  We investigate the response to temperature of a well-known colloid-polymer
mixture. At room temperature, the critical value of the second virial
coefficient of the effective interaction for the Asakura-Oosawa model predicts
the onset of gelation with remarkable accuracy. Upon cooling the system, the
effective attractions between colloids induced by polymer depletion are
reduced, because the polymer radius of gyration is decreases as the
theta-temperature is approached. Paradoxically, this raises the effective
temperature, leading to "melting" of colloidal gels. We find the Asakura-Oosawa
model of effective colloid interactions with a simple description of the
polymer temperature response provides a quantitative description of the
fluid-gel transition. Further we present evidence for enhancement of
crystallisation rates near the metastable critical point.
