ORDERING AND STRUCTURE AT INTERFACES OF COLLOIDAL DISPERSIONS UNDER FLOW

The structure of dispersions of sterically stabilised poly(methyl meth acrylate) (PMMA) particles under flow conditions are presented. Data h ave been obtained with a light scattering apparatus in which the sampl e can be sheared. The colloidal dispersions were prepared from particl es with a significant density difference to the suspending medium whic h gives rise to sedimentation. The scattering patterns indicate fluid- like structures coexisting with ordered arrays of particles. The order ed phase, which has a higher number density than the fluid, is found t o undergo significant structural changes under steady shear. Diffracti on from ordered and disordered layers of particles in finite-sized sta cks is discussed. Quantitative interpretation of the scattering patter ns in terms of particle arrangements are presented. The sedimentation gives rise to density and velocity gradient variations across the samp le with slip regions. Such important effects would go undetected in rh eological measurements, leading to misinterpretation of the rheologica l behaviour.