Wolf 2007 Shear Thickening Of

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See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/238994851 Shear thickening of an emulsion stabilized with hydrophilic silica particles ARTICLE in JOURNAL OF RHEOLOGY · JANUARY 2007 Impact Factor: 3.28 · DOI: 10.1122/1.2714642 CITATIONS 14 DOWNLOADS 379 VIEWS 88 4 AUTHORS, INCLUDING: Bettina Wolf University of Nottingham 54 PUBLICATIONS 483 CITATIONS SEE PROFILE Mark Kirkland Deakin University 14 PUBLICATIONS 562 CITATIONS SEE PROFILE Available from: Bettina Wolf Retrieved on: 22 July 2015

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  • Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/238994851

    ShearthickeningofanemulsionstabilizedwithhydrophilicsilicaparticlesARTICLEinJOURNALOFRHEOLOGYJANUARY2007ImpactFactor:3.28DOI:10.1122/1.2714642

    CITATIONS14

    DOWNLOADS379

    VIEWS88

    4AUTHORS,INCLUDING:

    BettinaWolfUniversityofNottingham54PUBLICATIONS483CITATIONS

    SEEPROFILE

    MarkKirklandDeakinUniversity14PUBLICATIONS562CITATIONS

    SEEPROFILE

    Availablefrom:BettinaWolfRetrievedon:22July2015

  • Shear thickening of an emulsion stabilized with hydrophilic silica particlesBettina Wolf, Sylvie Lam, Mark Kirkland, and William J. Frith

    Citation: J. Rheol. 51, 465 (2007); doi: 10.1122/1.2714642 View online: http://dx.doi.org/10.1122/1.2714642 View Table of Contents: http://www.journalofrheology.org/resource/1/JORHD2/v51/i3 Published by the The Society of Rheology

    Additional information on J. Rheol.Journal Homepage: http://www.journalofrheology.org/ Journal Information: http://www.journalofrheology.org/about Top downloads: http://www.journalofrheology.org/most_downloaded Information for Authors: http://www.journalofrheology.org/author_information

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  • Shear thickening of an emulsion stabilized withhydrophilic silica particles

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    DownBettina Wolfa) and Sylvie Lam

    Unilever Corporate Research, Colworth Science Park,Bedford MK44 1LQ, United Kingdom

    Mark Kirkland

    Measurement Sciences, Colworth Science Park,Bedford, MK44 ILQ, United Kingdom

    William J. Frith

    Unilever Corporate Research, Colworth Science Park,Bedford MK44 1LQ, United Kingdom

    (Received 21 September 2006; final revision received 6 February 2007

    Synopsis

    he flow behavior of particle stabilized oil-in-water emulsions with different dispersed volumeractions was analyzed in steady shear on a rotational rheometer employing a coaxial cylindereometry. The dispersed phase of the emulsion was a mixture of equal volumes of a polar oil,sopropyl myristate, and a nonpolar oil, dodecane. The continuous phase was an aqueoususpension of hydrophilic colloidal silica particles of 8 nm diameter with the pH adjusted to pH2n order to stabilize the emulsion Binks and Whitby, Colloids Surf., A 253, 105115 1995.roplet diameters were of the order of a few micrometers, and droplet surfaces apparently showense particle coverage. We show that the markedly different interfacial structure in particletabilized emulsions when compared to surfactant stabilized emulsions is reflected in theheological behavior. To illustrate these differences, the rheological behavior of a comparableurfactant stabilized emulsion with the particles in the aqueous phase replaced by Tween 20, waslso investigated. The rheological characterization revealed a domain of shear thickening in thearticle stabilized emulsions at high droplet phase volumes that is not observed for the classicalurfactant stabilized emulsions. 2007 The Society of Rheology. DOI: 10.1122/1.2714642

    . INTRODUCTIONEmulsions are dispersions of deformable droplets normally stabilized by surface-

    ctive species such as surfactant molecules, amphiphilic polymers or proteins. Theirheological behavior is typically shear thinning where, depending on the phase volume ofhe internal droplet phase and the material properties of the continuous phase, a low shear

    Author to whom correspondence should be addressed. Present address: Division of Food Sciences, Universityof Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD U.K.; electronic mail:[email protected] by The Society of Rheology, Inc.465. Rheol. 513, 465-478 May/June 2007 0148-6055/2007/513/465/14/$27.00

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    466 WOLF et al.

