Non-equilibrium behaviour of colloid-polymer mixtures

Moderate concentrations of a small non-adsorbing polymer cause a suspension of colloidal particles to phase-separate into coexisting colloidal fluid and crystal via the ‘depletion’ mechanism. At higher polymer concentrations, crystallization is suppressed, and a variety of non-equilibrium aggregation behaviour is observed. We report the results of small-angle laser light scattering studies of aggregation in a model system - colloidal PMMA + polystyrene. In all cases, ‘rings’ in the small-angle scattering are observed. The different temporal behaviour of these small-angle scattering rings allowed us to classify the non-equilibrium region into three regimes, ‘nucleation-like’, ‘spinodal-like’ and ‘transient gelation’. It is suggested that non-equilibrium behaviour is ‘switched on’ by a hidden, metastable gas-liquid binodal. Different regimes of aggregation behaviour are controlled by the nucleation-spinodal cross-over and the transient percolation lines within this binodal.