Effect of primary particle morphology on the structure of gels formed in intense turbulent shear.

We study the effect of primary particle morphology on intense shear-induced gelation without adding electrolytes. The primary particles are composed of a rubbery core grafted with a polystyrene shell. Depending on the shell-to-core mass ratio, the core can be partially covered by the shell, leading to strawberry-like morphology. It is found that at a fixed core mass the fractal dimension of the clusters constructing the gel decreases (i.e., more open cluster structure) as the shell mass increases, until reaching a plateau. The SEM pictures of the gels reveal that the structure variations are due to the occurrence of partial coalescence among particles, which decreases as the shell mass increases. In the region where the fractal dimension reaches a plateau, the coalescence is negligible. The conversion of the primary particles to gels is incomplete and increases as the extent of coalescence decreases. This is related to the fact that the smaller the extent of coalescence, the larger the cluster size. Thus, because of its cubic dependence on the cluster size, the aggregation rate increases as the extent of coalescence decreases, leading to increased conversion. It is therefore evident that the key parameter controlling the gel structure and the particle conversion is the core surface coverage by the shell. To further verify this conclusion, we have carried out the shear-induced gelation of another set of particles with varying core mass. It is found that the only parameter that can well correlate the values of the fractal dimension and particle conversion from the two sets of particles is the core surface coverage.