Particle migration in a concentrated suspension flowing between rotating parallel plates: Investigation of diffusion flux coefficients

This paper reports an experimental and numerical study conducted to investigate the behavior of macroscopic monomodal concentrated suspensions (40%) undergoing a creeping torsional flow between two rotating plates. An experimental technique based on the detection of tracers by measurement of light absorption is developed and used to quantify the time evolution of the particles concentration profiles. Contrarily to results reported in the literature, an outward migration of the particles is observed. This shear-induced migration is confirmed by viscometric measurements where an increase in the apparent viscosity of the suspension has been observed for long periods of shear. Moreover, this increase is found to depend solely on the value of the applied strain, which is consistent with a shear-induced migration phenomenon. Experimental results are reproduced using a semi-quantitative model involving the balance of three diffusion fluxes induced, respectively, by the gradient of viscosity (Jeta), the gradient of the collision rate between particles (Jc), and the flow curvature (Jr). Steady and transient numerical profiles are obtained using a finite volume approach. The coefficients of the diffusion fluxes (Keta,Kc,Kr) are determined by optimizing the numerical profiles to fit the experimental data. The ratios of these coefficients (Keta/Kc and Kr/Kc) are found to be independent of the flow geometry with their absolute values being tightly coupled to the direction of particles drift. In particular, the migration coefficients in the direction of the velocity gradient (in a cylindrical Couette flow) are found to be almost five times larger than those along the direction of vorticity (in a rotating parallel-plate flow).