Master curves for elastic and plastic properties of highly concentrated emulsions

Critical comparison of dependences of elastic and plastic properties of highly concentrated emulsions (so-called “compressed” emulsions) on the concentration and droplet sizes is performed. The studied emulsions of water-in-oil type are so-called “liquid explosives.” They are characterized by different mean sizes and different droplet size distributions of the dispersed phase. Different average values (D _av, D ₃₂, and D ₄₃) are used as characteristics of droplet sizes. Experiments are carried out with emulsions of two concentrations. Aqueous phase (dispersed droplets) is presented by supercooled solutions of inorganic salt in water in a metastable state. The concentration limit of the existence of highly concentrated emulsions is determined by the condition of the closest packing of liquid droplets, which lies in the φ* = 0.77–0.80 range. In addition, there is a limiting value of the maximal size of droplets. This limiting value depends on the concentration and meets the requirement that droplets should be small enough for the solution to exist in a supercooled state. The elastic modulus and the yield stress of emulsions studied are proportional to the square of the reciprocal linear size of droplets, which contradicts some theoretical models, according to which these parameter should be proportional to the reciprocal size of droplets. Using the obtained experimental data, we constructed generalized dependences of the elastic modulus and the yield stress on the concentration and size of droplets. These characteristics are in good agreement with the experimental data.     

Rheological transition of concentrated emulsions during successive shearing cycles

Rheological investigations have identified a shear viscosity transition from shear thinning to Newtonian at low to moderate shear rates for concentrated polydimethylsiloxane emulsions during successive shearing cycles. The viscosity "flattening" behavior is dependent on the maximum shear rate applied and on droplet deformation. Atomic force microscopy measurements indicate attraction between the "repulsive" emulsion droplets under deformation. The results suggest formation of stable droplet layers due to deformation under high shear hydrodynamic compression. Based on these findings, unique methods to control the post-shear rheology of concentrated emulsions can be envisaged.     

Non-Invasive Rheo-MRI Study of Egg Yolk-Stabilized Emulsions: Yield Stress Decay and Protein Release

A comprehensive understanding of the time-dependent flow behavior of concentrated oil-in-water emulsions is of considerable industrial importance. Along with conventional rheology measurements, localized flow and structural information are key to gaining insight into the underlying mechanisms causing time variations upon constant shear. In this work, we study the time-dependent flow behavior of concentrated egg-yolk emulsions with (MEY) or without (EY) enzymatic modification and unravel the effects caused by viscous friction during shear. We observe that prolonged shear leads to irreversible and significant loss of apparent viscosity in both emulsion formulations at a mild shear rate. The latter effect is in fact related to a yield stress decay during constant shearing experiments, as indicated by the local flow curve measurements obtained by rheo-MRI. Concurrently, two-dimensional D-T₂ NMR measurements revealed a decrease in the T₂ NMR relaxation time of the aqueous phase, indicating the release of surface-active proteins from the droplet interface towards the continuous water phase. The combination of an increase in droplet diameter and the concomitant loss of proteins aggregates from the droplet interface leads to a slow decrease in yield stress.     

Droplet surface properties and rheology of concentrated oil in water emulsions stabilized by heat-modified beta-lactoglobulin B

