We have investigated with small angle light scattering and optical mic
roscopy transient gelation phenomena which occur in phase-separating c
olloid-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 sh
ow an alternating pattern of dark and bright domains. These findings s
uggest that the polymer-induced depletion forces lead to the formation
of a bicontinuous network of colloid-rich and colloid-poor domains, v
ia a spinodal decomposition process. This bicontinuous network rapidly
attains a gellike character as indicated by the arrest of speckle flu
ctuations. The occurrence of the gel is ascribed to polymer-induced ag
gregation between the colloids in the colloid-rich phase. Due to the r
eversible nature of the aggregation the network restructures and event
ually the gel collapses, as is manifested by the rapid separation of t
he colloid-rich phase from the colloid-poor phase.