Rotating Doublets of Wetted, Solid Particles

Collisions between wetted particles appear in nature in examples such as pollen capture, landslides, and avalanche flow.  Furthermore, wetted collisions have industrial applications in granulation, coagulation, and water filtration.  Unlike previous investigations that focused on head-on collisions between multiple, wetted particles or oblique collisions between a particle and a wetted wall, oblique particle-particle collisions form a doublet that rotates.  Therefore, the centrifugal force plays an essential role as to whether the particles will stick or rebound.  Using both experiments and theory, we have explored the possible outcomes associated with oblique collisions between wetted particles for the case of Stokes flow in the liquid layer. The wet particles are observed to stick for low Stokes numbers and low impact angles (i.e., nearly head-on collisions).  However, at low Stokes numbers and high impact angles, particles initially stick, rotate for a long time (relative to the collision time of a head-on collision), but then rebound due to centrifugal forces. Bouncingcollisionswith short collision times have been observed for high Stokes numbers;in this case, both centrifugal forces and elastohydrodynamics play a significant role in the dynamics of the collision.  The model predictions of the critical Stokes number, the normal restitution coefficient, and the rotation angle are consistent with experiments in which parameters such as oil viscosity and ball materials are varied.