is reasonably close to the measured periodicity (Fig. 4c).
4. Discussion We manipulated the stability of concentrated aqueous emulsions of silica nanoparticle-stabilised oil drops to shear-induced coalescence by varying the salt concentration in the water. Emulsions formed at high salt concentrations are very stiff and resist deformation. They have an unusually large elasticity compared to surfactant-stabilised emulsions (Fig. 3). The particles attached to the drops make their surfaces solid-like under shear stress. Treating the particles attached to oil–water interface like a solid layer encapsulating the drops means that the interfacial pressure, 3, of the layer can be estimated. This was done assuming that the rheology data follow the relationship G0 d/3 f f(f frcp). At a given salt concentration, 3 does not vary signicantly with f and an average value of the interfacial pressure was calculated. Fig. 5a shows the elasticity moduli normalised by the ratio 3/d. It should be noted that the elasticity measured at drop volume fractions below maximum packing is due to occulation of the drops. The scaled data do not fall onto a single curve (at f < frcp) since the occulation is dependent on the salt concentration. At CNaCl ¼ 0.1 M, the layer needs to be characterised by 3 3 N m1 for the elastic moduli to follow the expected scaling behaviour. This large value of 3 must be due to the particles aggregating together. van der Waals forces, capillary forces, and attractive interactions due to interpenetration of the alkyl chains graed on neighbouring particles will all contribute to strong lateral attractive interactions between the particles26 that cause aggregation in the interfacial layer. Repulsive electrostatic interactions between the particles will oppose the attractive interactions and hinder particle aggregation. Emulsion elasticity at a given f decreases as the salt concentration and hence the ionic composition of the aqueous phase decreases. This suggests that particle aggregation and hence the characteristic pressure of the interfacial layer depends on the distance over which electrostatic interactions are screened in the aqueous phase. The thickness of the ionic layer that screens electrostatic interactions, the Debye length, pffiffiffiffiffiffiffiffiffiffiffi can be approximated27 by k1 ¼ 0:304 CNaCl . Fig. 5b shows that the elasticity of the concentrated emulsions decreases linearly with the square root of the salt concentration.
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(a) Drop volume dependence of Pickering emulsion elastic storage moduli scaled by the ratio of the interfacial pressure (3) to the average drop diameter (d). Data is shown for emulsions at NaCl concentrations of 0.001 ( ), 0.01 ( ) and 0.1 ( ) M. The blue solid line shows data for surfactant-stabilised emulsions obtained by Mason et al.16 (b) Salt concentration (CNaCl) dependence of the interfacial pressure, 3, of the emulsions. The purple solid line is a linear fit to the data. Fig. 5
Emulsions formed at high salt concentrations (0.1 M) are also very stiff due to occulation of the drops. Particle aggregation causes drops to adhere together. Drop adhesion in the emulsions was visualised by confocal microscopy (Fig. 6a). The thin lms that form between adhering drops have a nite contact angle (2q). Large contact angles were observed at high salt concentrations, indicating strong attractive interactions in the particle layers between the drops. Fig. 6b shows the decrease in the droplet contact angle as the salt concentration in the emulsions is reduced. In contrast, there is little adhesion between drops in dilute salt solutions (0.001 M). The adhesive energy of the lms can be calculated by 2g(1 cos q) using the Young–Dupr´ e equation.28 Assuming that the attached particles do not alter the oil–water interfacial tension it was estimated that the adhesion energy decreases by an order of magnitude as the salt concentration decreases from 0.1 to 0.001 M.
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Fig. 6 (a) Confocal fluorescence image highlighting the locations of particles coating a flocculated pair of drops in an emulsion at CNaCl ¼ 0.001 M. The drop surfaces intersect at an angle, 2q. (b) Salt concentration (CNaCl) dependence of the contact angle, q, of the thin films that form between flocculated drops. The purple solid line is a linear fit to the data.
