Shear-induced yielding and ordering in concentrated particle suspensions

At high enough concentrations, assemblies of microscopic colloidal particles suspended in a liquid behave as weak amorphous solids. We have studied the way in which these “soft solids” yield and reorder themselves under the application of an oscillatory shear strain, using the new technique of dynamic light scattering echo. Light scattering echo measures the degree of irreversible rearrangement in the suspension induced by the oscillatory strain. It operates by autocorrelating the intensity of laser light scattered by the sheared sample. At delay times corresponding to integral multiples of the oscillation period, “echoes” appear in the autocorrelation function as the particles in the suspension return to near their original positions. The amplitudes of the echoes provide a measure of the irreversible rearrangement. The echo technique and its application to the yielding of colloidal glasses are described.