SCATTERING DICHROISM MEASUREMENTS OF FLOW-INDUCED STRUCTURE OF A SHEAR THICKENING SUSPENSION

Shear thickening in concentrated colloidal dispersions of sterically stabilized PMMA particles has been investigated systematically, both mechanically and optically. Two types of shear thickening were distinguished. At the highest volume fractions, a discontinuous increase of the viscosity occurs at some critical point (in either the value of the stress or the velocity gradient). A further increase of the applied stress results in an erratic flow behavior. At lower concentrations the viscosity increases more gradually and the flow remains smooth. Dichroism experiments performed on a flowing dispersion and probing the structure in the 1-2 plane of simple shear flow do not show marked differences between a dispersion sheared in the upper Newtonian region and a continuously shear thickening sample. Moreover, it was also demonstrated that the type of shear thickening has no influence on the equilibrium values of the dichroism. However, a discontinuously shear thickening sample, sheared beyond the critical conditions, shows erratic, time-dependent transitions. The relaxation of the dichroism, after cessation of the flow, shows a marked increase in time scale compared to the relaxation time for shear rates smaller than the critical value. This enhanced relaxation process suggests that the discontinuity in viscosity is associated with the formation of very large, anisotropic aggregates of particles, which relax by both disorientation and disintegration. This behavior is closely related to the relaxation of the shear stress for a sample sheared beyond the critical conditions, which also shows two relaxation mechanisms. © 1993 by Academic Press, Inc.