Dynamics of meniscus-bound particle clusters in extensional flow*

Capillary suspensions are three-phase mixtures containing a solid particulate phase, a continuous phase liquid, and a second immiscible liquid. Capillary suspensions have a diverse array of applications in materials science including 3D printing, porous materials, and food formulations.  Despite recent progress, the micromechanics of capillary-bound clusters in external flow is not well understood. Here, we study the dynamics of meniscus-bound particle clusters in extensional flow using a Stokes trap, which is an automated flow-based technique that allows for precise control of fluid flow for manipulating and studying freely suspended particle clusters. We observe how clusters rearrange in extensional flow, quantify the capillary number Ca required for rupture, and determine steady-state configurations where particles maintain a finite separation without rupture. Initial experiments focus on the simple case of a two-particle doublet and further extend to multiparticle clusters. Cluster relaxation experiments are performed to observe the time required for clusters to return to their original state starting from a stretched configuration after the flow is stopped. In all cases, experiments are complemented by a mathematical model involving capillary and hydrodynamic drag forces acting on the particles, and good agreement is obtained with experiments. We further aim to determine a critical capillary number Ca_cr versus relative viscosity phase diagram for particle cluster dynamics, which is analogous to the classic Grace curve for droplet breakup.