Excluded Volume in Microrheological Models of Structured Suspensions
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 4, July, 2020
EXCLUDED VOLUME IN MICRORHEOLOGICAL MODELS OF STRUCTURED SUSPENSIONS E. E. Bibik, E. V. Sivtsov, and V. D. Rodinova
UDC 532.135
An exact solution of rheology equations, which is possible for a layered two-phase material simplest of structure, is used as a tool for testing concepts and rheological equations obtained on the basis of the existing structural models of disperse systems and for ranking the models by the extent to which the excluded-volume effect is taken account of in them. Consideration has been given to the features of flow of layered systems in which the dispersed phase is a continuous impermeable medium, an elastic material, and a coagulation structure permeable to a dispersion medium. In the context of the model of flow of layered systems, the authors interpret the flow regime observed experimentally, the so-called "stress plateau," which is characteristic of liquid crystalline structures and polymer melts. Keywords: rheology, viscosity, free volume, shear, elastoplasticity. Introduction. Rheology is the science dealing with the behavior of materials under deformation. Microrheology is a branch of theoretical rheology whose objective is to find rheology laws of heterogeneous materials on the basis of actual data or a priori ideas of their structure [1]. The main product of microrheology is a rheological equation, i.e., a formula describing the dependence of the viscosity η of a heterogeneous material and of its deformation ε or the deformation rate γ on the stresses τ acting in the material. The subsequent presentation refers to systems capable of being unboundedly deformed without failure, i.e., of flowing. These are suspensions, colloidal solutions, polymer solutions and melts, and, certainly, individual liquid substances. The rheology of highly dispersed heterogeneous systems is the most informative source of data on the structure of these systems and on the influence of their composition and production technology on the structure and structurally dependent properties. They include the uniformity of distribution of the highly dispersed component in composite materials and their strength, the level of internal stresses in the presence of the temperature difference and acquired in the process of heat treatment, the stability of the properties with time, and many other characteristics. Quantitative information is contained in the numerical value of rheological parameters. Decoding their dependence on the formulation and technology is based on theoretical models of the rheological behavior of variously structured systems [2]. Certain progress has been made in this field in recent decades [3]. It has become more obvious that the existing rheological equations fail to cover the whole range of regularities observed experimentally. Therefore, it is still topical to search for novel ideas and approaches to solving problems of applied rheology of disperse systems. Rheology Equations of a Disperse System. Describing the field of
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