Thus there are two major sources of the emulsion elasticity. The viscoelasticity of the interfacial particle layers and the stickiness of the particle-stabilized thin lms between the drops contribute to Pickering emulsion elasticity. These two factors are interrelated. They both arise from interactions between the particles attached to the drops. It is possible that the large value of surface pressure estimated at the highest salt concentration (3 3 N m1 at CNaCl ¼ 0.1 M) is partly due to strong adhesive interactions between the drops which enhance jamming in the emulsions. The sensitivity of the elasticity to the salt concentration is due to the small size of the particles (2rp 20 nm) coating the drops. The particles are deeply immersed in the water side of the oil–water interface, given their relatively small oil–water contact angle (qow ¼ 60 ). They are comparable in size to the thickness of the layer of ions that screens Coulombic interactions (k1 1–10 nm) in water at the salt concentrations studied (0.001 M # CNaCl # 0.1 M). Thus the charges on the water side contribute signicantly to the electrostatic interactions between the particles and reduce particle aggregation at low salt concentrations. This means that at low ionic strengths the response of the drop surfaces to deformation is not affected by the attached
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particles. Instead it is determined by the drop Laplace pressure, which is given by 2gow/d 600 Pa. Large perturbations (of the same order of magnitude as the Laplace pressure) presumably deform the drops and increase their total surface area, so that the fraction of the surface covered by particles is reduced. Exposing oil–water interface that is not coated by particles causes neighbouring drops to coalesce together, rather than ow past each other under shear stress. Denkov et al.29 proposed that the thin lms in concentrated Pickering emulsions rupture if there are defects, like fractures or vacancies, in the particle layer coating the drops. We have shown that increasing the repulsive interactions between particles in the interfacial layer favours rupture and destabilisation when emulsions are deformed by shear stress. Our results show that the emulsion drops become nonspherical and their surfaces wrinkle (or buckle) as they coalesce together. The structures observed in the sheared emulsions are similar to those formed when colloidal monolayers at planar or curved interfaces are compressed. Aveyard and co-workers30,31 demonstrated that when a layer of particles at a planar uid interface is compressed, it bends and forms an undulating surface with a characteristic wavelength. Pawar et al.32 found that the coalescence of two oil drops could be halted at some intermediate stage if micrometre-sized silica particles at the drop surfaces form a close-packed, jammed layer. The results presented here demonstrate that interfacial particles in bulk Pickering emulsions behave like a 2-dimensional solid and provide a mechanical barrier to droplet coarsening. They are direct experimental evidence of the solid-like nature of the drop surfaces. The rigidity of particle-coated interfaces is due to the particle aggregation. Previous studies33,34 have identied the types of interactions that contribute to the total attractive potential between interfacial particles. Maurice et al.34 examined concentrated water-in-oil emulsions stabilised by hard sphere colloids. They showed that capillary interactions between particles make a signicant contribution to interfacial rigidity. Arditty et al.33 found that capillary forces and steric interactions contribute to the surface elasticity of oil-in-water emulsions stabilised by silanised silica particles. We have shown that the mechanical properties of silica nanoparticle-stabilised oil-inwater emulsions are sensitive to the salt concentration in the water. This means that repulsive electrostatic interactions can inuence particle aggregation in the interfacial layer and hence the emulsion stability to drop coalescence.
5.
Conclusions
We have investigated the yielding behaviour of concentrated aqueous emulsions of oil drops stabilized by silanised silica particles. The stability of the thin lms between closely packed drops depends on the interactions in the interfacial layer of particles coating the drops. Reducing the salt concentration in the emulsions increases the repulsive interactions between the particles. These conditions favour destabilisation, as deformation of the drops exposes interfacial area that is free of particles. Large shear strains cause the emulsions to yield and coalesce,
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rather than ow. The particles attached to the surfaces of merging drops make them sufficiently solid-like in response to compression that complete fusion can be arrested at an intermediate stage.
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Acknowledgements We acknowledge the confocal microscopy and cryo-SEM facilities, and the technical assistance of the Australian Microscopy & Microanalysis Research Facility at Adelaide Microscopy, The University of Adelaide. CPW acknowledges receipt of an Australian Research Council Future Fellowship. This research was also supported under Australian Research Council's Discovery Projects funding scheme (DP110104179).
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Coalescence in concentrated Pickering emulsions under shear† Published on 27 May 2014. Downloaded by Brown University on 27/10/2014 01:29:58.
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Catherine P. Whitby* and Melinda Krebsz We have investigated the rheology of concentrated oil-in-water emulsions stabilised by silanised silica nanoparticles. The emulsions behave like highly elastic solids in response to small, uniform strains. They become unstable and begin to break down, however, on yielding. We show that the emulsion elasticity is correlated with the salt concentration in the water and hence the particle aggregation in emulsions at a Received 4th March 2014 Accepted 2nd May 2014
given drop volume fraction. A supporting observation is that destabilisation is favoured by minimising the attractive interactions between the particles. Microscopic observations revealed that coalesced drops
DOI: 10.1039/c4sm00491d
have anisotropic shapes and wrinkled surfaces, direct evidence of the interfacial particle layer acting like
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a mechanical barrier to bulk emulsion destabilisation.
