Temperature as an external field for colloid-polymer mixtures: 'quenching' by heating and 'melting' by cooling

We investigate the response to temperature of a well-known colloid-polymer mixture. At room temperature the gas-liquid critical value of the second virial coefficient of the effective pairwise colloid-colloid interaction for the Asakura-Oosawa model predicts the onset of gelation observed experimentally with remarkable accuracy. Upon cooling the system the effective attraction between colloids induced by polymer depletion is reduced, because the polymer radius of gyration decreases as the theta-temperature is approached. Paradoxically this raises the effective temperature, leading to 'melting' of colloidal gels. We find that the Asakura-Oosawa model of effective colloid interactions, together with a simple description of the polymer temperature response, provides a quantitative description of the observed location of the fluid-gel transition in the colloid volume fraction polymer reservoir number density plane. Further, we present evidence for enhancement of crystallization rates in the vicinity of the metastable critical point.