(317j) An Experimental and Numerical Investigation of Collisions of Wet Particles

Collisions and flows of wet particles (i.e., solid particles coated with thin liquid layers) occur in a variety of natural (e.g., avalanches, river sediments, landslides, and pollen capture on wet leaves) and industrial (e.g., filtration, slurry transport, particle coating, and agglomeration) processes. These granular materials constitute a significant portion of products and raw materials in the U.S. When particles or surfaces are coated with a thin layer of fluid, collision outcomes differ significantly from dry collisions, as they involve the coupling of fluid dynamics with solid mechanics. Most of the experimental studies involving normal and oblique collisions between a sphere and a plane have generally involved dropping dry spheres onto wetted surfaces from different heights or have relied on pendulums for investigations involving collisions of two wetted spheres. On the other hand, experimental studies involving three wetted spheres have been limited solely to head-on colinear collisions. Thus, there exists a gap in literature, for experimentally studying the dynamics of oblique collisions of three wetted spheres. Here, we experimentally and theoretically investigate oblique collision dynamics of both two and three wetted spheres. We perform experiments using a combined air table and pendulum launching apparatus and validate them numerically with the hard-sphere model of RH Davis, Physics of Fluids, 2023. The physics of the collision process is primarily governed by the Stokes number, St, measuring the inertia of the colliding spheres relative to viscous forces in the thin fluid layer between them. For oblique collisions of two wet spheres, we observe agglomeration at low Stokes number, contact-bounce phenomenon at high Stokes number and stick-rotate-separate phenomenon at intermediate Stokes number where the two spheres initially stick together due to viscous losses, rotate as a doublet due to conservation of angular momentum, and then slowly separate due to centrifugal forces. On the other hand, collisions between three wet spheres exhibit an even wider variety of collision outcomes, including full agglomeration of all spheres at low Stokes number, full separation at high Stokes number, and partial agglomeration where one sphere separates from the remaining pair at intermediate Stokes number. More importantly, we show that when the Stokes number is less than a critical value, the particles sticks together with a zero value of wet coefficient of restitution (ratio of relative normal separation velocity and relative normal approach velocity); whereas separation occurs when the Stokes number exceeds the critical value, with a non-zero value of the wet coefficient of restitution, that increases with increasing Stokes number. The results from our study will help to understand interactions between wetted particles at the microscopic level, which will help determine the principal physical mechanisms that are crucial for comprehending diverse systems, ranging from pharmaceutical manufacturing to interstellar grains, at a macro-level.