Collisions and flows of solid particles covered with thin liquid layers occur in natural phenomena such as avalanches, mud slides, and pollen capture. They are also prevalent in industrial processes such as coating pharmaceutical powders, fertilizer pellets, and chocolate candies for storage and controlled release of these products. To better understand these phenomena, it is necessary to predict the conditions under which the particles stick together or bounce apart after colliding. Similarly, it is important to know when a dense suspension of wet particles will flow when subject to forces and when it will instead lock or jam. This award will use laboratory experiments and computer simulations to elucidate the fundamental physics of collisions of wet particles and to develop predictive models of flows involving wet particles.

The goals of this award are (1) to develop new microphysical models for collisions of two or more wet particles, (2) to utilize new experimental approaches to verify and refine the models, (3) to incorporate the microphysical models in a discrete element method for simulating the flows of many wet, colliding particles, and (4) to train students who are well prepared as future researchers and leaders. The modeling approach will employ a natural coordinate system for each interacting pair of particles, resolved into normal and tangential motions and including lubrication, capillary, centrifugal and solid-contact forces. It will determine microphysical outcomes such as sticking and bouncing, so that macroscale phenomena of wet granular flow, including jamming and agglomeration, may be predicted. The experiments will use an air table and high-speed camera to observe collisions of wetted spheres mounted on hockey-type pucks, thereby avoiding limitations imposed by strings in pendulum-based experiments. The outcome of this award will allow researchers to effectively model wet granular flows and provide new understanding of the role of liquid forces in substantially changing the behavior of these dense particulate systems.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.