Colloidal Aggregation in Mixtures of Partially Miscible Liquids by Shear-Induced Capillary Bridges

We have studied shear-induced aggregation of colloidal silica particles suspended in a variety of partially miscible liquid mixtures. The shared characteristic of the investigated systems is that after liquid-liquid phase separation of the binary liquid mixtures one phase completely wets the particles. We have explored compositions where there are insufficient quantities of the particle wetting component to induce phase separation. As the proportion of the wetting component is increased, we find a significant concentration range where shear-induced aggregation takes place. The macroscopic characteristics of this phenomenon are demonstrated, for which observations were greatly facilitated by mostly using liquid pairs partially miscible at room temperature. Measurements revealing the adsorption of the minority component to colloidal particles show that capillary condensation between particles causes the observed aggregation. The likely microscopic features underlying this aggregation behavior are then discussed. Finally, the overall picture of these systems is sketched as a nonequilibrium liquid-liquid phase diagram, in which outside the binodal there is a region of shear-induced aggregation.