    Downiscosity plateau or yield behavior and, occasionally, a high shear viscosity plateau isbserved Barnes 1994; Tadros 1994. Reports of classical emulsions showing shearhickening, or dilatancy, are scarce. In fact, there appears to be only the work by Otsubond Prudhomme 1994 who studied the rheology of oil-in-water emulsions. For theirmulsions, Otsubo and Prudhome varied the interfacial tension between 0.9 and.9 mN m1 by using different oils and surfactants. The oils were either a mixture ofritolyl phosphate and dioctyl phthalate which was a close density match for the aqueoushase, or paraffin oil. The surfactants were either Alipal CD-128 or Triton X-100, each0% in the aqueous phase. Shear thickening was only observed for the one emulsionystem that was comprised of paraffin oil, which incidentally had the highest interfacialension of the emulsions studied, and the largest droplet size. They observed that at oilolume fractions of 0.20.53 the viscosity initially decreased, passing through a mini-um which was followed by an increasing viscosity as the shear rate increased. At the

    igher volume fractions, for which this behavior was observed, a viscosity plateau waseached upon further increase of the shear rate. The authors tentatively ascribed thehear-thickening behavior to interfacial elasticity causing an inhibition of fluid circulationithin the drops, and hence increasing the energy dissipation within the emulsion as ahole. The observations by Otsubo and Prudhomme 1994, however, are questioned byal 1996 who, for emulsions, reported the effect of using certain measurement geom-tries in a rotational rheometer on the apparent rheological behavior. Pal 1996 demon-trated that creaming and sedimentation of the droplet phase of an emulsion measured incone-and-plate geometry, or a plate-and-plate geometry can lead to the detection of

    pparent shear-thickening and time-dependent effects which are not present when theame sample is measured in a concentric cylinder geometry. The work by Otsubo andrudhomme 1994 is quoted by Pal 1996 as an example from the published literature

    n which reports of shear thickening are based on rheological characterization in a cone-nd-plate geometry. The recommendation was to employ coaxial cylinder geometries tonsure a negligible effect of sedimentation or creaming on the rheological data. It isorth noting that the only system in which Otsubo and Prudhomme observed the shear-

    hickening behavior was also the only system in which the oil phase was significantly lessense than the aqueous phase, making creaming more likely.

    As introduced in this paper, it appears, however, that emulsions can be designed suchhat they exhibit true shear thickening when stabilizing the emulsion droplets with solidarticles. With reference to the above cited paper by Pal 1996, all rheological measure-ents reported here were conducted using coaxial cylinder geometries. Evidence of such

    ehavior, i.e., shear thickening or true suspension-like rheological behavior, has not beeneported previously though there are a small number of recent publications discussingheological data for well characterized particle stabilized emulsions Binks et al. 2005;rditty et al. 2005. Binks et al. 2005 studied the rheological behavior of water-in-oil

    mulsions prepared from dispersions of hydrophobic clay particles in oils. Depending onhe type of oil the continuous phase fluid used, the emulsions microstructure resembledclassical emulsion, that may or may not flocculate, or showed a network in the oil phase

    continuous phase formed by the clay particles to which the droplet surfaces are largelyonnected. In the former case, a rheological response, including steady shear and oscil-atory shear data, that was classical for emulsions is reported. In the latter case, theheological behavior is more complicated and appears to be determined by the networktructure in the continuous phase in which the droplets are embedded. Rheological datan particle stabilized emulsions have also been reported by Arditty et al. 2005. Theyerformed oscillatory measurements in the linear viscoelastic regime, and while the data

    annot be compared to those reported here, obtained under steady flow conditions, we