1. Introduction Concentrated emulsions resist owing and only give way under pressure.1 Mayonnaise and moisturiser cream, for example, can behave like elastic solids and hold their shape. They consist of drops packed closely together so they touch all their neighbours.2 The drop volume fraction is so high that they can support their own weight and resist mechanical shear. Attractive interactions between the drops can cause them to stick together as they come into close contact.3 The drops remain separated by thin lms of continuous phase. Emulsions stabilised by solid particles (Pickering emulsions4–9) are remarkably robust structures. Particle-coated drops can be squeezed together and the liquid evaporated to form a porous solid.10–13 The drops may coalesce, however, under shear stress. The mechanisms that cause the thin lms to rupture are not well understood. In this paper we investigate the shear response of aqueous Pickering emulsions as the drop volume fraction increases. Droplet adhesion was varied by controlling the salt concentration and hence the particle aggregation. The conditions leading to destabilisation under shear stress are of particular interest. The shear elasticity of concentrated emulsions is due to the drops being jammed together.14 Emulsion drops are geometrically conned at drop volume fractions (f) larger than about 64 vol% (fm). Princen15 argued that the drops are compressed by an osmotic pressure greater than their Laplace pressure (which is given by the ratio of the oil–water interface tension to the average drop diameter, 2gow/d). The drops deform into
Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia. E-mail: [email protected] † Electronic supplementary 10.1039/c4sm00491d
information
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(ESI)
available.
See
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polyhedral shapes with attened areas of contact. Small perturbations stretch the thin liquid lms between the drops. The additional surface area created determines the elastic response of the emulsion, which is characterised by the elastic storage modulus, G0 . Mason et al.16 demonstrated that the elastic shear moduli of surfactant-stabilised silicone oil-inwater emulsions (at a given f) scale with the drop Laplace pressure. They showed that G0 depends on the increase in the drop volume fraction above the maximum packing fraction16 G0 z ðf fm Þ ðgow =dÞ
(1)
Flow only occurs in concentrated emulsions above a critical value of applied stress, the yield stress (sy). At the solid–liquid transition, the yield stress is given by sy ¼ Ggy, where G is the emulsion elasticity and gy is the yield strain. Princen17 proposed that emulsions yield by irreversible changes in drop neighbours (rearrangements). Arditty et al.18 found that the elastic moduli of concentrated emulsions of micrometre-sized oil drops stabilised by silanised silica nanoparticles (of radius, rp ¼ 25 nm) were signicantly higher than the values measured for surfactant-stabilised emulsions at a given f. They proposed18 that the particles coating the drops aggregate and behave collectively like an elastic network (or membrane). Assuming that the network was a two dimensional solid layer, Arditty et al.18 estimated that its tension (the elastic energy required to increase the layer area) was an order of magnitude larger than the oil–water interfacial tension. This makes it the dominant contribution to the interfacial energy of the particle-coated drops and their response to deformation. Hermes and Clegg19 investigated the yielding behaviour of oil drops (d ¼ 15 mm) in water stabilised by silica nanoparticles
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(rp ¼ 330 nm). Adding high concentrations of salt (44.8 wt%) caused the drops to occulate and they saw evidence19 for this being due to aggregation between particles attached to neighbouring drops. The occulated emulsion (f ¼ 0.5 < fm) owed once sufficient strain was applied to break the drop clusters apart. Compressing the emulsion to f 0.95 ([fm) increased its elasticity by two orders of magnitude. The drops in the compressed emulsion coalesced instead of owing when the emulsion yielded. Hermes and Clegg19 argued that this was due to the particle stabilisation of the interface failing at high strains. We investigated the role of particle aggregation in the stability of Pickering emulsions deformed by shear stress. The emulsions were stabilised by silanised fumed silica particles which have been shown to aggregate and form weak particle gels at high concentrations (0.5 M NaCl) of salt.20 Reducing the salt concentration in the emulsions increased the repulsive interactions between the interfacial particles. We found that this minimises the contribution of the particle layer tension to the interfacial energy of the drops. Applying a stress of the order of the Laplace pressure destabilises the emulsions. We show that droplet coalescence is arrested at intermediate stages. The particle layers coating the merged drops buckle (wrinkle) due to the reduction in the total interfacial area.
