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Dynamic Simulations of Soft Particle Pastes

Jyoti R. Seth, University of Texas, Austin, 1 university station C0400, Austin, TX 78712 and Roger T. Bonnecaze, Department of Chemical Engineering, The University of Texas at Austin, 1 University Station C0400, Austin, TX 78712.

Soft particle pastes (SPP) are concentrated suspensions or emulsions of soft, deformable particles compressed against one another. Soft particle pastes appear in ceramic processing, coatings and food and cosmetic products. Types of particles making up a SPP include microgels, compressed emulsions, polymer-coated sand and star polymers and micelles. These dense suspensions behave like complex fluids and have interesting flow properties. They are shear thinning, have a yield stress, show normal stresses, aging and memory and also exhibit slip at smooth shearing surfaces. Here we present a computational methodology to study the rheology of SPPs and results of a theoretical study using the simulation.

We model SPPs as a concentrated suspension of elastic spheres. The particles are packed against one another and interact though strong elastohydrodynamic lubrication forces generated due to compression and relative motion between them. The 3D molecular dynamics-like simulations capture the particle dynamics and also its effect on bulk rheology. Constant stress and constant shear-rate simulations are used to define the complete stress tensor of these materials. The simulation results for the yield, shear and normal stresses are presented as function of concentration and the ratio of viscous to elastic forces. The macroscopic rheology of these materials is related to the microstructural changes that occur during and upon cessation of flow. The theoretical predictions of the bulk rheology are found to compare favorably to previous experimental observations.