Alexander Ya. Malkin

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...

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...

Viscoplasticity and stratified flow of colloid suspensions

Investigation of the rheology of concentrated colloid suspensions and direct observation of their flow allowed us to find several effects inherent to these media as typical soft matter. So, at low stress amplitudes, these colloids behave as mild gels with frequency independent elastic modulus and low mechanical losses. Meanwhile, suspensions demonstrate dualism of properties: at a given shear rate, they behave as viscoplastic media with clearly expressed yielding, while at a given low stress the pronounced Newtonian plateau is detected. The increase in shear rates and stresses leads to the sharp drop of the apparent viscosity, which usually is treated as the yielding effect. Transition through the yield stress is of a dynamic nature because the threshold stress depends on time and suspensions are thixotropic yielding materials. In the transient shear rate range, an unstable regime of deformation appears. It manifests itself either as deformation thickening up to jamming, or as the excitation of self-oscillations. The measuring of rheological properties in varying volume-to-surface ratio of a sample proves that flow of a suspension with high velocity at constant shear stress actually proceeds in a narrow layer inside the instrument gap. This conclusion has been confirmed by direct visual observations demonstrating that a flux is separated into three layers. A wide almost motionless layer is seen near a stationary surface. Near a moving surface, a narrow band with linear velocity profile is detected. Between them, a rather wide transient layer is observed and shear rate in this layer exceeds the average (global) shear rate by several times. Approximately, only a half of the total volume of a suspension is involved in flow. So, we observed a three-band flux of a suspension not described before. Shearing leads to an anisotropic structure of a solid phase.

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.

A modern look on yield stress fluids

A concept of viscoplasticity advanced exactly one century ago by Bingham appears very fruitful because there are many natural and artificial materials that demonstrate viscoplastic behavior, i.e., they are able to pass from a solid to a liquid state under the influence of applied stress. However, although this transition was originally considered as a jump-like phenomenon occurring at a certain stress—the yield stress—numerous subsequent studies have shown that the real situation is more complicated. A long-term discussion about the possibility of flow at low stresses less than the yield stress came to today’s conclusion denying this possibility as being opposite to the existence of the maximal Newtonian viscosity in viscoelastic polymeric fluids. So, there is a contradiction between the central dogma of rheology which says that “everything flows” and the alleged impossibility for flow at a solid-like state of viscoplastic fluids. Then, the concept of the fragile destruction of an inner structure responsible for a solid-like state at the definite (yield) stress was replaced by an understanding of the yielding as a transition extending over some stress range and occurring in time. So, instead of the yield stress, yielding is characterized by the dependence of durability (or time-to-break) on the applied stress. In this review, experimental facts and the new understanding of yielding as a kinetic process are discussed. Besides, some other alternative methods for measuring the yield stress are considered.

Non-linearity in rheology —an essay of classification

Different non-linear phenomena (such as non-Newtonian flow, large elastic deformations, instabilities of different types and many others) are the heart of rheology. Therefore many attempts were carried out to find quantitative, or at least qualitative, models of non-linear behavior. The general or perhaps most attractive way of developing rheological constitutive equations consists in the search for the most general method to describe everything in the framework of a single approach. Naturally, this leads to very complicated and ambiguous equations. Meanwhile, it is reasonable to try another way based on separating observed phenomena into different types depending on observed phenomena into different types depending on their physical origin. An attempt to propose such their physical origin. An attempt to propose such classification of nonlinear rheological effects is made.According to the assumed scheme three levels of non-linearity are distinguished. There is a group of phenomena which originate as a consequence of finite elastic deformations. This is weak non-linearity related to equilibrium properties of a matter. The second level can be characterized as strong non-linearity. It is related to reversible structure changes, developing in time and connected with changes in relaxation properties of a matter. This group of effects can be treated as kinetic phenomena. Lastly, the third level of non-linearity is connected with breaking or phase transitions induced by deformations. This leads to the most severe consequences and can be treated as effects of thermodynamic nature. It is shown that some well known rheological effects can be explained if we consider them as a superposition of non-linearity of different types.

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...

Some Inverse Problems in Rheology Leading to Integral Equations

In various applications we meet the necessity to solve inverse problems, that means to find the properties of the matter ( expressed by some constants or functions) from the experimentally observed macro-behaviour of a sample. Such inverse problems can be differential ( the unknown constants being the coefficients of differential equations) or integral. The last case consists in existance of relationships between inherent properties of the matter and gross properties of a body, expressed by integral equations or functions. In this lecture two typical rheological themes leading to inverse integral problems are discussed. The first one represents rheology as a method of reflection of molecular composition of a matter. It is flow curve — molecular weightdistribution correlation. It is shown that this problem has the exact solution, but this solution is unstable in principle and it means that the inverse problem is incorrect by its nature. The second inverse problem, discussed in the lecture, lies in its proper field of rheology. It is the calculations of creep from relaxation functions and vice versa. This problem has a correct solution but the possibility to find such a solution depends upon the method of approximation of an experimental curve.

Rheological properties of polyethylene/metaboric acid thermoplastic blends

The rheological properties of molten low-density polyethylene/metaboric acid blends were studied. It was found that the blend behavior can be rather different, depending on volume fraction of the inorganic component. Specifically, at some concentration of metaboric acid, the dynamic moduli and the Newtonian viscosity of the blends demonstrate a jump-like change. The concentration threshold depends on temperature and equals to 21.9 and 14.1 vol %, at 150 and 180 ∘C, respectively. In the concentration range below the threshold, the gain in the content of inorganic component results in an enhancement of the blend dynamic moduli and viscosity, without changing the general character of the rheological behavior of composition in the region of linear response. On the other hand, at higher concentrations of metaboric acid, the yield stress is observed, and the elastic modulus in the linear region of mechanical behavior becomes virtually independent of frequency. It was suggested that the rheological behavior of blends is related to a spontaneous change in their structure as well as planar molecular structure of the inorganic component.

The rheological characterisation of typical injection implants based on hyaluronic acid for contour correction

Systematic rheological characterisation of several injection implants based on hyaluronic acid (Belotero®, Teosyal®, Glytone® and Juvéderm® brands) has been carried out. All these dermal fillers are viscoelastic media with the storage modulus exceeding the loss modulus. So at low deformations, they are gel-like materials, but at increasing shear stress, they can flow demonstrating typical non-Newtonian behaviour. In some cases, though not always, the yield stress is expressed rather clearly. The application of the technique of large amplitude oscillation shear (LAOS) allowed us to distinguish two groups of rheological behaviour characterised by shear thinning or strain overshoot. The Belotero® fillers belong to the first group. Similar strong changes in the storage and loss moduli as a function of frequency and fuzzy yielding are characteristic of these materials. The Teosyal® and Juvéderm® Voluma filler belong to the second group. Their typical rheological features are existence of the minimum for the frequency dependence of the loss modulus and clearly expressed the yield stress. The Glytone® and Juvéderm® Ultra fillers occupy the intermediate position. When the dermal fillers of the second group were diluted, they demonstrated effects similar to the phase separation. As a result of the rheological characterisation, some assumptions have been advanced regarding correlations between the objective rheological parameters of dermal fillers and conditions of their application.