2.
The particle-coated oil drops in a concentrated Pickering emulsion (f ¼ 0.75) are closely packed together as shown in (a) scanning electron microscope and (b) confocal fluorescence images. (c) Schematic of a concentrated emulsion contained between two parallel plates (separated by a distance of z) as a shearing stress (s) is applied. When the upper plate is moved relative to the lower one over a small distance (Dx/z # 10%), the emulsion is strained uniformly throughout its mass. When the stress is released, the drops relax back into (roughly) spherical shapes. Applying larger stresses causes adjacent drops to coalesce when uncoated drop surfaces are exposed by the deformation. (d) Non-spherical structures form as drops are trapped in an intermediate stage of coalescence in sheared emulsions. Fig. 1
Experimental section
2.1. Emulsion preparation and characterisation Silica nanoparticles modied by reaction with hexadecylsilane were supplied by Evonik (Aerosil R816). A sample of the powder was compressed into a disc-shaped pellet to form a planar substrate for wetting measurements. The contact angle of water drops under oil on the powder surface (qow, measured through aqueous phase) is about 60 . Dispersions of nanoparticles in solutions of NaCl (Chem Supply, 99%) in water (resistivity 18.2 MU cm, pH 5.3–5.8) were sonicated in an ultrasound bath (Soniclean 160T, 70 W power, 44 kHz operating frequency) for 20 minutes. TEM images revealed that the approximately spherical nanoparticles had radii between 10 and 30 nm. The particles had a mean radius (rp) of 12 nm and a polydispersity of 30%. Emulsions were prepared by homogenising equal volumes of isopropyl myristate (Sigma Aldrich, 98%, passed through chromatographic alumina twice to remove polar impurities) with the aqueous silica dispersions (1.5 wt%) using a X1000D homogenizer with a 19 mm diameter sha (Ingenieurbuero CAT, M. Zipper GmbH) operated at 10 400 rpm for 2 min. The oil volume fraction in the emulsion (fo) was then increased in small steps (10 vol%), rehomogenising for 30 s aer each addition of oil. Scanning electron micrograph images of emulsion surfaces produced by fracturing samples of frozen emulsions were obtained using a Philips XL30 Field Emission Scanning Electron Microscope tted with a CT1500 HR Low Temperature Cryo system. A typical image of an emulsion surface is shown in Fig. 1a. SEM images showed that the drops were coated with layers consisting of particle aggregates about 50 nm in size (ESI
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Fig. S1†). The emulsion drop size distributions were determined by static light scattering using a Malvern Mastersizer 2000 (assuming a relative refractive index of 1.1 and a particle absorption coefficient of 0). The emulsions were stored in screw-cap vials at 25 C.
2.2. Confocal microscopy Confocal uorescence microscopy (CFM, Leica SP5 spectra scanning confocal microscope) was used to visualise droplet adhesion in the emulsions. An o-ring xed on a glass cover slip was lled with emulsion. A second cover slip placed on top of the o-ring sealed the cell. The particles were stained with Nile Blue (1 mM in 0.1 M NaCl) and excited at 633 nm. Nile Red (1 mM in isopropyl myristate) was used for staining the oil and the sample was excited at 496 nm. Preliminary studies conrmed that the presence of the dyes did not alter emulsion structure or stability. Fluorescence intensity data for Nile Blue and Nile Red were collected in two separate channels corresponding to 520– 570 and 653–750 nm, respectively. Each line of pixels in an image was scanned sequentially for Nile Red and Nile Blue uorescence to avoid interference due to cross-uorescence. Fig. 1b shows a typical image slice obtained at a depth of 260 mm in an emulsion sample. The two-dimensional confocal images were analysed using the ImageJ soware. Each image contained 1024 1024 pixels, with a grey value ranging from 0 to 255. The images were processed by converting them to a binary form and then segmented by ‘thresholding’ using an iterative selection procedure based on the composite average of the averages of the background and
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object pixels. The perimeters of pairs of drops in close contact in the Nile Blue images were used to measure the contact angles formed at the junction between adhesive interfaces (attened areas of contact) and isolated ones.