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    467PICKERING EMULSIONS

    Downuote them since they are one of the scarce sets of rheological data published for particletabilized emulsion. The emulsions studied by Arditty et al. 2005 were strongly floc-ulated and the surface coverage of the droplets with particles is reported to be below thealue expected for a densely packed layer of adsorbed particles. This is important withegards to the interpretation of the rheological data. The authors state that the results areeminiscent of those classically obtained for surfactant-stabilized emulsions. The pres-nce of classical emulsion behavior is underlined by the fact that the authors succeeded ineforming and ultimately rupturing the droplets in an emulsion initially prepared with anverage droplet diameter of 8 m by applying sufficiently high shear stresses in theone-and-plate gap of a rheometer. This indicates that the droplets were deformable, and,ence, the overall system should behave like an emulsion. Taking into consideration theact that the authors report the surface coverage of the droplets with particles is below thealue expected for a densely packed layer, the occurrence of droplet deformation is notoo surprising. In contrast to these findings, we report on a particle stabilized emulsionhowing suspension-like rheological behavior in steady shear which is hypothesized to beresult of dense surface coverage of the droplets with adsorbed particles. To emphasize

    he unusualness of the flow behavior found here for an emulsion system, albeit a particletabilized emulsion, data for an equivalent surfactant stabilized emulsion prepared fromhe same fluid phases are also presented.

    I. EXPERIMENT

    . MaterialsParticle stabilized oil-in-water emulsions were prepared from a mixture of equal vol-

    mes of isopropyl myristate IPM, 98%, Sigma Aldrich, UK and dodecane 98%, Sigmaldrich, UK, de-ionized water, and alkaline dispersions of silica particles Ludox SM30

    x Grace Davison, UK. All materials were used as supplied.The pH of the aqueous phase of the emulsions containing the silica particles at

    %w/w was adjusted to pH2 using hydrochloric acid. As reported previously by Binksnd Whitby 2005, emulsification at neutral or alkaline pH did not yield a stable emul-ion. Also, the choice of mixing a nonpolar oil, dodecane, with a polar oil, IPM, is basedn the aforementioned publication. However, heptane was replaced with dodecane toircumvent changes in the oil composition due to the volatility of heptane. The viscosityf the oil mixture as used was 4 mPa s at 20 C.

    The silica particles were used as received. Dispersions of Ludox SM30 are of pH10,nd sodium is used as the stabilizing counter ion. As given on the material data sheet, thepecific surface area of the particles was 350 m2 g1. Particle size distributions of theudox particles were measured at a particle concentration of 1%w/w in aqueous suspen-ion at pH10, pH7 and pH2 using dynamic light scattering Zetasizer Nano ex Malvernnstruments; see Fig. 1 for the results. The corresponding particle diameters at the peakf the distributions were 7.8, 11.1, and 14.2 nm at pH10, pH7, and pH2, respectively.articles were also imaged using transmission electron microscopy TEM; see Fig. 2,hereby the sample for the imaging was taken straight from the bulk dispersion at pH10.he increase in measured particle diameter, although not large, with decreasing pH may

    ndicate particle aggregation, or weak flocculation, which was previously concluded to berequirement for the preparation of stable particle stabilized emulsions Binks 2002.rom the TEM micrographs see Fig. 2, it appears that the primary silica particles areainly spherical in shape, and relatively narrow in size distribution with a minor fractionf the particles aggregated or fused.

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    468 WOLF et al.

    DownSurfactant stabilized oil-in-water emulsions were prepared from the same mixture ofqual volumes of isopropyl myristate and dodecane as used for the particle stabilizedmulsion, and de-ionized water containing the nonionic surfactant Tween 20 Sigmaldrich, UK, used as supplied at 1%w/w. The choice of Tween 20 as surfactant was

    olely based on the fact that it is known to be a good surfactant for oil-in-water emul-ions.

    . Methods. Emulsion preparation and characterization

    The particle stabilized emulsion was prepared at 20 %v /v oil by direct emulsificationf the oil phase and the aqueous phase in batches of 200 ml using a high speed rotaryixer Silverson L4R, 8000 rpm for 5 min.

    IG. 1. Particle size distribution for silica particles Ludox SM30 as determined by dynamic light scattering.IG. 2. TEM micrographs of silica particles Ludox SM30, samples taken from dispersion at pH10, scale barquals 100 nm on the left, and 50 nm on the right.

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    469PICKERING EMULSIONS

    DownThe surfactant stabilized emulsion was prepared following the same protocol omittinghe acidification step. The same oil phase volume, V=0.2.

