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.     

Electron Paramagnetic Resonance in HoxLu1−xB12 Dodecaborides

JETP Letters - The iron based superconductor FeSe0.5Te0.5 has been studied applying Mössbauer spectroscopy and ab initio density functional theory calculations of hyperfine parameters of iron...     

Relaxation spectra of polymers and phenomena of electrical and hydrophobic recovery: Interplay between bulk and surface properties of polymers

Low‐density polyethylene, polypropylene, and polycarbonate were exposed to cold air plasma treatment. The decay of electret response, hydrophobic recovery, and mechanical relaxation of polymers were studied experimentally. The three‐exponential decay kinetic model was used for the treatment of mechanical and electret responses. The characteristic time scales of mechanical and electret responses turned out to be very close. The “longest” relaxation time, extracted from the experimental study of the hydrophobic recovery, was also close to the corresponding characteristic time spans of electret and mechanical responses. The kinetics of surface processes taking place in polymers is controlled by the mobility of their functional groups, represented by the bulk relaxation spectra. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 198–205     

The elastic problem of a homogeneous circular ring acted upon by equally spaced concentrated twists of equal magnitude

Conclusions With the reservation and assumptions stated at the beginning of the preceding Section 8 the results derived in the present paper can be briefly formulated as follows:A homogeneous circular ring acted upon by equally spaced concentrated twists of equal magnitude is always free of torques. The bending moment B ₀, represented by a vector normal to the original plane of the ring, vanishes, if one of the two cross sectional principal axes is situated in the plane just mentioned; the case that the two principal moments of inertia of the cross sectional area of the ring are equal to each other is a special case of the one just taken care of. If the angle t is essentially different from zero [cf. definition of case (I) in Section 8], which means that C ₁ and C ₂ are actually different from each other, then the bending moment B ₀ is different from zero and given by our formula (64). This is basically the solution of our problem of a circular ring under concentrated twists, because the effects of the bending moments B, see formula (2), can be studied by means of elementary methods.The present paper is dedicated by its author to the memory of his teacher Professor Dr. Richard von Mises (April 19, 1883–July 14,1953).     

Viscoelastic and relaxation properties of a polystyrene melt in axial extension

On axial extension of polymer melts at constant deformation rates, the development of high-elastic deformation is of predominant importance during the initial period. High-elastic deformation is accompanied by a rise in viscosity and in the modulus of high-elasticity and by retardation of the relaxation processes in the region of large relaxation times. At relatively low deformation rates, the rise in viscosity and high-elasticity modulus and the retardation of relaxation processes may give way to a decrease in viscosity and high-elasticity modulus and acceleration of relaxation processes, so that stationary flow regimes are attained. The transition from strain regimes with increasing viscosity and modulus of high elasticity to those with a decrease of these quantities corresponds to an increase in the rate of accumulation of irreversible deformation. Accordingly, a competing influence due to the orientation effect and to destruction of the network of intermolecular bonds becomes evident while stationary flow is being attained. The orientation effect must be responsible for the retardation of the relaxation processes, whereas rupture of the intermolecular network bonds results in structural relaxation accelerating relaxation processes. In contrast to shearing, during extension the orientation effect is of predominant importance. Hence in stationary flow regimes the viscosity may not only remain independent of the rate of strain, but even increase with it. In this case the contribution of the large relaxation times to the relaxation spectrum increases with increasing stress in stationary flow regimes. The fact that the longitudinal viscosity and the modulus of high elasticity are independent of the stress in stationary flow regimes does not guarantee linearity of the mechanical properties of the polymer in the prestationary stage of deformation when complex changes occur in its relaxation characteristics. At high deformation rates the viscosity and the modulus of high elasticity keep rising with increasing deformation until rupture occurs. Determination of the strength of polystyrene samples vitrified after extension showed that it is due not to the entire degree of extension, but only to the value of accumulated high-elastic deformation. The strength of the vitrified samples is to a first approximation independent of the rate at which the melt was extended.     

Viscous and high-elasticity properties of solutions of monodispersed polybutadienes in the region of critical concentrations

It is shown that there is a critical concentration corresponding to the formation of a compact, fluctuating network, which results in a sharp change in the concentration dependence of the viscosity, first difference of normal stresses and high-elasticity deformations. It is found that at concentrations below the critical concentration high-elasticity deformations do not develop, although normal stresses exist. In consequence, the relationship whereby the modulus of high elasticity is equivalent to the ratio between the square of the viscosity and the coefficient of normal stress is fulfilled only for concentrations higher than the critical.A. V. Topchiev Institute of Petrochemical Synthesis, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 896–902, September–October, 1972.     

Viscosity,rubber elasticity,and viscoelasticity of concentrated polymer solutions

The results of a comparative investigation of the effect of the nature of the polymer and the solvent on the viscosity, rubber elasticity, and viscoelasticity of concentrated polymer solutions in the region of linear mechanical behavior are presented. It is established that in the case of nonpolar polymers the solvent affects only the free volume of the solution, whereas for polar polymers it also affects the entanglement network. This leads to the equality of the viscosities of solutions of nonpolar polymers in different solvents when compared in corresponding states (relative to the glass transition temperature) and to the dependence of the shear modulus of solutions of polar polymers on the nature of the solvent and temperature. It is shown that there is a universal relaxation spectrum in the flow zone for solutions of different polydisperse polymers after normalization with the shear modulus and the natural relaxation time determined as the ratio of the viscosity to the shear modulus.A. M. Gor'kii Ural State University, Sverdlovsk, A. V. Topchiev Institute of Petrochemical Synthesis, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 4, pp. 729–736, July–August, 1973.     

Normal stresses associated with the flow of non-Newtonian polymer systems

It has been experimentally confirmed that the method of calculating the shear rate dependence of the normal stresses from the flow curve, proposed in [1], gives good results consistent with the experimental data. A series of simple relations proposed for estimating the initial normal stress coefficient are compared with the experimental results and it is shown that there is good agreement between calculation and experiment. In the linear region the Lodge relation, which equates the high-elastic strains to half the ratio of the normal to the shear stresses is satisfied.For communication 1 see [1].Topchiev Institute of Petrochemical Synthesis, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 4, pp. 714–721, July–August, 1971.