Restitution coefficient models for collisions of airborne particles and drops

Restitution models for solid and liquid particles colliding with surfaces or other particles have high relevance to a variety of aerosol systems, where the net change in translation and angular velocities (before and after impact) of colliding particles is dependent on the normal and tangential restitution coefficients. These restitution coefficients are influenced by a number of different effects, including elastic rebound, viscous fluid damping, viscoelastic damping within the solid, adhesive forces, and plastic deformation of the particle material. A full restitution description includes determination of quantities such as the particle sticking velocity, viscoelastic restitution, plastic velocity, and the impact Stokes number. For oblique collisions, tangential restitution is dominated by particle rolling for near-normal collisions and by particle sliding for grazing collisions, respectively. For solid particles, surface roughness and non-spherical particle shapes lead to stochastic restitutions and increased conversion of translational energy into par-ticle spin energy. For droplets impacting each other or a wall, additional complexity rises through surface tension and viscosity effects (via the Weber and Ohnesorge numbers), which can deter-mine whether bounce, coalescence or break-up occurs. While the models presented herein can reasonably predict restitution in many cases, more research is needed to better understand the sticking velocity, plastic velocity, and tangential friction coefficient for solid particles and the dissipation and wetting effects for liquid particles.