Daniela Asnaghi

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 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 gel-like 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.

Transient gels in colloid–polymer mixtures studied with fluorescence confocal scanning laser microscopy

We study the structure and the time evolution of transient gels formed in colloid–polymer mixtures, by means of fluorescence Confocal Scanning Laser Microscopy (CSLM). This technique is used in conjunction with novel colloidal silica particles containing a fluorescent core. The confocal micrographs reveal that there exist large differences 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.

Light scattering studies of aggregation phenomena

We present recent results in the field of colloidal aggregation. We first present static light scattering data of aggregation in intermediate regimes between RLCA and DLCA. In particular, we show that working with dense solutions, new and unexpected features are observed. In fact, we measure intensity distributions strongly peaked at q≠0, which scale according to the same scaling law typical of spinodal decomposition. Furthermore, we show that by only varying the initial conditions, the appearance of the peak can be completely inhibited, and that this result provides new information about the intermediate regimes. Finally, we present depolarized dynamic light scattering data from aggregates grown in the RLCA regime. We show that using optically anisotropic spheres it is possible to determine in a unique experiment all the features characterizing an aggregation process. Moreover, we present evidence that the asymptotic form expected for the cluster mass distribution is attained in the very early phases of the aggregation process.

Small angle light scattering studies concerning aggregation processes

Substantial work, both experimental and theoretical, has been performed on aggregation processes in dense solutions. Aggregation driven by depletion interactions has recently been studied in a variety of systems. The phase diagrams that have been obtained are extremely complex, and they include aggregation and gelation phenomena.

Kerr effect from fractal aggregates of polystyrene microspheres

SummaryWe study the slow aggregation of aqueous suspensions of optically isotropic microspheres by means of transient electric birefringence. Aggregating clusters exhibit large Kerr constants, and consequently we provide direct experimental evidence that the clusters do posses form anisotropy. From the birefringence relaxation time we derive information on the average cluster size and on the size distribution, and determine their evolution during the aggregation process. Static and dynamic light scattering data confirm the validity of the transient electric birefringence results.

Colloidal aggregation in the intermediate regimes

We have studied salt induced aggregation of polystyrene latex in an intermediate region between DLCA and RLCA. The average cluster mass is found to grow according to a power law ( ?α tz) with an exponent z varying with the salt concentration C. Furthermore we find that at high C (DLCA) the fractal dimension d f is stable in time and equal to 1.65. On the contrary, for lower concentrations d f surprisingly decreases during each run, starting from values typical of RLCA. An interpretation of this trend is presented.