Large-Scale Bundle Ordering in Sterically Stabilized Latices

Time-resolved small-angle light scattering and linear conservative dichroism measurements are presented for concentrated, sterically stabilized, aqueous latices under simple shear flow. At low stress levels, flow causes a mild distortion of the liquid-like structure in colloidally stable dispersions, which is quite well understood. In this paper flow-induced structures are investigated in concentrated dispersions when the system is brought far from equilibrium by means of hydrodynamic forces. At high stress levels various structural changes have been predicted by numerical simulation, among others string phases oriented in the flow direction. Here, experimental results are reported on a bundle-like ordering in very dense systems, which involves a length scale much larger than that of a single string of particles. Two latices, with different particle sizes and different thicknesses of the stabilizing layer, are compared. The occurrence of the bundle-like ordering is related to the rheological behavior: it causes a significant decrease in viscosity. It is shown that the presence of this phase results in a structural hysteresis, which explains a thixotropic behavior that is encountered in some stable colloidal suspensions. Also the relaxation behavior of the bundle-like phases has been studied. Interparticle forces are found to have a very strong effect on the relaxation time scales. Copyright 1999 Academic Press.