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2.3. Rheology Rheological measurements were made using a Rheometric Scientic Dynamic Stress Rheometer SR2000 at a xed temperature of (25 0.1) C and parallel plate (25 mm diameter) or a cone and plate geometry (25 mm diameter, 2 angle). The ow behaviour obtained using at plates with either smooth hydrophobic or sand blasted steel at large separations (2 mm) was similar to that measured using a cone and plate geometry, indicating an absence of wall slip. A solvent trap was used to prevent evaporation. For oscillatory measurements, the amplitude was increased in a series of logarithmic steps (30 s in duration) at a constant frequency (typically 0.1 Hz). The moduli in the linear viscoelastic regions were determined from scans of the oscillation frequency at a constant stress. For shearing experiments, a xed shear stress or rate was applied for a xed time (typically 300 s in duration) as illustrated schematically in Fig. 1c. Aer shearing, the rotating plate was slowly removed and samples of the sheared emulsion were collected. Optical microscopy (Olympus BX51 microscope) was used to visualise the microstructure of sheared emulsions.
3.
Results
Mechanically mixing aqueous dispersions of the fumed silica nanoparticles with oil drives the particles to assemble into layers at the freshly created oil–water interface. Over the range of oil volume fractions used (f # 0.8), monomodal distributions of spherical oil drops in water are formed. The mean drop diameter (d) and hence the total interfacial area is determined by the mass of particles present. At the low particle concentration used (Cp ¼ 1.5 wt% in the aqueous phase), coalescence dominates emulsion formation. During emulsication, drops merge together until they are completely coated and protected by a dense layer of particles. Assuming that all the particles attach to the oil–water interface, a simple mass-balance model predicts that the nal drop surface area will be proportional to the drop volume fraction (f). In the special case of a xed particle concentration in the aqueous phase, the mass of particles present is proportional to the water volume fraction (1 f). Thus the mean drop diameter is given by21 d ¼ (8f/1 f)(rprpfCP/Cp)
(2)
where rp is the particle density. fCP is the fraction of the surface area covered by attached particles. Consistent with this equation, the average emulsion drop size is a linear function of the ratio f/1 f – ESI Fig. S2.† By geometrical arguments, it can be shown that the fraction of a planar surface coated by hexagopffiffiffiffiffi nally closely packed spheres is given by p= 12 z 0:907. Assuming that fCP 0.9, it was estimated that the drops are coated with sufficient particles to form a bilayer of fumed silica particles using the value of the tted slope in Fig. S2.† This
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accounts for the drop stability until shear is applied (shown later). It should be noted that the nanoparticles occulate in water and thus attach to the oil drop surfaces in the form of aggregates. Careful inspection of the emulsions showed no evidence for there being excess particles in the water, supporting the assumption that all the particles attach to the drops. The attached particles are not easily displaced under quiescent conditions. Particle attachment changes the free energy of the system by reducing the total oil–water interfacial area. The energy required to detach the particles22,23 (E) is given by E ¼ pgowrp2(1 + cos qow)2, where gow is the interfacial tension. Assuming that particle attachment does not alter the interfacial tension, it can be calculated that the capillary energy trapping the particles is about two orders of magnitude larger than their thermal energy. The particle-coated drops pack together into structures that resist ow. Fig. 2 summarises the response of emulsions at different CNaCl to oscillatory strain. The drop volume fraction and hence the size of the drops was xed in these emulsions. At the lowest salt concentration studied (0.001 M, black squares in Fig. 2), the emulsions show a linear viscoelastic response at strain amplitudes (the relative deformation) of g < 10%. The component of the stress that is in phase with the applied strain (represented by the modulus G0 ) is at least an order of magnitude larger than the component out of phase by 90 (G00 ). This means that signicantly more energy is stored elastically by the emulsions (per unit volume) than is lost by viscous dissipation during each cyclic period of strain. G0 decreases as the strain amplitude increases above 10%. G00 increases with increasing strain up to a maximum at which G00 ¼ G0 . Both moduli decrease as g increases at strains larger than the critical (crossover) strain (gc 100%) and the emulsions yield. The emulsions become stiffer as the salt concentration increases (0.01 M and 0.1 M, green circles and blue triangles, respectively, in Fig. 2). They show a linear viscoelastic response only at g < 1% at higher salt concentrations. The strain at which
Fig. 2 Strain (g) dependence of the emulsion elastic storage moduli (G0 , filled symbols) and viscous loss moduli (G00 , open symbols) of 60 vol% emulsions at NaCl concentrations of 0.001 ( ), 0.01 ( ) and 0.1 ( ) M.
