5:15 PM - 7:15 PM
[MGI30-P03] Temperature dependence of head-on collisions between dust monomers explored by molecular dynamics simulations
Keywords:Planet formation, Dust, Molecular dynamics
Collisional sticking between dust is the initial stage of planet formation. Dust is considered to be an aggregate of submicron-sized particles, which are called monomers, and simulation studies have been conducted treating dust as a powder. Dust collision processes have been investigated using powder simulations (e.g., Wada et al., 2013), in which the interactions between monomers in contact are calculated. The JKR model (Johnson et al., 1971), which is a model of contact interaction between elastic spheres, is used for the interaction, but at small scales such as submicron, viscous effects due to molecular motion must be taken into account. Molecular dynamics simulations, which simulate physical processes by reducing them to molecular motion, are effective for this problem. Previous molecular dynamics simulation studies have investigated collision processes with large impact velocities, such as 1000 m/s for small particles of about 10 nm (Nietiadi et al., 2020). However, for these collision conditions, monomers are too small, and collisional velocities are too large compared to the environment of protoplanetary disks. Therefore, we have simulated collision processes between low-velocity submicron monomers at extremely low temperatures. We evaluated the energy dissipation due to viscous effects by comparison with the JKR model and developed a new model (Yoshida et al., 2024).
We have clarified the collisional process at low temperatures, but the temperature in protoplanetary disks varies with location and has a temperature distribution from 100 K to 1000 K. Although the physical properties of monomers are expected to change with temperature, the effects of temperature have not been clarified. Therefore, we used molecular dynamics simulations to study the temperature dependence of the dust monomer collision process.
We prepared monomers with a radius of about 30 nm and varied the temperature of the monomers from 0 K to about 1/3 of the triple point. The Young's modulus of the monomers tends to decrease at higher temperatures as a result of Hertz contact simulations. The surface energy is based on an empirical expression, which is derived from the temperature dependence of surface tension using molecular dynamics simulations (Baidakov et al., 2007), which shows a decreasing behavior of surface energy at higher temperatures. In our study, the molecules in the monomer have a face-centered cubic crystal structure, and we performed simulations for 20 runs with different crystal orientations to investigate the interparticle forces and coefficient of restitution (CoR). As a result, we found that the interparticle forces at all temperatures are in good agreement with the JKR model during the loading phase, but are smaller than the JKR model during the unloading phase, which is the same as the case of low temperature. This difference between the loading and unloading phases is called hysteresis and suggests that the kinetic energy of the monomer motion is dissipated. Next, the CoR tends to be smaller at higher temperatures, indicating that the higher the temperature, the more the kinetic energy of the monomers is dissipated. In addition, the impact velocity dependence of the CoR has a peak at all temperatures, and a significant decrease in the CoR is observed for the impact velocity where plastic deformation occurs. However, the temperature dependence of energy dissipation cannot be explained by the temperature dependence of monomer properties, and the temperature dependence is considered to exist in the viscosity effect.
In this presentation, we will present these results and discuss their influence on the planet formation process.
We have clarified the collisional process at low temperatures, but the temperature in protoplanetary disks varies with location and has a temperature distribution from 100 K to 1000 K. Although the physical properties of monomers are expected to change with temperature, the effects of temperature have not been clarified. Therefore, we used molecular dynamics simulations to study the temperature dependence of the dust monomer collision process.
We prepared monomers with a radius of about 30 nm and varied the temperature of the monomers from 0 K to about 1/3 of the triple point. The Young's modulus of the monomers tends to decrease at higher temperatures as a result of Hertz contact simulations. The surface energy is based on an empirical expression, which is derived from the temperature dependence of surface tension using molecular dynamics simulations (Baidakov et al., 2007), which shows a decreasing behavior of surface energy at higher temperatures. In our study, the molecules in the monomer have a face-centered cubic crystal structure, and we performed simulations for 20 runs with different crystal orientations to investigate the interparticle forces and coefficient of restitution (CoR). As a result, we found that the interparticle forces at all temperatures are in good agreement with the JKR model during the loading phase, but are smaller than the JKR model during the unloading phase, which is the same as the case of low temperature. This difference between the loading and unloading phases is called hysteresis and suggests that the kinetic energy of the monomer motion is dissipated. Next, the CoR tends to be smaller at higher temperatures, indicating that the higher the temperature, the more the kinetic energy of the monomers is dissipated. In addition, the impact velocity dependence of the CoR has a peak at all temperatures, and a significant decrease in the CoR is observed for the impact velocity where plastic deformation occurs. However, the temperature dependence of energy dissipation cannot be explained by the temperature dependence of monomer properties, and the temperature dependence is considered to exist in the viscosity effect.
In this presentation, we will present these results and discuss their influence on the planet formation process.