Molecular-dynamics study on characteristics of energy and tangential momentum accommodation coefficients

Gas-surface interaction is studied by the molecular dynamics method to investigate qualitatively characteristics of accommodation coefficients. A large number of trajectories of gas molecules colliding to and scattering from a surface are statistically analyzed to calculate the energy (thermal) accommodation coefficient (EAC) and the tangential momentum accommodation coefficient (TMAC). Considering experimental measurements of the accommodation coefficients, the incident velocities are stochastically sampled to represent a bulk condition. The accommodation coefficients for noble gases show qualitative coincidence with experimental values. To investigate characteristics of these accommodation coefficients in detail, the gas-surface interaction is parametrically studied by varying the molecular mass of gas, the gas-surface interaction strength, and the molecular size of gas, one by one. EAC increases with increasing every parameter, while TMAC increases with increasing the interaction strength, but decreases with increasing the molecular mass and the molecular size. Thus, contradictory results in experimentally measured TMAC for noble gases could result from the difference between the surface conditions employed in the measurements in the balance among the effective parameters of molecular mass, interaction strength, and molecular size, due to surface roughness and/or adsorbed molecules. The accommodation coefficients for a thermo-fluid dynamics field with a temperature difference between gas and surface and a bulk flow at the same time are also investigated.

Figure 1

A numerical coordinate system.

Figure 2

The initial incident velocity vectors towards colliding points for gas molecules are plotted. They are sampled from the Maxwell distribution for EAC and the shifted Maxwell distribution in the $x$ direction for TMAC.

Figure 3

Typical convergences of EAC and TMAC against the number of scattering samples.

Figure 4

The calculated EAC and TMAC for He, Ne, Ar, and Kr are plotted against the molecular weight.

Figure 5

The calculated EAC and TMAC, changing only the molecular mass $m_g$ artificially.

Figure 6

The calculated NMAC (left axis) and adsorption probability (right axis), changing only the molecular mass $m_g$ artificially.

Figure 7

The calculated EAC and TMAC by changing only the interaction strength between gas and surface ${\epsilon }_{gs}$ artificially.

Figure 8

The calculated NMAC (left axis) and adsorption probability (right axis) by changing only the interaction strength between gas and surface ${\epsilon }_{gs}$ artificially.

Figure 9

The calculated EAC and TMAC, changing only the size of a gas molecule ${\sigma }_{gs}$ artificially.

Figure 10

The calculated NMAC (left axis) and adsorption probability (right axis), changing only the size of a gas molecule ${\sigma }_{gs}$ artificially.

Figure 11

The scattering polar angle distributions of TMAC for the smallest ${\sigma }_{gs}=2.0\AA$ and for the largest ${\sigma }_{gs}=3.5\AA$ conditions.

Figure 12

The EAC and TMAC for He, Ne, Ar, and Kr are obtained by using the shifted Maxwell distribution with the bulk flow $u\ne 0$ and the temperature difference between gas and surface $T_g\ne T_s$, and they are plotted against the molecular weight.

Figure 13

The calculated NMAC (left axis) and adsorption probability (right axis) by using the shifted Maxwell distribution with the bulk flow $u\ne 0$ and the temperature difference between gas and surface $T_g\ne T_s$.