Irina Masalova

Rheopexy in highly concentrated emulsions

This work describes a detailed study of the rheopectic effect in the flow of highly concentrated emulsions at low stresses. Experiments with the shear rate sweep demonstrated that the upward and downward branches of the flow curves coincide above some specific shear rate value. The upward experiments show the existence of the Newtonian part on the flow curve in the low-shear-rate domain, while the effect of yielding is observed in the downward curve. Restoration of the initial structure (and properties) after cessation of loading occurs very quickly. This allows associating the rheopexy with elastic deformations and the relaxation process (observed in frequency dependence of dynamic moduli) with characteristic times of about 0.03 s. Transient processes proceed in the range of shear deformation of the order of several units. Some quantitative measures of the rheopectic behavior are proposed and discussed. Normal stresses are constant in the low shear stress domain, but decrease sharply above the range of r...

Peculiarities of rheological properties and flow of highly concentrated emulsions: The role of concentration and droplet size

Results of a complete study of the rheological properties of highly concentrated emulsions of the w/o type with the content of the dispersed phase up to 96% are reported. The aqueous phase is a supersaturated solution of nitrates, where the water content does not exceed 20%. Dispersed droplets are characterized by a polyhedral shape and a broad size distribution. Highly concentrated emulsions exhibit the properties of rheopectic media. In steady-state regimes of shearing, these emulsions behave as viscoplastic materials with a clearly expressed yield stress. Highly concentrated emulsions are characterized by elasticity due to the compressed state of droplets. Shear storage modulus is constant in a wide range of frequencies that reflect solid-like behavior of such emulsions at small deformations. The storage (dynamic) modulus coincides with the elastic modulus measured in terms of the reversible deformations after the cessation of creep. Normal stresses appear in the shearing. In the low shear rate domain, normal stresses do not depend on shear rate, so that it can be assumed that they have nothing in common with normal stresses arising owing to the Weissenberg effect. These normal stresses can be attributed to Reynolds’ dilatancy (elastic dilatancy). Normal stresses sharply decrease beyond some threshold value of the shear rate and slightly increase only in a high shear rate domain. Observed anomalous flow curves and unusual changes of normal stresses with shear rate are explained by the two-step model of emulsion flow. Direct optical observations show that emulsions move by the mechanism of the rolling of larger droplets over smaller ones without noticeable changes of their shape at low shear rates, while strong distortions of the droplet shape is evident at high shear rates. The transition from one mechanism to the other is attributed to a certain critical value of the capillary number. The concentration dependence of the elastic modulus (as well as the yield stress) can be described by the Princen-Kiss model, but this model fails to predict the droplet size dependence of the elastic modulus. Numerous experiments demonstrated that the modulus and yield stress are proportional to the squared reciprocal size, while the Princen-Kiss model predicts their linear dependence on the reciprocal size. A new model based on dimensional arguments is proposed. This model correctly describes the influence of the main structural parameters on the rheological properties of highly concentrated emulsions. The boundaries of the domain of highly concentrated emulsions are estimated on the basis of the measurement of their elasticity and yield stress.

Evolution of rheological properties of highly concentrated emulsions with aging —Emulsion-to-suspension transition

The effect of aging on the rheological properties and physical structure of highly concentrated water-in-oil emulsions with dispersed phase of 82–90v% is the subject of this study. It was proven by various experimental techniques that aging leads to the emulsion-to-suspension transition. Significant shift of rheological properties to the solid-like behavior is the result of the emulsions aging, which shows itself as an increase of the storage modules with time as well as Newtonian viscosity measured in the upward sweeping shear rate mode. Comparison of flow curves measured in the upward and downward sweeping shear rate modes shows that the rheopectic effect at low stresses is observed for both fresh and aged emulsions. Viscosity measurements in the downward mode demonstrate transition to the elastic-like behavior at low stresses with appearance of strongly pronounced yield stress. Dependencies of the characteristic rheological parameters on aging have been investigated by using different analytical method...

