Tuning suspension rheology using capillary forces

When a small amount (less than 1%) of a second immiscible liquid is added to the continuous phase of a suspension, the rheological properties of the admixture are dramatically altered and it can change from a fluid-like to a gel-like state. The rheological properties can be modified through changes in the particle size, temperature induced changes to the interfacial tension, and through the addition of surfactants. These changes are experimentally investigated in this current work, concentrating on the capillary (non-wetting) state with additional experiments in the pendular (wetting) state. The yield stress scales with the interfacial tension and reciprocal radius (Gamma/r) as expected due to the capillary nature of the attractive force leading to network formation and gelation. The addition of surfactants leads to a drastic reduction in the yield stress and viscosity, which is attributed to a partial trapping of the secondary fluid within surfactant micelles.