Effects of substituting native beta-lactoglobulin B (beta-lactoglobulin) with heat-treated beta-lactoglobulin as emulsifier in oil in water emulsions were investigated. The emulsions were prepared with a dispersed phase volume fraction of Phi=0.6, and accordingly, oil droplets rather closely packed. Native beta-lactoglobulin and beta-lactoglobulin heated at 69 degrees C for 30 and 45 min, respectively, in aqueous solution at pH 7.0 were compared. Molar mass determination of the species formed upon heating as well as measurements of surface hydrophobicity and adsorption to a planar air/water interface were made. The microstructure of the emulsions was characterized using confocal laser scanning microscopy, light scattering measurements of oil droplet sizes, and assessment of the amount of protein adsorbed to surfaces of oil droplets. Furthermore, oil droplet interactions in the emulsions were quantified rheologically by steady shear and small and large amplitude oscillatory shear measurements. Adsorption of heated and native beta-lactoglobulin to oil droplet surfaces was found to be rather similar while the rheological properties of the emulsions stabilized by heated beta-lactoglobulin and the emulsions stabilized by native beta-lactoglobulin were remarkably different. A 200-fold increase in the zero-shear viscosity and elastic modulus and a 10-fold increase in yield stress were observed when emulsions were stabilized by heat-modified beta-lactoglobulin instead of native beta-lactoglobulin. Aggregates with a radius of gyration in the range from 25 to 40 nm, formed by heating of beta-lactoglobulin, seem to increase oil droplet interactions. Small quantities of emulsifier substituted with aggregates have a major impact on the rheology of oil in water emulsions that consist of rather closely packed oil droplets.     

Materials based on solid-stabilized emulsions

Solid-stabilized emulsions are obtained by shearing a mixture of oil, water, and solid colloidal particles. In this article, we present a large variety of materials, resulting from a limited coalescence process. Direct (o/w), inverse (w/o), and multiple (w/o/w) emulsions that are surfactant-free and monodisperse were produced in a very wide droplet size range, from micrometers to centimeters. These materials exhibit original properties compared with surfactant-stabilized emulsions: outstanding stability with respect to coalescence and unusual rheological behavior. For such systems, the elastic storage modulus G' is not controlled by interfacial tension but by the interfacial elasticity resulting from the strong adhesion between the solid particles adsorbed at the oil-water interface. Due to the wide accessible droplet size range, concentrated emulsions can be extremely fluid while emulsions with low droplet volume fraction can behave as solids.     

Study on the flow of highly concentrated emulsions

An investigation was performed into the flow of highly concentrated water-in-oil emulsions. The viscosity of the low shear rate region in the downward curve was much higher than the viscosity of the upward curve due to the refining effect of the intensive shear. However, this refining effect lasted for just a short time. After that, the structure of highly concentrated emulsion (HCE) can return to its original state. The flow of HCE depends on the shear rate and droplet size of the dispersed phase. The viscosity curve of HCE, which is measured in the initial upward shearing sweep, had two platforms, whereas the region of shear rate was 10^?4?s^?1?～?10³?s^?1. The water-in-oil structure was destroyed by intensive shear and much solid ammonium nitrate (AN) was observed in the image of HCE. The small droplets can enhance the capacity of HCE to prevent the breakage of structure under shearing. The microstructure of HCE was closer to its original situation when the droplet size was small.     

Shear Stability of Highly Concentrated Emulsions

Shear stability of water-in-oil highly concentrated emulsions was characterized by the rate of the droplet size decrease at a constant shear rate. Samples of different concentration (ranging from 0.85 to 0.94 wt %), prepared with different surfactants and three types of oils were analysed. The emulsions under study are visco-plastic media with a clearly expressed yield stress. The usually used Capillary number is not valid for such systems but instead Bingham number (ratio of the yield stress to interfacial forces) was used to characterise their stability. Within the frames of our experiment, it has been proven that the correlation between shear stability of emulsions and the Bingham number exists.     

The structure recovery capacity of highly concentrated emulsions under shear flow via studying their rheopexy

An investigation was performed into the structure recovery of highly concentrated water-in-oil emulsions (HCEs) under shear flow via studying their rheopexy. Experiments with the shear rate sweep in the up and down modes demonstrate that HCE has rheopexy. Restoration of the initial structure after cessation of shearing needs a period of time. The recovery time and ratio depend on the shear rate and the droplet size of the dispersed phase. A high shear rate results in a high probability of structure break of HCE. Thus, it is difficult to return to its initial structure. The structure of HCE that underwent shearing is closer to its original situation when the droplet size is small.     

