Colloid-polymer mixtures may undergo either fluid-fluid phase separation or
gelation. This depends on the depth of the quench (polymer concentration) and
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 the 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. On deeper quenches the system undergoes
a continuous crossover to gel formation. We identify two regimes, `classical'
phase separation, where single particle relaxation is 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.Comment: 8 page
