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 Porod 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.

We investigate the kinetics and fractal morphology of aggregating polystyrene latex in an intermediate region between diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA). The measurements are made by means of a low-angle elastic-light-scattering setup covering two decades in scattering wave vectors. For each aggregation run, the salt concentration c is changed and the samples are prepared under isopycnic conditions, to avoid differential sedimentation. The average cluster mass is found to grow according to a power law with an exponent z varying in a continuous fashion in the range of salt concentration used. At high c (DLCA), we find for the fractal morphology df =1.65. For lower concentrations, df is at first larger than 2 and therefore close to the typical RLCA fractal dimension, but then it gradually reverts to lower values not far from those typical of DLCA. This behavior is also exhibited at the lowest values of c explored, although the decay to DLCA is much slower. An interpretation of the results is presented on the basis of the available theories and simulation work on the cluster morphology in the intermediate regimes.

We study the structure and the time evolution of transient gels formed in colloid-polymer mixtures, by means of uorescence Confocal Scanning Laser Microscopy (CSLM). This technique is used in conjunction with novel colloidal silica particles containing a uorescent core. The confocal micrographs reveal that there exist large di erences in the local structure within a single system. At a given time there are regions where the gel structure consists of alternating patterns of colloid-rich and colloid-poor regions with a characteristic length scale and regions where the gel structure becomes disrupted by the formation of fractures. The number of fractures increases with time. It is speculated that the increase of the number of fractures leads to a weakening of the strength of the gel such that it eventually collapses under gravity.

Exceptionally large pore sizes have recently been reported for polyacrylamide gels grown in the presence of polyethylene glycol. We present small angle static light scattering measurements performed during the evolution of this peculiar gelation process. We give evidence that the large pores are generated by a microsegregation process caused by the competition between gelation and a phase separation of the polymers solution. The separation occurs via spinodal decomposition, and the size of the pores is determined by the stage at which the decomposition is stopped by the gelation process.

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