Ordering and structure at interfaces of colloidal dispersions under flow

Abstract

The structure of dispersions of sterically stabilised poly(methyl methacrylate)(PMMA) particles under flow conditions are presented. Data have been obtained with a light scattering apparatus in which the sample can be sheared. The colloidal dispersions were prepared from particles with a significant density difference to the suspending medium which gives rise to sedimentation. The scattering patterns indicate fluid-like structures coexisting with ordered arrays of particles. The ordered phase, which has a higher number density than the fluid, is found to undergo significant structural changes under steady shear. Diffraction from ordered and disordered layers of particles in finite-sized stacks is discussed. Quantitative interpretation of the scattering patterns in terms of particle arrangements are presented. The sedimentation gives rise to density and velocity gradient variations across the sample with slip regions. Such important effects would go undetected in rheological measurements, leading to misinterpretation of the rheological behaviour.