The effect of enclosed water–ice pockets on porous silica cluster collisions

Abstract

Collisions between dust particles play an important role in the initial stages of the formation of rocky planets. In space, silica dust exhibits a porous structure due to its formation history. Outside the ice line, water–ice will be present on and in silica dust particles, such that collisions between water-ice-filled porous silica grains are relevant. Molecular dynamics simulations are used to simulate central collisions of such grains (diameter 44 nm), focusing on the analysis of bouncing events. While naked bulk silica particles bounce at high velocities, empty porous silica particles always stick since plastic deformation of the porous structure adds to energy dissipation during the collision. Ice-filled porous silica particles exhibit bouncing collisions already at smaller velocities than naked bulk silica particles of the same size. Collision-induced heating of the ice in the collision zone and also in the pores assists the bouncing process. The coefficient of restitution shows good agreement with available macroscopic models, if both the bouncing velocity and the energy dissipation during the collision are used as fitting parameters. Our results give insights into the agglomeration and bouncing of heterogeneous silica–ice particles, relevant for energetic collision processes outside of the ice line in protoplanetary and planetary systems.