Memory impairment in flowing suspensions. II. Experimental results

Abstract

Experiments on the reversibility of particle orientation distributions in suspensions of nearly monodisperse rigid rods and hardened red blood cells (HRBC) subjected to cyclic shearing are described.The orientation distributions of initially aligned rods measured by direct observation exhibited a gradual loss in memory as flow was reversed back and forth which was attributed mainly to non-reversible particle interactions. When an electric field was applied continuously during cyclic shear flow, the rate of memory loss was increased. However, if an electrical shock was applied momentarily between each reversal of flow, the suspension maintained some memory. These results are generally in agreement with the theory presented in the preceding paper.With suspensions of HRBC, the particles were initially randomly oriented and the light transmittance across the planes of shear was used to measure changes in orientation distribution. Since HRBC are small enough to exhibit appreciable rotary Brownian motion, memory loss was inevitable and increased with the duration of each shear cycle and the cell concentration.Memory loss was also observed for rods and HRBC in non-Newtonian media and was attributed to drift in the rotational orbit constants of the particles.