The effect of particle size and migration on the formation of flow-induced structures in viscoelastic suspensions

Flow-induced structures in suspensions contain- ing spheres in viscoelastic suspending media were investi- gated by microscopy and rheo-optical methods. Suspensions of monodisperse polystyrene spheres of different diameters (1-3µ m) dispersed in aqueous solutions of hydroxypropy- lcellulose were studied in simple shear flows. Optical mi- croscopy observations as well as small angle light scattering (SALS) experiments were performed using a parallel plate geometry. In agreement with previous work, aligned strings of particles were observed when shearing faster then a criti- cal shear rate, which was found to be independent of particle size. In contrast to earlier work, however, the role of parti- cle migration was found to be of prime importance. Particles were shown to migrate towards the plates where the particles assembled and aligned in strings running in the flow direc- tion. For the smallest particles (1 µ m diameter), the align- ment of particle doublets or short strings along the vorticity direction was observed at low shear rates, which flipped to a an orientation into the flow direction of longer strings at higher shear rates. This phenomenon never detected before for spherical particles and maybe attributed to an interplay between the hydrodynamic and the attractive inter-particle forces. Finally,SALS experiments were used to quantify the degree of alignment and its dependence o particle size, shear rate and gap. For the system under investigation, the degree of alignment was found to increase with increasing shear rate and particle size and with decreasing gap. The present results suggest that, depending on the details of the suspend- ing medium and the size and nature of the suspending media, the formation of aligned structures is affected by the relative magnitude of the colloidal and hydrodynamic forces and the kinetics of string formation versus the kinetics of migration