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, ϕ, spanning the regime from a low volume fraction (ϕ = 0.25) to a highly filled regime (ϕ = 0.65) using dynamic oscillatory measurements. While for ϕ ≤ 0.4 with a fixed water volume fraction (ϕw) 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 ϕ ≥ 0.45 showed a two-step yielding behavior. On plotting elastic stress (G′γ) as a function of applied strain (γ), two distinct peaks, indicating two yield stresses, were observed. Both the yield stresses and storage modulus at low strains were found to increase with ϕ following a power law dependence. With increasing ϕw (0 – 0.08) at a fixed ϕ = 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 ϕw following a power law dependence, while the second yield stress was found to increase exponentially with ϕw. Transient steady shear tests were also performed. The single stress overshoot for ϕ ≤ 0.4 with ϕw = 0.06 reflected one-step yielding behavior. In contrast, for high ϕ (≥ 0.45) values with ϕw = 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.