The role of interdroplet interaction in the physics of highly concentrated emulsions

The osmotic pressure and shear modulus of highly concentrated emulsions were modelled by considering both interfacial energy and interdroplet interaction. This was performed for two- and three-dimensional cases and by optimization and approximation methods of predicting film thickness. The results show that even a small source of interaction can result in non-superimposition of scaled osmotic pressure and shear modulus by Laplace pressure for different droplet sizes, and also significant deviation from the models which consider interfacial interaction as the sole source of energy. The model was used to explain the reciprocal squared diameter dependency of elastic modulus: an interaction similar to the van der Waals type can be responsible for this observation. The model can also be used to analyze the interdroplet interactions in highly concentrated emulsions.     

Rheology of concentrated carbon nanotube suspensions

The rheological properties of non-Brownian carbon nanotube suspensions are measured over a range of nanotube volume fractions spanning the transition from semidilute to concentrated. The polymer-stabilized nanotubes are "sticky" and form a quiescent elastic network with a well-defined shear modulus and yield stress that both depend strongly on nanotube volume fraction with different but related critical exponents. We compare controlled-strain-rate and controlled-stress measurements of yielding in shear flow, and we study the effect of slow periodic stress reversal on yielding and the arrest of flow. Our measurements support a universal scaling of both the linear viscoelastic and steady-shear viscometric response. The former allows us to extract the elastic shear modulus of semidilute nanotube networks for values that are near or below the resolution limit of the rheometers used, while the latter provides a similar extrapolation of the yield stress. A simple scaling argument is used to model the dependence of yield stress and elastic modulus on concentration.     

Droplet structure instability in concentrated emulsions

Dynamic rheological measurements are reported on concentrated emulsions of monodispersed sodium dodecyl sulfate-stabilized polydimethylsiloxane droplets with different cross-linking levels (i.e., controllable deformability and either viscous or viscoelastic) and over a volume fraction range 0.5 to 0.72. Emulsion structure instability is revealed at a volume fraction of 0.7 and is represented by an anomalously low G(')/G(') crossover stain, gamma(co) (G('), elastic modulus; G('), viscous modulus). This phenomenon is independent of the droplet cross-linking level and not observable for hard-sphere silica sols of volume fractions from 0.54 to 0.63. It is suggested that the structural instability arises from deformation-induced formation of "slip planes" between droplet layers specific to the repulsive droplets at the specific volume fraction, which may be dependent on the droplet packing configurations for the given polydispersity of the system. The gamma(co) value may be considered as an in situ index of the structural stability and interdroplet interaction balance in concentrated emulsions.     

Shear field modification of strongly flocculated suspensions — Aggregate morphology

Rheological and microscopical studies have been made to elucidate the effects of shear fields on the morphology of concentrated, aggregated model colloids. The models employed are well-characterised, predominantly chargestabilised polymer latices, coagulated by the addition of excess electrolyte. Continuous shear rheological and viscoelastic measurements indicate a very significant decrease in shear yield stress, apparent viscosity and shear modulus following prolonged shearing.Electron microscopy reveals the source of these changes. Freshly coagulated suspensions form networks that are porous, strong and qualitatively similar to simulated structures for diffusion limited aggregation. Following protracted shearing, the network structure is rearranged to yield discrete, tightly packed aggregates with a characteristic size, which is principally a function of the primary particle size.     

Influence of interfacial characteristics on Ostwald ripening in hydrocarbon oil-in-water emulsions

The influence of the nature of the interfacial membrane on the kinetics of droplet growth in hydrocarbon oil-in-water emulsions was investigated. Droplet growth rates were determined by measuring changes in the droplet size distribution of 1 wt % n-tetradecane or n-octadecane oil-in-water emulsions using laser diffraction. The interfacial properties of the droplets were manipulated by coating them with either an SDS layer or with an SDS-chitosan layer using an electrostatic deposition method. The emulsion containing SDS-coated octadecane droplets did not exhibit droplet growth during storage for 400 h, which showed that it was stable to Ostwald ripening because of this oils extremely low water-solubility. The emulsion containing SDS-coated n-tetradecane droplets showed a considerable increase in mean droplet size with time, which was attributed to Ostwald ripening associated with this oils appreciable water-solubility. On the other hand, an emulsion containing SDS-chitosan coated n-tetradecane droplets was stable to droplet growth, which was attributed to the ability of the interfacial membrane to resist deformation because of its elastic modulus and thickness. This study shows that the stability of emulsion droplets to Ostwald ripening can be improved by using an electrostatic deposition method to form thick elastic membranes around the droplets.     