    The emulsions were visually inspected using a standard optical light microscopePolyvar. Droplet size distributions were determined by static light scattering using a

    alvern Instruments Mastersizer 2000. A few drops of the emulsion were added toe-ionized water in the small sample dispersion unit, to achieve an appropriate scatteringntensity. The results were then obtained using Fraunhoffer analysis. Care was taken tonsure that the use of Fraunhoffer or Mie analysis using the measured RI of the oilhase did not make any significant difference to the size distributions obtained. Addi-ionally, values characterizing the droplet size distributions, the volume based values10,3, d50,3 and d90,3, as well as the volume weighted mean diameter, d4,3, and the surfaceeighted mean diameter, d3,2, are provided in the results section below. The latter wassed to estimate the surface coverage of the droplets with particles.

    Emulsions with an oil phase volume of less than 20% or more than 20% were preparedrom the originally processed emulsion by diluting with continuous phase fluid and con-entrating via creaming, respectively. This procedure of preparing emulsions with a rangef oil phase volumes was chosen to keep the droplet size distribution in emulsions withifferent oil phase volumes constant. Preparation of emulsions at different oil phaseolumes in the homogenizer would almost certainly have led to a variation in droplet sizeistribution and interfacial characteristics. The procedure was as follows: The droplethase of the emulsions was allowed to cream followed by removal, or addition of theppropriate amount of continuous phase fluid. The creaming process was accelerated byentrifugation 90 min at 300 g. It was ensured that this treatment in the centrifuge didot influence the properties of the emulsion.

    . Structure of the oil-water interfaceThe structure of the particle layers adsorbed at the oil-water interface was imaged by

    ryogenic scanning electron microscopy JEOL 6301F field emission scanning electronicroscope with a Gatan alto2500 low temperature preparation system fitted. This tech-

    ique has been applied previously for the visualization of the interfacial structure inarticle stabilized emulsions as reported by Binks and Kirkland 2002. There, however,he particle diameters were large compared to the diameters of the particles used in thistudy, and the experimental protocol was adapted as outlined below.

    A drop of emulsion was frozen in a nitrogen slush and then placed in the preparationhamber of the cryo scanning electron microscope SEM held at 90 C. The sampleas fractured and etched for 75 s followed by cooling down to 110 C in order to only

    emove part of the water. At 110 C, where the sublimation was stopped, the exposedurface area of the emulsion was coated with a thin layer 2 nm of a mixture of platinumnd palladium. The sample was then transferred onto the cold stage of the microscopend imaged at 150 C.

    . Rheological measurementsThe rheological characterization of the emulsions was conducted in a strain-rate con-

    rolled rotational rheometer ARES ex Rheometrics Scientific now available from TAnstruments, UK. A concentric cylinder geometry with an inner diameter of 32 mm andn outer diameter of 34 mm was used. The length of the measurement gap was 33 mm.hear viscosities were measured by applying a constant shear rate for 30 s followed bypplication of the same shear rate in the opposite shear direction. The data recorded

    orrespond to an average of the last 10 s of each measurement direction. The applied

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    Downhear rate was increased, and 5 measurement points per shear rate decade were recorded.amples showing an apparent yield stress see Results section were also analyzed in atress controlled rotational rheometer DSR ex Rheometrics Scientific, UK using a con-entric cylinder geometry of similar dimensions. Here, each shear stress was applied for80 s, and 5 measurement points were applied per shear stress decade. All measurementsere conducted at 20 C. There was no indication of slip in any of the results obtained.lso, the shear history did not influence the results established by repeating measure-ents without changing the sample. Viscosities reported in the results section can be

    ssumed to be steady state viscosity values with the exception of the data that wereecorded for emulsions showing apparent yield stress behavior. Here, it might be ex-ected that data points in the region of the apparent yield stress will show more timeependence, and hence not represent a true steady state.

    II. RESULTS. Bulk microstructureIn Fig. 3, the graph on the left hand side shows the droplet size distribution for the

    article stabilized emulsion as prepared at 20 %v /v oil. Also plotted is the droplet sizeistribution for a second batch of base emulsion to demonstrate the reproducibility of themulsion preparation. The result for the surfactant stabilized emulsion as prepared at0 %v /v oil is shown in the second graph in Fig. 3, and the values for characteristicarameters of the droplet size distribution are provided in Table I. The disperse structuref the two emulsions compare well, justifying a comparison of their rheological behavior.