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the emulsions yield and move irreversibly decreases as CNaCl increases. The crossover strain is about 20% for emulsions at 0.01 M NaCl and about 3% for emulsions at 0.1 M salt. Over the range of drop volume fractions studied (0.4 < f < 0.8) the emulsions behave like solids at small strains (g # 1%). In the linear viscoelastic regime, the moduli increase with the drop volume fraction, as shown in Fig. 3 (only G0 is plotted, for clarity). This is unlike the volume fraction dependence of the elasticity of surfactant-stabilised emulsions,16 which shows a sharp increase at volume fractions corresponding to random close packing of the drops, f frcp. Fig. 3 shows that the elasticity of the particle-stabilised emulsions varies with the salt concentration in the aqueous phase, CNaCl. The moduli are normalised by the Laplace pressure of the drops (given by the ratio 2gow/d). At the highest salt concentration studied, CNaCl ¼ 0.1 M, the elastic storage moduli of the concentrated emulsions (f $ frcp ¼ 0.64) is up to two orders of magnitude larger than G0 of surfactant-stabilised emulsions. The layers of particles attached to the drop surfaces enhanced the emulsion elasticity. Emulsion elasticity at a given f decreases as the salt concentration and hence the ionic composition of the aqueous phase decreases. At the lowest salt concentration studied, CNaCl ¼ 0.001 M, the (normalised) elastic moduli of the concentrated emulsions (f $ frcp ¼ 0.64) are similar to those measured for surfactant-stabilised emulsions (Fig. 3a). The yielding behaviour of the emulsions is also sensitive to CNaCl. This was characterised by microscopic examination of the emulsions aer shearing at stress values above the yield stress. Emulsions formed at high salt concentrations (0.01 M and 0.1 M) resist deformation. The drop size distribution in the emulsions does not change, indicating that the emulsions remain stable to coalescence. Emulsions formed at at CNaCl ¼ 0.001 M do not withstand the disruption associated with yielding. Applying larger shear stresses (s > 100 Pa) induces drop coalescence in closely-packed emulsions.
(a) Drop volume fraction (f) dependence of the Pickering emulsion elastic storage moduli, G0 at NaCl concentrations of 0.001 ( ), 0.01 ( ) and 0.1 ( ) M. The elasticity is scaled by the Laplace pressure, the ratio of the oil–water interfacial tension (g) to the average drop diameter (d). The blue solid line shows the variation in elasticity of surfactant-stabilised emulsions obtained by Mason et al.16
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Coalesced drops tend to have shapes that resemble an intermediate stage of the coalescence process, as shown in Fig. 1d. This indicates that coalescence is arrested before the merging drops fused into a single spherical drop. The morphology of coalesced drops changes with the proportion of oil in the emulsions. As f increases, anisotropic shapes formed by clusters of more than two droplets are observed (Fig. 4a). Coalesced drops that relax into (relatively) spherical shapes have wrinkled (buckled) surfaces, as shown in Fig. 4b. The wrinkles are prominent on larger coalesced drops and tend to orientate in a xed direction. Their periodicity (l) was determined to be about 10 mm by estimating the wavelength of the undulations (Fig. 4c). Staining sheared emulsions with uorescent dyes that bind preferentially to the particles revealed that the coalesced drop surfaces are completely coated with particles – Fig. S3.† Wrinkling occurs because the area occupied by the attached particles remains constant although the droplet surface area has decreased. Lateral compression causes the coating to expand (wrinkle) in the perpendicular directions. The particles are acting collectively, like a thin, two-dimensional solid attached to a thicker foundation (the oil drop). The response of the solid can be characterised by the ratio of the perpendicular and parallel strains, the Poisson ratio (n). Assuming the particles attached to the oil–water interface are hexagonally close
Fig. 3
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Fig. 4 Optical images of (a) partially merged drop cluster and (b) the crumpled surface of a merged drop in a sheared concentrated emulsion (f ¼ 0.80). (c) Line profile of the intensity extracted along the solid path shown in (b).
