A light scattering study of colloid-polymer mixtures

Abstract

A detailed light scattering study of non-equilibrium states found in a model colloid-polymer mixture is presented. Conventional light scattering is used to examine the average structure of the phase, over a wide range of wavevectors. For all non-equilibrium samples a 'ring' of scattered radiation is found at small angles, whose temporal evolution is used to investigate aggregation kinetics and the growth of structure. For low concentrations of colloid and just sufficient polymer to induce non-equilibrium behaviour a small angle ring is observed after a initial 'lag time', which remains stationary and brightens rapidly, behaviour reminiscent of classical nucleation. At higher colloid concentrations one finds a continuously collapsing and brightening ring, similar to that found in classical spinodal decomposition. Upon the addition of further polymer the system gels and the small angle ring becomes arrested at a finite wavevector. Direct visual observation of these different regimes is also presented, via video-enhanced microscopy and direct time-lapse video recordings, to complement the light scattering data. The particle dynamics within the aggregates are measured using dynamic light scattering, The results obtained suggest that the onset of non-equilibrium behaviour is caused by the presence of a metastable gas-liquid phase boundary, which can be calculated using a recently-developed mean-field theory.