Desorption-induced fragmentation of silica aggregates

Critical Casimir forces for colloidal assembly

Critical Casimir forces attract increasing interest due to their opportunities for reversible particle assembly in soft matter and nano science. These forces provide a thermodynamic analogue of the celebrated quantum mechanical Casimir force that arises from the confinement of vacuum fluctuations of the electromagnetic field. In its thermodynamic analogue, solvent fluctuations, confined between suspended particles, give rise to an attractive or repulsive force between the particles. Due to its unique temperature dependence, this effect allows in situ control of reversible assembly. Both the force magnitude and range vary with the solvent correlation length in a universal manner, adjusting with temperature from fractions of the thermal energy, k B T, and nanometre range to several ten kT and micrometer length scale. Combined with recent breakthroughs in the synthesis of complex particles, critical Casimir forces promise the design and assembly of complex colloidal structures, for fundamental studies of equilibrium and out-of-equilibrium phase behaviour. This review highlights recent developments in this evolving field, with special emphasis on the dynamic interaction control to assemble colloidal structures, in and out of equilibrium.

Quantitative Description of Nonhomogeneous Mass Distribution in Controlled Aggregation and Fragmentation of Hydrated Colloids

The destabilization of hydrated colloidal latex particles in an aqueous electrolyte solution was found to produce aggregates of unusual mass frequencies. Analysis of the mass frequency curves revealed the presence of at least three aggregate populations growing at different rates and allowed determination of the variation with time of the number and weight average masses of these populations and their relative concentrations within the suspension. The fragmentation of aggregates of large mass was induced by decreasing the ionic strength of the suspending medium at constant pH. This instantaneous fragmentation and the subsequent slow breakup of the resulting fragments were investigated by following the variation with time of the suspension characteristics. The aggregation mechanism presented the features of a reversible diffusion-limited process. On the other hand, since electrical interactions were of short range in the concentrated electrolyte medium and hydration forces could be considered to oppose attractive forces, it was possible to attribute the formation of three aggregate populations at least in part to the unusually low energy of the attractive forces involved in aggregate formation. Model structures were designed for each population: aggregates containing only linear interparticle bonds were assigned to population 1, aggregates containing double or triple interparticle contact points to population 2, and aggregates containing multiple contact points or circular links to population 3. Thus, the combined effects of internal stability and collision efficiency, both dependent on aggregate structure, could be assumed to be responsible for the well-differentiated growth of three distinct populations. The preservation of these populations during fragmentation was attributed to their different internal stabilities. In fact, the structure of large aggregates appeared to be very complex since their fragments could be ranked among different populations. Finally, the absence of self-similarity of the mass frequencies determined at different times led us to suppose that the aggregation mechanism would escape analysis on the basis of the mean field approach using Smoluchowski's equation with the usual form of the kernel. Copyright 1998 Academic Press.