Droplet size scaling of water-in-oil emulsions under turbulent flow

The size of droplets in emulsions is important in many industrial, biological, and environmental systems, as it determines the stability, rheology, and area available in the emulsion for physical or chemical processes that occur at the interface. While the balance of fluid inertia and surface tension in determining droplet size under turbulent mixing in the inertial subrange has been well established, the classical scaling prediction by Shinnar half a century ago of the dependence of droplet size on the viscosity of the continuous phase in the viscous subrange has not been clearly validated in experiment. By employing extremely stable suspensions of highly viscous oils as the continuous phase and using a particle video microscope (PVM) probe and a focused beam reflectance method (FBRM) probe, we report measurements spanning 2 orders of magnitude in the continuous phase viscosity for the size of droplets in water-in-oil emulsions. The wide range in measurements allowed identification of a scaling regime of droplet size proportional to the inverse square root of the viscosity, consistent with the viscous subrange theory of Shinnar. A single curve for droplet size based on the Reynolds and Weber numbers is shown to accurately predict droplet size for a range of shear rates, mixing geometries, interfacial tensions, and viscosities. Viscous subrange control of droplet size is shown to be important for high viscous shear stresses, i.e., very high shear rates, as is desirable or found in many industrial or natural processes, or very high viscosities, as is the case in the present study.     

Effects of Surface Properties on Rheological and Interfacial Properties of Pickering Emulsions Prepared by Fumed Silica Suspensions Pre-Adsorbed Poly(N-Isopropylacrylamide)

The hydrophobic fumed silica suspensions physically pre-adsorbed poly(N-isopropylacrylamide) (PNIPAM) in water could prepare oil dispersed in water (O/W) Pickering emulsion by mixing of silicone oil. The resulting Pickering emulsions were characterized by the measurements of volume factions of emulsified silicone oil, adsorbed amounts of the silica suspensions, oil droplet size, and some rheological responses, such as stress-strain sweep curve and dynamic viscoelastic moduli as a function of the added amount of PNIPAM. Moreover, their characteristics were compared with those of the O/W Pickering emulsions prepared by the hydrophilic fumed silica suspensions pre-adsorbed PNIPAM. For the emulsions prepared by the hydrophobic silica suspensions, an increase in the added amount of PNIPAM led to (1) a decrease in the volume fraction of the emulsified oil in the emulsified phase, (2) both the size of oil droplets and the adsorbed amount of the corresponding silica suspensions being almost constant, except for the higher added amounts, and (3) both the storage modulus (G′) and the yield shear strain being constant. The term of 1 is the same for the emulsions prepared by the hydrophilic silica suspensions, whereas both the adsorbed amount of the corresponding silica suspension and the G′ value increase and both the droplet size and the yield shear strain decrease with an increase in the added amount of PNIPAM. The differences between the rheological properties of the emulsions prepared by the hydrophilic silica suspensions and those by the hydrophobic ones are attributed to the hydrophobic interactions of the flocculated silica particles in the Pickering emulsions.     

Shear-induced disruption of dense nanoemulsion gels

The structural evolution and rheology of dense nanoemulsion gels, which have been formed by creating strong attractions between slippery nanodroplets, are explored as a function of steady shear rate using rheological small-angle neutron scattering (rheo-SANS). For applied stresses above the yield stress of the gel, the network yields, fracturing into aggregates that break and reform as they tumble and interact in the shear flow. The average aggregate size decreases with increasing shear rate; meanwhile, droplet rearrangements within the clusters, allowed by the slippery nature of the attractive interaction, increase the local density within the aggregates. At the highest shear rates, all clusters disaggregate completely into individual droplets.     

