Collisions of spheres with wet and dry porous layers on a solid wall

A theory that combines Darcy's law for flow in porous media with inelastic solid mechanics, to model collisions of solid spheres with wet or dry porous layers placed on a solid wall, is found to closely describe the trends in data collected from particle-collision experiments. An exponential-hardening, stress–strain model is used for the porous layer, validated with dynamic mechanical analyzer measurements. Low-velocity collisions were performed in the low-gravity environment afforded during parabolic flight of a KC-135 aircraft, and also under normal gravity with a pendulum-based setup. Both theory and experiments show a decrease in the dry restitution coefficient with an increase in impact velocity, mainly due to increased inelastic losses in the porous material. The wet restitution coefficient is also found to decrease with an increase in the impact velocity, in contrast to the wet restitution coefficient for collisions of a solid sphere with a wet wall without a porous layer. Moreover, a critical impact velocity (below which no rebound occurs) is observed for wet collisions without a porous layer but not with a porous layer. The wet restitution coefficient is always found to be lower than the dry restitution coefficient, due to the viscous losses associated with fluid flow in addition to the inelastic losses associated with the porous layer.