Phase separation dynamics in colloid-polymer mixtures: the effect of interaction range

Colloid-polymer mixtures may undergo either fluid-fluid phase separation or gelation. This depends on the depth of the quench (polymer concentration) and the polymer-colloid size ratio. We present a real-space study of dynamics in phase separating colloid-polymer mixtures with medium- to long-range attractions (polymer-colloid size ratio q(R) = 0.45-0.89), with the aim of understanding the mechanism of gelation as the range of attraction is changed. In contrast to previous studies of short-range attractive systems, where gelation occurs shortly after crossing the equilibrium phase boundary, we find a substantial region of fluid-fluid phase separation. Upon quenching deeper, the system undergoes a continuous crossover to gel formation. We identify two regimes, 'classical' phase separation, where single particle relaxation is much faster than the dynamics of phase separation, and 'viscoelastic' phase separation, where demixing is slowed down appreciably due to slow dynamics in the colloid-rich phase. Particles at the surface of the strands of the network exhibit significantly greater mobility than those buried inside the gel strand which presents a method for coarsening.