11:00 AM - 11:15 AM
[MGI33-07] Interaction between dust monomers by Molecular Dynamics simulation
Keywords:planetesimal formation, process of dust growth, molecular dynamics simulation
Dust in protoplanetary disks is the material for planets. Dust is considered to be monomers, spheres of about 0.1 μm in early phase, and to be an aggregate of monomers. It is considered that dust particles smaller than cm are directly coalesced by intermolecular forces, while objects larger than km are coalesced by self-gravity. However, the growth process of intermediate sizes is still unknown, and the possibility of growth by direct coalescence or by some instability is being studied. For these studies, it is important to know the evolution of dust properties such as size and density. The dust growth process has been investigated by numerical simulations and laboratory experiments. And in the numerical simulation, the monomer interactions are modeled and calculated based on a contact theory that is called JKR theory.
However, some differences between the simulation results and the experimental results have been suggested. In the collision experiments of monomers and aggregates, it has been pointed out that the critical velocity of bounce is larger than the theoretical value and that there is a temperature dependence on the critical velocity (e.g., Poppe et al., 2020, Gundluch & Blum, 2015). It has been pointed out that the difference between numerical calculations and laboratory experiments is due to the energy dissipation to molecular motion during the dust collision process (Krijt et al. 2013; Tanaka et al. 2015). However, the JKR theory does not take into account microscopic physics such as energy dissipation due to molecular motion. Therefore, it is necessary to extend the JKR theory to incorporate microscopic physics.
Molecular dynamics (MD) simulation is an effective method to investigate physical phenomena by reducing them to the molecular level. In MD simulation, an object is composed of many molecules, and the motion of each molecule is solved. There is a previous study that investigated the particle collision process using MD simulation, but the model parameters are outside the range of the dust growth process in the protoplanetary disk. In the disk, the monomer size is about 0.1 μm, and the collision velocity is thought to be less than 100 m/s. However, previous studies have dealt with particles of 1-10 nm size or with collision velocity higher than 100 m/s (e.g., Takato & Sen, 2014; Takato et al., 2015; Nietiadi et al., 2017). Also, these previous studies dealt with head-on collisions at specific temperatures. Oblique collision and the temperature dependence of the collision process have not yet been investigated.
In this study, we use MD simulations to simulate monomer collisions and clarify the interactions between monomers. First, we reproduced the head-on collision by changing the collision conditions and environment, such as monomer size, collision velocity, and temperature, and investigated the forces acting on the monomers. We found that the larger the monomer size, the smaller the critical bounce velocity. This size dependence agrees with Tanaka et al. (2012). The JKR theory predicts that the monomer will bounce back when the collision velocity in the initial state exceeds the critical velocity. However, in the present study, a coalescence phenomenon, which cannot occur in the JKR theory, was observed in higher velocity collisions. This coalescence is thought to be caused by the change of collision energy into internal motion and binding energy. We also found that the higher the temperature, the larger the oscillation in the monomer interaction. This is due to an oscillatory mode of the monomer radius caused by the crystal structure of the monomer. In addition, the distance between the centers of mass of the monomers at maximum compression became smaller at higher temperatures. This is thought to be due to the softening of the monomer at higher temperatures.
However, some differences between the simulation results and the experimental results have been suggested. In the collision experiments of monomers and aggregates, it has been pointed out that the critical velocity of bounce is larger than the theoretical value and that there is a temperature dependence on the critical velocity (e.g., Poppe et al., 2020, Gundluch & Blum, 2015). It has been pointed out that the difference between numerical calculations and laboratory experiments is due to the energy dissipation to molecular motion during the dust collision process (Krijt et al. 2013; Tanaka et al. 2015). However, the JKR theory does not take into account microscopic physics such as energy dissipation due to molecular motion. Therefore, it is necessary to extend the JKR theory to incorporate microscopic physics.
Molecular dynamics (MD) simulation is an effective method to investigate physical phenomena by reducing them to the molecular level. In MD simulation, an object is composed of many molecules, and the motion of each molecule is solved. There is a previous study that investigated the particle collision process using MD simulation, but the model parameters are outside the range of the dust growth process in the protoplanetary disk. In the disk, the monomer size is about 0.1 μm, and the collision velocity is thought to be less than 100 m/s. However, previous studies have dealt with particles of 1-10 nm size or with collision velocity higher than 100 m/s (e.g., Takato & Sen, 2014; Takato et al., 2015; Nietiadi et al., 2017). Also, these previous studies dealt with head-on collisions at specific temperatures. Oblique collision and the temperature dependence of the collision process have not yet been investigated.
In this study, we use MD simulations to simulate monomer collisions and clarify the interactions between monomers. First, we reproduced the head-on collision by changing the collision conditions and environment, such as monomer size, collision velocity, and temperature, and investigated the forces acting on the monomers. We found that the larger the monomer size, the smaller the critical bounce velocity. This size dependence agrees with Tanaka et al. (2012). The JKR theory predicts that the monomer will bounce back when the collision velocity in the initial state exceeds the critical velocity. However, in the present study, a coalescence phenomenon, which cannot occur in the JKR theory, was observed in higher velocity collisions. This coalescence is thought to be caused by the change of collision energy into internal motion and binding energy. We also found that the higher the temperature, the larger the oscillation in the monomer interaction. This is due to an oscillatory mode of the monomer radius caused by the crystal structure of the monomer. In addition, the distance between the centers of mass of the monomers at maximum compression became smaller at higher temperatures. This is thought to be due to the softening of the monomer at higher temperatures.