Dual yielding in capillary suspensions

Rheological measurements were performed to examine the yielding behavior of capillary suspensions prepared by mixing cocoa powder as dispersed phase, vegetable oil as the continuous primary fluid, and water as the secondary fluid. Here, we investigated the yielding behavior of solid-fluid-fluid systems with varying particle volume fraction, I center dot, spanning the regime from a low volume fraction (I center dot = 0.25) to a highly filled regime (I center dot = 0.65) using dynamic oscillatory measurements. While for I center dot 0.4 with a fixed water volume fraction (I center dot (w) ) of 0.06 as the secondary fluid, capillary suspensions exhibited a single yield point due to rupturing of aqueous capillary bridges between the particles, while capillary suspensions with I center dot 0.45 showed a two-step yielding behavior. On plotting elastic stress (G(') gamma) as a function of applied strain (gamma), two distinct peaks, indicating two yield stresses, were observed. Both the yield stresses and storage modulus at low strains were found to increase with I center dot following a power law dependence. With increasing I center dot (w) (0 - 0.08) at a fixed I center dot = 0.65, the system shifted to a frustrated, jammed state with particles strongly held together shown by rapidly increasing first and second yield stresses. In particular, the first yield stress was found to increase with I center dot (w) following a power law dependence, while the second yield stress was found to increase exponentially with I center dot (w) . Transient steady shear tests were also performed. The single stress overshoot for I center dot 0.4 with I center dot (w) = 0.06 reflected one-step yielding behavior. In contrast, for high I center dot ( 0.45) values with I center dot (w) = 0.06, two stress overshoots were observed in agreement with the two-step yielding behavior shown in the dynamic oscillatory measurements. Experiments on the effect of resting time on microstructure recovery demonstrated that aggregates could reform after resting under quiescent conditions.