10:00 AM - 10:15 AM
[MGI29-05] Dust monomer interactions explored by molecular dynamics simulations
Keywords:planetesimal formation, dust growth, molecular dynamics simulation
The initial stage of planet formation is the growth of dust. Dust is thought to be an aggregate composed of submicron-sized particles, which are called monomers. Dust aggregates are thought to grow by collisional sticking, but it has been suggested that dust fragmentation occurs as their size increases and that the dust falls to the central star before growing into planetesimals due to the gas drag. To understand dust growth, it is essential to know the details of the dust collision process, and numerical simulations have been performed to investigate the collision process, including the probability of collisional coalescence, the critical velocity of dust fragmentation, and the dust size, density, and number of fragments after collisional coalescence (e.g., Wada et al. 2008,2013; 2008,2012).
The numerical simulation of dust aggregate collision is an N-body calculation of monomers, which calculates the interaction between monomers in contact. The numerical simulations have used the Johnson-Kendall-Roberts (JKR) theory and the Dominik & Tielens (DT) model, which provide the forces and moments between contacting elastic spheres. Although it has been pointed out that the presence of molecules causes translational kinetic energy dissipation (Tanaka et al. 2012), these models do not consider molecular motion. Therefore, it is necessary to extend the existing models to include microphysics, such as molecular motion. Dust aggregate collisions depend well on the interaction model; for example, the interaction normal to the contact surface has been investigated to be related to the critical velocity of dust sticking (Wada et al. 2013). The rolling motion of monomers is also an important interaction because it causes the most energy dissipation in the dust aggregate collision (Arakawa et al. 2022). When two monomers roll on each other's surface, the contact surface is peeled off, and a new contact surface is formed, which results in energy dissipation. However, the amount of energy dissipation differs between the DT model (Dominik & Tielens 1995) and experiments (Heim et al. 1999), and its uncertainty remains in the contact model.
In this study, we investigate the head-on collision process and rolling motion between two monomers using molecular dynamics (MD) simulation, which is a method to simulate physical processes by calculating the N-body molecular system by constructing an object with a large number of molecules. We first performed MD simulations of head-on collisions of two monomers by preparing equal-mass monomers with varying sizes and impact velocities. As a result, we find that the stronger translational energy is dissipated for smaller sizes or higher impact velocities. We then consider that the dissipation is related to the contact surface pressure, and succeeded in reproducing the MD simulation results by constructing a new stress-dependent dissipation force model.
We next perform MD simulations of the rolling motion of the contacting monomer and investigate the time evolution of the rotational angle and angular velocity while varying the initial angular velocity. The time evolution of the rolling motion of the monomers can be divided into two motions. Initially, the monomers roll on each other's surfaces, and the angular velocity approaches zero, followed by an oscillatory motion with the contact surface fixed. From the rolling angle and the time variation of the angular velocity, we find that the magnitude of energy dissipation during rotation is equivalent to the value of the DT model. In addition, the oscillatory motion exhibits damping, which suggests energy dissipation. This damped oscillation is successfully reproduced by a model that introduced a dissipative torque proportional to the angular velocity.
I will present these results.
The numerical simulation of dust aggregate collision is an N-body calculation of monomers, which calculates the interaction between monomers in contact. The numerical simulations have used the Johnson-Kendall-Roberts (JKR) theory and the Dominik & Tielens (DT) model, which provide the forces and moments between contacting elastic spheres. Although it has been pointed out that the presence of molecules causes translational kinetic energy dissipation (Tanaka et al. 2012), these models do not consider molecular motion. Therefore, it is necessary to extend the existing models to include microphysics, such as molecular motion. Dust aggregate collisions depend well on the interaction model; for example, the interaction normal to the contact surface has been investigated to be related to the critical velocity of dust sticking (Wada et al. 2013). The rolling motion of monomers is also an important interaction because it causes the most energy dissipation in the dust aggregate collision (Arakawa et al. 2022). When two monomers roll on each other's surface, the contact surface is peeled off, and a new contact surface is formed, which results in energy dissipation. However, the amount of energy dissipation differs between the DT model (Dominik & Tielens 1995) and experiments (Heim et al. 1999), and its uncertainty remains in the contact model.
In this study, we investigate the head-on collision process and rolling motion between two monomers using molecular dynamics (MD) simulation, which is a method to simulate physical processes by calculating the N-body molecular system by constructing an object with a large number of molecules. We first performed MD simulations of head-on collisions of two monomers by preparing equal-mass monomers with varying sizes and impact velocities. As a result, we find that the stronger translational energy is dissipated for smaller sizes or higher impact velocities. We then consider that the dissipation is related to the contact surface pressure, and succeeded in reproducing the MD simulation results by constructing a new stress-dependent dissipation force model.
We next perform MD simulations of the rolling motion of the contacting monomer and investigate the time evolution of the rotational angle and angular velocity while varying the initial angular velocity. The time evolution of the rolling motion of the monomers can be divided into two motions. Initially, the monomers roll on each other's surfaces, and the angular velocity approaches zero, followed by an oscillatory motion with the contact surface fixed. From the rolling angle and the time variation of the angular velocity, we find that the magnitude of energy dissipation during rotation is equivalent to the value of the DT model. In addition, the oscillatory motion exhibits damping, which suggests energy dissipation. This damped oscillation is successfully reproduced by a model that introduced a dissipative torque proportional to the angular velocity.
I will present these results.