Initial dust is called a monomer with < μm size, and monomers form aggregates. Dust growth larger than cm-size is not understood well. Numerical simulations of aggregate collisions have investigated the critical velocity of dust compression and disruption, and the dust size evolution (e.g., Wada et al. 2013; Suyama et al. 2012). They used the JKR theory to calculate the monomer interactions. However, dust collision experiments showed that the bouncing velocity is larger than the theoretical value (e.g., Poppe et al. 2000, Wada et al. 2008). It is suggested that this difference is due to molecules (Krijt et al. 2013; Tanaka et al. 2015). The JKR theory does not consider microscopic physics. Therefore, we need a contact model considering molecules. In this case, Molecular Dynamics (MD) simulation is an effective method, which solves the N-body problem of molecules.
In this study, we use MD simulations to reproduce monomer collisions. We simulated the head-on collisions of monomers and investigated the dependence of the coefficient of restitution on the monomer size, impact velocity, and temperature. The results show that the coefficient of restitution increases with the monomer size and that it has a peak at ~50 m/s. Monomer deformation weakens the repulsive force and decreases the coefficient of restitution. We also confirmed that the bulk kinetic energy converted to molecular kinetic energy and potential energy.
Next, we extended the contact model by adding dissipative forces to the JKR theory to reproduce MD simulations. We considered the dissipative force model varying the dependence on contact radius and relative velocity. We found that a dissipative force model proportional to the cube of the relative velocity and the 3/2 power of the contact radius reproduced the MD calculation well. However, another energy dissipation is required to reproduce the MD simulations for high-velocity collisions.