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packed, n z 1/3.24 Following the general theory for wrinkling of elastic sheets, the periodicity (l) of the wrinkles will depend on the balance between the bending stiffness of the solid (B) and the elastic stiffness (K) of the oil drop, l z (B/K)1/4.25 The elasticity of a drop surface is given by its capillary ratio, rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi . ffi Lc ¼ gow rg , where r is the oil density. Thus for interfacial particle layers l can be estimated24 by Published on 27 May 2014. Downloaded by Brown University on 27/10/2014 01:29:58.
l¼
4
3ð1 fCP Þð1 þ nÞ
1
4 pffiffiffiffiffiffiffiffiffi
Lc rp
(3)
This gives a result of 33 mm, which is reasonably close to the measured periodicity (Fig. 4c).
4. Discussion We manipulated the stability of concentrated aqueous emulsions of silica nanoparticle-stabilised oil drops to shear-induced coalescence by varying the salt concentration in the water. Emulsions formed at high salt concentrations are very stiff and resist deformation. They have an unusually large elasticity compared to surfactant-stabilised emulsions (Fig. 3). The particles attached to the drops make their surfaces solid-like under shear stress. Treating the particles attached to oil–water interface like a solid layer encapsulating the drops means that the interfacial pressure, 3, of the layer can be estimated. This was done assuming that the rheology data follow the relationship G0 d/3 f f(f frcp). At a given salt concentration, 3 does not vary signicantly with f and an average value of the interfacial pressure was calculated. Fig. 5a shows the elasticity moduli normalised by the ratio 3/d. It should be noted that the elasticity measured at drop volume fractions below maximum packing is due to occulation of the drops. The scaled data do not fall onto a single curve (at f < frcp) since the occulation is dependent on the salt concentration. At CNaCl ¼ 0.1 M, the layer needs to be characterised by 3 3 N m1 for the elastic moduli to follow the expected scaling behaviour. This large value of 3 must be due to the particles aggregating together. van der Waals forces, capillary forces, and attractive interactions due to interpenetration of the alkyl chains graed on neighbouring particles will all contribute to strong lateral attractive interactions between the particles26 that cause aggregation in the interfacial layer. Repulsive electrostatic interactions between the particles will oppose the attractive interactions and hinder particle aggregation. Emulsion elasticity at a given f decreases as the salt concentration and hence the ionic composition of the aqueous phase decreases. This suggests that particle aggregation and hence the characteristic pressure of the interfacial layer depends on the distance over which electrostatic interactions are screened in the aqueous phase. The thickness of the ionic layer that screens electrostatic interactions, the Debye length, pffiffiffiffiffiffiffiffiffiffiffi can be approximated27 by k1 ¼ 0:304 CNaCl . Fig. 5b shows that the elasticity of the concentrated emulsions decreases linearly with the square root of the salt concentration.
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(a) Drop volume dependence of Pickering emulsion elastic storage moduli scaled by the ratio of the interfacial pressure (3) to the average drop diameter (d). Data is shown for emulsions at NaCl concentrations of 0.001 ( ), 0.01 ( ) and 0.1 ( ) M. The blue solid line shows data for surfactant-stabilised emulsions obtained by Mason et al.16 (b) Salt concentration (CNaCl) dependence of the interfacial pressure, 3, of the emulsions. The purple solid line is a linear fit to the data. Fig. 5
Emulsions formed at high salt concentrations (0.1 M) are also very stiff due to occulation of the drops. Particle aggregation causes drops to adhere together. Drop adhesion in the emulsions was visualised by confocal microscopy (Fig. 6a). The thin lms that form between adhering drops have a nite contact angle (2q). Large contact angles were observed at high salt concentrations, indicating strong attractive interactions in the particle layers between the drops. Fig. 6b shows the decrease in the droplet contact angle as the salt concentration in the emulsions is reduced. In contrast, there is little adhesion between drops in dilute salt solutions (0.001 M). The adhesive energy of the lms can be calculated by 2g(1 cos q) using the Young–Dupr´ e equation.28 Assuming that the attached particles do not alter the oil–water interfacial tension it was estimated that the adhesion energy decreases by an order of magnitude as the salt concentration decreases from 0.1 to 0.001 M.
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Fig. 6 (a) Confocal fluorescence image highlighting the locations of particles coating a flocculated pair of drops in an emulsion at CNaCl ¼ 0.001 M. The drop surfaces intersect at an angle, 2q. (b) Salt concentration (CNaCl) dependence of the contact angle, q, of the thin films that form between flocculated drops. The purple solid line is a linear fit to the data.
