(337f) Dynamics and Stability of Liquid-Bound Particle Clusters in Flow

Capillary suspensions are ternary systems consisting of a solid particulate phase, a continuous phase liquid, and a second immiscible liquid. These materials are commonly encountered in a wide array of applications including food formulations, printed electronics, and 3D print inks. Despite recent progress, we lack a complete understanding of the stability and dynamics of liquid-bound clusters in flow. In this talk, I will present an experimental and theoretical analysis of the dynamics of liquid-bound particle clusters in extensional flow, which reveals a new set of criteria for capillary suspension stability in flow. Cluster breakup is first characterized for a two-particle doublet in extensional flow using a Stokes trap, an automated flow technique that allows for precise manipulation of freely suspended objects in flow. The time required for cluster breakup is determined as a function of capillary number Ca and meniscus volume. Experiments are complemented by an analytical model that accounts for capillary forces, hydrodynamic drag, and hydrodynamic interactions (HI) acting on the particles. In all cases, results from the analytical model are found to be in good agreement with experiments, but only when accounting for full interparticle HI. A linear stability analysis reveals a critical capillary number Ca_c for cluster breakup that depends on the initial cluster separation, and these observations are rationalized by constructing a phase diagram for cluster stability in flow. Cluster relaxation experiments are also performed by deforming particle clusters in flow, followed by flow cessation before breakup and observation of cluster relaxation dynamics under zero-flow conditions. Moving beyond the case of two-particle doublets, the dynamics of multi-particle liquid-bound clusters (N = 3, 4, 5, 6, ...) are studied in extensional flow. By systematic observation of cluster reorientation, deformation, and breakup, the experiments reveal that multi-particle clusters generally reduce to the two-particle doublet during a successive stepwise breakup process in extensional flow. Initial efforts at modeling multi-particle cluster dynamics and breakup using Stokesian dynamics simulations will also be discussed. Overall, our work provides a new understanding of the deformation dynamics of liquid-bound particle clusters in non-equilibrium flows.