    For the particle stabilized emulsion, based on the surface weighted mean diameter,3,2, it was estimated that per 100 ml emulsion the total available surface area is 37.5 m2

    IG. 3. Particle size distribution for particle stabilized emulsion left hand side and surfactant stabilizedmulsion right hand side.

    ABLE I. Characteristic parameters of the droplet size distribution.

    Sampled10,3m

    d50,3m

    d90,3m

    d4,3m

    d3,2m

    article stabilized batch 1 2.0 3.6 6.3 3.9 3.2article stabilized batch 2 1.9 3.4 6.0 3.7 3.1

    urfactant stabilized 1.5 2.8 5.3 3.1 2.5

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    471PICKERING EMULSIONS

    Downompared to a total cross sectional area of the silica particles present of 87.5 m2 calcu-ated using the specific surface area of the Ludox particles as provided by the manufac-urer and ignoring the fact that the contact angle at the interface is not 90 which intro-uces only a small error. Hence, there are more particles available than needed to fullyover the total surface area of the droplets, in fact there are sufficient particles to fullyover droplets of half the size with a monolayer. In the surfactant stabilized emulsions, its also true that there is a large excess of surfactant, and that smaller droplets could beormed and fully coated with surfactant, while still maintaining a concentration in solu-ion above the CMC. Hence, in these two emulsions particle and surfactant stabilized its likely that the droplet size is determined by the process conditions in the homogenizer,ather than being limited by the available surfactant or particle concentrations. Thereakup process in the homogenizer is determined to a first approximation by two mate-ial parameters; namely the viscosity ratio which is the same in both emulsions and thenterfacial tension which is similar. Hence, it is not surprising that the two emulsionypes have similar size distributions, indeed, the surfactant stabilized emulsion drops arelightly smaller, which probably arises from them having a somewhat lower surfaceension.

    . Interfacial structure of the particle stabilized emulsionThe SEM micrographs depicted in Fig. 4 appear to demonstrate that the droplet inter-

    ace is close packed with particles. This is particularly evident from the bottom micro-raph showing what appears to be a particle shell on the surface of a droplet fractured byhe sample preparation routine. Also visible are clusters of particles on the outside surfacef the shell. Although the resolution of the image is not sufficient to image individualarticles, it seems clear that the droplets are coated with a dense layer which is likelyomposed of the Ludox particles.

    . Flow behavior in steady shear. Particle stabilized emulsion

    The rheological behavior of the particle stabilized emulsions as measured in steadyhear is plotted in Fig. 5 as the relative viscosity

    r =sm

    1

    ersus the applied shear stress, s: viscosity of the suspension/emulsion; m: viscosityf the suspension medium/continuous phase fluid. When calculating r, the viscosity ofhe suspension medium was taken to be equal to the viscosity of water, 1 mPa s mea-ured values for the aqueous emulsion phase prior to emulsification did not significantlyeviate from 1 mPa s. Each curve corresponds to a different oil phase volume as givenn the legend.

    For a dispersed phase volume of 0.2, and below, the rheological behavior of themulsions is Newtonian. With increasing dispersed phase volume, however, shear-hickening as well as shear-thinning regions are present in each of the viscosity curves.his behavior is most pronounced in the emulsions with an oil phase volume of 0.5 and.55. For an even higher internal phase volume, see data for V=0.6 in Fig. 5, the initialhear-thinning region is not resolved. It is assumed that the increase in viscosity mea-ured for shear stresses of up to roughly 10 Pa corresponds to the shear-thickening region

    s detected for the emulsions with slightly smaller values of V. Further rheological

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    472 WOLF et al.

    Downeasurements, also in oscillatory shear, are required to confirm this hypothesis. Themulsion with an internal phase volume of 0.7 was characterized in a stress controlledheometer since this emulsion apparently exhibited yield stress-like behavior. In order toelate the results of the two instruments, the emulsion with =0.6 was also analyzed in

    IG. 4. SEM micrographs of emulsion stabilized with 1%w /w particles with increasing magnification from topo bottom.Vhe stress controlled rheometer; see Fig. 6. For comparison, the previous results for the

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    473PICKERING EMULSIONS

    Downmulsion with V=0.6 as measured in the strain-rate controlled instrument are re-plotted.he sharp decline in viscosity found for the emulsion with V=0.7 is indicative of yieldehavior.

