A model for surface effects in slow collisions of icy grains

Abstract

A model is presented which describes the change of the relative velocity of slowly colliding solid grains in planar motion. It takes into account elasticity and viscosity of the material, effects of soft surface layers and adhesion, and friction between the surfaces. The equations of motion of the colliding bodies are solved numerically. Normal and tangential restitution coefficients are computed, and their dependence on the components of the relative velocity before impact is determined. The material constants in the examples have been chosen to reflect the behaviour of water ice as an important material in the outer solar system. Soft surface layers as well as surface energy lead to aggregation for low collision velocities, both normal and tangential relative velocity can be reduced to zero at the same time. A critical velocity exists below which rebound is inhibited. It can be related to the thickness and properties of the surface layer or the surface energy of the collision partners. The results are expected to be important for the evolution of the size distribution in granular media, in particular for the formation of larger bodies from centimetre-sized grains.