Thus there are two major sources of the emulsion elasticity. The viscoelasticity of the interfacial particle layers and the stickiness of the particle-stabilized thin lms between the drops contribute to Pickering emulsion elasticity. These two factors are interrelated. They both arise from interactions between the particles attached to the drops. It is possible that the large value of surface pressure estimated at the highest salt concentration (3 3 N m1 at CNaCl ¼ 0.1 M) is partly due to strong adhesive interactions between the drops which enhance jamming in the emulsions. The sensitivity of the elasticity to the salt concentration is due to the small size of the particles (2rp 20 nm) coating the drops. The particles are deeply immersed in the water side of the oil–water interface, given their relatively small oil–water contact angle (qow ¼ 60 ). They are comparable in size to the thickness of the layer of ions that screens Coulombic interactions (k1 1–10 nm) in water at the salt concentrations studied (0.001 M # CNaCl # 0.1 M). Thus the charges on the water side contribute signicantly to the electrostatic interactions between the particles and reduce particle aggregation at low salt concentrations. This means that at low ionic strengths the response of the drop surfaces to deformation is not affected by the attached
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particles. Instead it is determined by the drop Laplace pressure, which is given by 2gow/d 600 Pa. Large perturbations (of the same order of magnitude as the Laplace pressure) presumably deform the drops and increase their total surface area, so that the fraction of the surface covered by particles is reduced. Exposing oil–water interface that is not coated by particles causes neighbouring drops to coalesce together, rather than ow past each other under shear stress. Denkov et al.29 proposed that the thin lms in concentrated Pickering emulsions rupture if there are defects, like fractures or vacancies, in the particle layer coating the drops. We have shown that increasing the repulsive interactions between particles in the interfacial layer favours rupture and destabilisation when emulsions are deformed by shear stress. Our results show that the emulsion drops become nonspherical and their surfaces wrinkle (or buckle) as they coalesce together. The structures observed in the sheared emulsions are similar to those formed when colloidal monolayers at planar or curved interfaces are compressed. Aveyard and co-workers30,31 demonstrated that when a layer of particles at a planar uid interface is compressed, it bends and forms an undulating surface with a characteristic wavelength. Pawar et al.32 found that the coalescence of two oil drops could be halted at some intermediate stage if micrometre-sized silica particles at the drop surfaces form a close-packed, jammed layer. The results presented here demonstrate that interfacial particles in bulk Pickering emulsions behave like a 2-dimensional solid and provide a mechanical barrier to droplet coarsening. They are direct experimental evidence of the solid-like nature of the drop surfaces. The rigidity of particle-coated interfaces is due to the particle aggregation. Previous studies33,34 have identied the types of interactions that contribute to the total attractive potential between interfacial particles. Maurice et al.34 examined concentrated water-in-oil emulsions stabilised by hard sphere colloids. They showed that capillary interactions between particles make a signicant contribution to interfacial rigidity. Arditty et al.33 found that capillary forces and steric interactions contribute to the surface elasticity of oil-in-water emulsions stabilised by silanised silica particles. We have shown that the mechanical properties of silica nanoparticle-stabilised oil-inwater emulsions are sensitive to the salt concentration in the water. This means that repulsive electrostatic interactions can inuence particle aggregation in the interfacial layer and hence the emulsion stability to drop coalescence.
5.
Conclusions
We have investigated the yielding behaviour of concentrated aqueous emulsions of oil drops stabilized by silanised silica particles. The stability of the thin lms between closely packed drops depends on the interactions in the interfacial layer of particles coating the drops. Reducing the salt concentration in the emulsions increases the repulsive interactions between the particles. These conditions favour destabilisation, as deformation of the drops exposes interfacial area that is free of particles. Large shear strains cause the emulsions to yield and coalesce,
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rather than ow. The particles attached to the surfaces of merging drops make them sufficiently solid-like in response to compression that complete fusion can be arrested at an intermediate stage.
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Acknowledgements We acknowledge the confocal microscopy and cryo-SEM facilities, and the technical assistance of the Australian Microscopy & Microanalysis Research Facility at Adelaide Microscopy, The University of Adelaide. CPW acknowledges receipt of an Australian Research Council Future Fellowship. This research was also supported under Australian Research Council's Discovery Projects funding scheme (DP110104179).
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