Transient gelation by spinodal decomposition in colloid-polymer mixtures

We have investigated with small angle light scattering and optical microscopy transient gelation phenomena which occur in phase-separating colloid-polymer mixtures. The scattering intensity distribution shows a peak at non-zero wave vector and satisfies the asymptotic q(-4) Pored behaviour. Consistent with these observations, optical micrographs show an alternating pattern of dark and bright domains. These findings suggest that the polymer-induced depletion forces lead to the formation of a bicontinuous network of colloid-rich and colloid-poor domains, via a spinodal decomposition process. This bicontinuous network rapidly attains a gellike character as indicated by the arrest of speckle fluctuations. The occurrence of the gel is ascribed to polymer-induced aggregation between the colloids in the colloid-rich phase. Due to the reversible nature of the aggregation the network restructures and eventually the gel collapses, as is manifested by the rapid separation of the colloid-rich phase from the colloid-poor phase.