    Figure 7 illustrates the concentration dependence of the relative viscosity by plottinghe viscosity at two shear stresses 0.2 and 2 Pa as a function of the oil volume fraction.hese data were also fitted with the Krieger-Dougherty equation Krieger and Dougherty

    1959; Krieger 1972

    r = 1 VV,max

    V,max 2or the relative viscosity of hard sphere suspensions V,max: maximum dispersed phaseolume, : intrinsic viscosity, =2.5 for a hard sphere suspension, or an emulsionith nondeforming droplets; see Fig. 7 for the fits at two different shear stresses. For

    IG. 5. Relative viscosity measured for particle stabilized emulsions at different dispersed phase volumes, V;ee legend.

    IG. 6. Relative viscosity for particle stabilized emulsions with the two highest oil phase volumes, V; see

    egend. Open symbols: measured with the stress controlled rheometer. Filled symbols: measured with strain-rateontrolled rheometer, data re-plotted from above.

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    474 WOLF et al.

    Downoth sets of data, the fitted value for was 3.3, and for V,max the results were 0.63 and.61 for the data set at 0.2 and 2 Pa, respectively. Values for 2.5 are found forarticle shapes other than spheres Barnes et al. 1989 or for perfect spheres featuringarticle/particle or particle/solvent interactions which form aggregates Yang et al.2001. In the present sample, aggregation can in principle occur between droplets,etween particles, and between droplets and particles, so the value of 3.3 for is notnreasonable.

    The shear-thickening effect observed here is not as dramatic as sometimes found forard sphere suspensions, or even suspensions of deformable microspheres Otsubo2001; Frith et al. 1996; Frith et al. 1998; Frith and Lips 1995. Despite only havinglimited set of data available, we have calculated the Peclet number

    Pec =a3cm

    kT3

    or the onset of shear thickening for V=0.5 and 0.55 where c=1.5 and 10 s1, respec-ively a: droplet radius, a=d3,2 /2; c: critical shear rate for the onset of shear-thickening;: Boltzman constant; T: absolute temperature. For clarity, c was taken as the shear rateorresponding to the first measured increased viscosity following the initial shear-hinning region. It follows that Pec15 and 8 for V=0.5 and 0.55 which is in the samerder of magnitude as reported for suspensions of sterically stabilized particles Frith etl. 1996 apparently substantiating our hypothesis that the particle stabilized emulsionsnvestigated here behave rheologically like suspensions. It is interesting to note, however,hat the increasing trend in Pec with V is the opposite to that observed in Frith et al.1996. Also, the Pec values obtained here are much lower than those observed foruspensions of starch granules and microgel particles Frith et al. 1998.

    As mentioned above, the shear-thickening region is followed by a shear-thinningomain. Attempting to interpret this finding locally on the scale of the drops, one mightssume that the observed decrease in viscosity for further increases in shear stressesriginates from deformation of the emulsion droplets. However, since we argue that themulsion droplets behave like rigid particles in suspensions, it appears unlikely thatroplet deformation is the origin of the observed effect. This is also demonstrated by theollowing estimation, assuming that droplet deformation is not inhibited by the presencef the particles at the interface. The deformation of individual drops can be estimatedsing the classical relation by Taylor for droplet deformation in shear flow Taylor

    FIG. 7. Krieger-Dougherty fit dashed line for data symbols at two different shear stresses.1932

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    475PICKERING EMULSIONS

    DownD = a

    19 dm

    + 16

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    + 1 , 4here is the interfacial tension and d: viscosity of the droplet phase fluid. The defor-ation parameter is defined as

    D =L BL + B

    5

    ssuming that the deformed droplet is of ellipsoidal shape L: dimension of the long axisf the deformed droplet; B: dimension of the short axis of the deformed droplet. Thetatic interfacial tension between the two emulsion phases IPM/dodecane and particleaden aqueous phase was 23 mN m1 as determined with the pendant drop technique.he shear stress, , acting at the interface of the droplets at the beginning of the secondhear-thinning region is probably higher than the bulk shear stress since the droplets arehought to form a transient network during shear thickening. In simulations of shear-hickening polymer coated spheres, Melrose and Ball 2004b found that the hydrody-amic forces between close neighboring spheres were exponentially distributed. Theharacteristic forces in the contact network dominating the thickening response scaled ashe shear stress times the diameter of the spheres squared with a pre-factor varyingetween 2 and 3 up the thickening curve. With =10 Pa, corresponding to the shear stresst which shear-thinning postshear thickening sets in, and taking into account the citedork by Melrose and Ball 2004b, a value of D of the order of 103 follows. This