Viscous behavior of concentrated emulsions of two immiscible Newtonian fluids with interfacial tension

A new equation for the relative viscosity of infinitely dilute emulsions of noncolloidal droplets is proposed using the analogy between shear modulus and shear viscosity. In the limit of capillary number -->0, the proposed equation reduces to the well-known Taylor viscosity law for infinitely dilute emulsions. Starting from the proposed equation for an infinitely dilute emulsion, new viscosity equations for concentrated emulsions are then developed using a differential scheme. The proposed equations for concentrated emulsions are evaluated in light of a large body of published experimental data on the viscosity of emulsions.     

Rheology of semi-dilute emulsions: viscoelastic effects caused by the interfacial tension

The steady state rheological properties of viscous emulsions are discussed in the dilute and semi-dilute concentration regions. In these systems the first normal stress differences can be measured as well. Such data have been collected over a wide range of ratios of droplet over matrix viscosity. In this manner data became available to evaluate the Choi-Schowalter model. Application of the latter to the normal stresses requires that the droplet diameter be known. At high shear rates the droplet diameter changes nearly inversely proportional to the shear rate. This results in a first normal stress difference proportional to shear rate and hence a ‘normal viscosity’ can be defined. This is used to compare the data with the available theoretical predictions. At low shear rates deviations from a constant normal viscosity can be observed. They are associated with a hysteresis region, where no single steady state droplet size can be defined anymore. Slightly viscoelastic components have been used as well to investigate whether this would result in deviations from the behaviour observed for mixtures of Newtonian fluids.     

THE INFLUENCE OF THE FILM CONTACT ANGLE OF EMULSION DROPLETS ON THE DISPERSIBILITY OF THEIR SEDIMENTS.

In certain W/0 emulsions, the films between the flocculated water droplets and the adjoining menisci show a very large contact angle θ. Because this phenomenon indicates a strong interaction of the droplets, we investigated its influence on the redispersion of sediments formed in such emulsions.

Two models describing the yield stress of such sediments were developed using calculations that relate the yield stress of a string of emulsion droplets to the value of θ. The first model estimates the maximum yield stress that can be accounted for by capillary forces. The second model considers the yield stress of sediments consisting of droplet aggregates joined by single chains of droplets.

These theories were found to describe satisfactorily cone penetration and redispersion experiments performed on two series of W/O emulsions that exhibited a wide range of values of Θ.     

Electroosmosis through a Cation-Exchange Membrane: Effect of an ac Perturbation on the Electroosmotic Flow

The viscous behavior of oil-in-water (O/W) emulsions is studied over a broad range of dispersed-phase concentrations (φ) using a controlled-stress rheometer. At low-to-moderate values of φ (φ<0.60), emulsions exhibit Newtonian behavior. The droplet size does not exert any influence on the viscosity of Newtonian emulsions. However, at higher values of φ, emulsions exhibit shear-thinning behavior. The viscosity of shear-thinning emulsions is strongly influenced by the droplet size; a significant increase in the viscosity occurs when the droplet size is reduced. With the decrease in droplet size, the degree of shear thinning in concentrated emulsions is also enhanced. The viscosity data of Newtonian emulsions are described reasonably well by the cell model of Yaron and Gal-Or (Rheol. Acta 11, 241 (1972)), which takes into account the effects of the dispersed-phase concentration as well as the viscosity ratio of the dispersed phase to continuous phase. The relative viscosities of non-Newtonian emulsions having different droplet sizes but the same dispersed-phase concentration are scaled with the particle Reynolds number. The high shear viscosities of non-Newtonian emulsions can be predicted fairly well by the cell model of Yaron and Gal-Or (Rheol. Acta 11, 241 (1972)). Copyright 2000 Academic Press.