Ageing and collapse in gels with long-range attractions

We report measurements on the ageing dynamics of a colloid-polymer mixture with a large polymer-colloid size ratio of 0.62. Quenched into a two-phase region the system gels and forms a network with a characteristic radius R-c. We find three distinct regimes in the time evolution of R-c(t), reminiscent of the linear, late and gravity-dominated regimes of coarsening seen in classical spinodal decomposition kinetics of binary fluids. In the early stages of gelation, we observe a peak in the time-dependent structure factor S(q, t) which is stationary in q and grows in intensity characteristic of a linear-Cahn regime. The domain size then coarsens continuously with the age t of the sample. In the late stages the domain size follows the approximate algebraic law, R-c similar to t(theta). The growth exponent theta is a strong function of the quench depth: for small polymer concentrations theta is significantly larger than for large polymer concentrations. The gel networks formed are transient, and in the final stages of phase separation, collapse under gravity when the correlation length of the gel becomes similar to 2 pi times the capillary length.