    ndicates that the droplets hardly deform, hence, the origin of the second shear-thinningegion does not appear to arise from droplet deformation. The effect of a system flippingrom thickening back to thinning is also seen in polymer coated hard sphere simulationsMelrose and Ball 2004a. It is a subtle effect for which Melrose and Ball 2004a offern explanation based on the origin of the preceding shear-thickening region. Thickeningccurs when the shear time, which is equal to the inverse shear rate, is shorter than theelaxation time of close particle approaches. Consequently, contacts cannot relax andetworks build up. As one moves up the shear-thickening curve to ever higher sheartresses, the particles get ever closer in their approach, and, if the conservative springorces stiffen up at very close approach, the relaxation time of the contact can becauseast again, and one drops out of the thickening region. However, this can only be aypothesis with respect to the emulsions investigated here, since we have no knowledgef the detailed particle interactions.

    . Comparison to surfactant stabilized emulsionAs mentioned in the Introduction, we compare the behavior of the particle stabilized

    mulsion system to that of a conventional surfactant stabilized emulsion system. Figureshows the relative viscosity as a function of shear stress for the surfactant stabilized

    mulsions. The emulsions display near Newtonian flow behavior for oil phase volumesp to 0.5. At an oil phase volume of 0.6, shear thinning is observed. To emphasize theifferent behavior of the particle stabilized emulsions, the data for three different phaseolumes were re-plotted in Fig. 9 together with the corresponding data of the particletabilized emulsion. Whereas the viscosity behavior of the two emulsion types is nearlydentical for =0.2, differences in behavior at elevated droplet phase volume are clearlyVvident. The data were also fitted with the Krieger-Dougherty relation see Fig. 10 and

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    476 WOLF et al.

    Downhe fitted model parameters were =2.5 and V,max=0.67 indicating that the droplets inhis emulsion system follow the Einstein relation, and that they show no sign ofggregation/flocculation. The latter is confirmed by the absence of shear thickening com-ared to the particle stabilized emulsions at a volume fraction of 0.6.

    V. CONCLUSIONSThe rheological behavior of particle stabilized oil-in-water emulsions with an average

    roplet size of a few microns and interfacial stabilization by nanometer sized silicaarticles effected by reducing the pH to pH2, and using an oil phase with a polar com-onent, shows a domain of slight shear thickening for elevated droplet phase volumes.he shear-thickening region is preceded and followed by a shear-thinning domain. Theresence of shear thickening indicates that the emulsion system behaves rheologicallyike a suspension, though it is characterized by a slow increase in viscosity with increas-ng shear rate/stress compared to sharp increases reported for other types of suspensions

    IG. 8. Relative viscosity measured for surfactant stabilized emulsions at different dispersed phase volumes,V; see legend.IG. 9. Relative viscosity. Open symbols: particle stabilized emulsions. Filled symbols: surfactant stabilizedmulsions.

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    477PICKERING EMULSIONS

    DownOtsubo 2001; Frith et al. 1996; Frith and Lips 1995. The second shear-thinningomain is likely not to originate from deformation of the individual emulsion droplets,ut might be the result of a competition between shearing time and relaxation time ofpproaching droplets/particles with the latter becoming fast compared to the former at theighest shear rates. While this study is only a first step towards understanding theicrostructure-rheology relationships for particle stabilized emulsions, it demonstrates a

    ovel material behavior in terms of being observed for an emulsion. Further rheologicalharacterization, including oscillatory shear measurements and creep tests, combinedith flow visualization and an attempt to quantify particle-particle interactions, are re-uired to build a more complete picture of the material behavior of particle stabilizedmulsions.

    CKNOWLEDGMENTSThe authors would like to thank D. A. Adams, D. B. Farrer, J. R. Melrose, S. Furze-

    and, D. Rossetti, and T. Weaver for their input in the form of valuable discussions orxperimental support. J. R. Melrose in particular is acknowledged for discussing thenset of the second shear-thinning region with us. Grace Davison U.K. is thanked forroviding the particle suspension.

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