SHEAR-INDUCED STRUCTURE DISTORTION IN NONAQUEOUS DISPERSIONS OF CHARGED COLLOIDAL SPHERES VIA LIGHT-SCATTERING

The effect of steady-shear flow on the microstructure of nonaqueous di spersions of colloidal silica spheres having medium-range repulsive po tentials is studied, with a newly developed two-dimensional light-scat tering set-up, as a function of shear rate gamma and particle volume f raction phi. When dispersed in a close refractive index matching solve nt mixture of ethanol and toluene, the dispersions exhibit an equilibr ium disorder-order transition at phi = 12.6%. It is found that the she ar deformation of the colloidal liquid structure at high shear rates i s a smooth process with no long-range string or layer ordering in the flow-vorticity plane. The structure distortion is in good qualitative agreement with an approximate solution of the two-particle Smoluchowsk i equation. In particular, the theoretically recognized boundary-layer effect, which has been implicitly known for its relation to the onset of the non-Newtonian behaviour of effective viscosity, is clearly sho wn in our experiment for the first time. In the crystal phase an appli ed shear flow leads to the gradual melting of the equilibrium solid-li ke structure into sliding layers, strings, and finally into a distorte d liquid-like structure as gamma is raised. There is also an indicatio n that the layer structure contains a significant number of free strin gs. No shear-thickening or discontinuous behaviour in dispersion visco sity in relation to these translational order transitions is detected. We conclude that the nonequilibrium order of our system is correlated to the corresponding microstructure in equilibrium.