(479c) Head-on and Oblique Collisions of Wet Particles

Wet granular flows arise in coating and agglomeration processes, pharmaceutical processes, avalanches, and other applications. To successfully model wet granular flows using discrete element methods (DEM), collisions rules between wet particles are needed. In this fundamental work, prior work on elastohydrodynamic collisions of wet particles by the author and others is modified and extended to simulate head-on and oblique collisions between two or three particles. Both squeezing and sliding lubrication forces are included during soft-sphere stages of approach and rebound. Solid-solid contact is treated as a nearly-instantaneous, hard-sphere collision, with a normal impulse that reverses relative motion as governed by the dry coefficient of restitution and a tangential impulse due to friction. For two-sphere oblique collisions, prior experimental results of agglomeration, rapid rebound, and stick-rotate-separate are demonstrated and shown to be in good agreement with the simulations. For three-sphere, head-on collisions, the outcomes of full agglomeration, full separation, and partial separation are also demonstrated and shown to be in good agreement with prior experiments. Wet coefficients of restitution are determined are shown to be nonzero only for a Stokes number (ratio of particle inertia to fluid viscous stress) above a critical value for each colliding pair. The collision rules developed for these two cases are then employed in simulations of three-sphere collisions using a framework that can be employed in DEM simulations of many particles. Where reasonable, analytical models are developed to simplify the description compared to numerical simulations.