Physical chemistry of highly concentrated emulsions

This review explores the physics underlying the rheology of highly concentrated emulsions (HCEs) to determine the relationship between elasticity and HCE stability, and to consider whether it is possible to describe all physicochemical properties of HCEs on the basis of a unique physical approach. We define HCEs as emulsions with a volume fraction above the maximum closest packing fraction of monodisperse spheres, φm=0.74, even if droplets are not of polyhedron shape. The solid-like rheological behavior of HCEs is characterized by yield stress and elasticity, properties which depend on droplet polydispersity and which are affected by caging at volume fractions about the jamming concentration, φj. A bimodal size distribution in HCEs diminishes caging and facilitates droplet movement, resulting in HCEs with negligible yield stress and no plateau in storage modulus. Thermodynamic forces automatically move HCEs toward the lowest free energy state, but since interdroplet forces create local minimums - points beyond which free energy temporarily increases before it reaches the global minimum of the system - the free energy of HCEs will settle at a local minimum unless additional energy is added. Several attempts have been undertaken to predict the elasticity of HCEs. In many cases, the elastic modulus of HCEs is higher than the one predicted from classical models, which only take into account spatial repulsion (or simply interfacial energy). Improved models based on free energy calculation should be developed to consider the disjoining pressure and interfacial rheology in addition to spatial repulsion. The disjoining pressure and interfacial viscoelasticity, which result in the deviation of elasticity from the classical model, can be regarded as parameters for quantifying the stability of HCEs.

The rheology of binary mixtures of highly concentrated emulsions: Effect of droplet size ratio

Binary mixtures of highly concentrated emulsions (HCE) with three droplet size ratios and different compositions were prepared. It was found that by the proper selection of droplet size ratio and composition of binary mixtures, the shear modulus, viscosity, yield stress, and yield strain can be dropped lower than mixing rules and even primary HCE. This effect is similar to what is known for dispersions with volume fraction less than 0.7 but has not been described for HCE. For such formulations, the caged mechanism of droplets dynamics is not dominant due to the provided free volume that can be occupied by smaller droplets during flow. This is originated from the increase in maximum closest packing and thus more efficient spatial packing. By studying the scaling behavior of shear modulus and yield stress, the significance of interdroplet interaction was distinguished.

A new mechanism of aging of highly concentrated emulsions: Correlation between crystallization and plasticity

The aging of highly concentrated w/o emulsions is studied upon variations in their compositions and the concentration of the dispersed phase. The dispersed phase consists of a supercooled aqueous solution of nitrate salts. The aging leads to an increase in the rigidity of a composition: the elastic modulus and yield stress increase, and the flow curve shifts toward higher viscosities. The evolution of emulsion structure during aging is studied by X-ray diffraction. It is shown that the instability of emulsions is due to the slow crystallization of the droplets of the dispersed phase that results in an increase in the yield stress. It is found that there is a direct correlation between the degree of crystallinity and the relative increase of the yield stress as a measure of the structure strength. The main result of the investigation is experimental evidence that there is a quantitative relation between the degree of crystallinity and an increase in the yield stress reflecting the transition from emulsion to suspension. An empirical equation describing this relation is proposed.

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.

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 (Dav, D32, and D43) 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.

Effect of Nanoparticle Hydrophobicity on Stability of Highly Concentrated Emulsions

The effect of hydrophobicity index (HI) of fumed nanosilica specimens on stability of water-in-oil (W/O) highly concentrated emulsions (HCE with ϕ = 90 vol%) with an overcooled dispersed phase was studied. A series of five silica with HI in the 0.60–1.34 range and HI > 3 were used separately and in combination with a low molecular weight traditional surfactant, Sorbitan MonoOleate (SMO). First, it was shown that SMO alone can stabilize W/O HCE whereas only silica nanoparticles with intermediate HI in the range 0.97 ≤ HI ≤ 1.34 could form W/O emulsions only up to 77–79 vol%. Then, on the contrary to SMO-based emulsions, Pickering emulsions are unstable under shearing. When mixed (silica plus SMO) emulsifier systems were used, firstly a thermodynamic consideration revealed that only SMO is likely to adsorb at the W/O interface and controls the emulsifying process by the decrease in the interfacial tension. Then, interestingly, all different kinds of emulsion stability investigated in this study demonstrate a breaking point (BP) at HI = 0.97. Below the BP the emulsions were found to be very unstable on shelf as well as under shear. Above the BP, a clear synergy between colloidal silica and SMO surfactant has been found.

Flow of Super‐Concentrated Emulsions

Super concentrated emulsions, e.g., emulsion explosives, are two‐phase systems consisting of aqueous droplets dispersed in an oil phase. The concentration of the disperse phase is 92–96 w.%, liquid droplets, containing a supersaturated aqueous solution of inorganic oxidizer salts. The flow of such emulsions is determined by their Theological properties as well as the time‐dependent processes of “aging” which take place due to the thermodynamic instability of these emulsions. This work presents the results of experimental studies of the main effects that accompany the flow of such materials: non‐Newtonian flow behavior, rheopexy which manifests as a slow increase of viscosity in the low shear rate domain, linear viscoelastic behavior, and the transition of elastic modulus to non‐linearity at high amplitudes of deformation.The emulsions under study are non‐Newtonian liquids. Experiments with the shear rate sweep demonstrate that the upward and downward branches of the flow curves coincide above some specifi...