PHYSICAL REVIEW E 96, 013116 (2017) Molecular-dynamics study on characteristics of energy and tangential momentum accommodation coefficients Hiroki Yamaguchi, 1 , * Yu Matsuda, 2 and Tomohide Niimi 1 1 Department of Micro-Nano Mechanical Science and Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, Aichi, 464-8603, Japan 2 Institute of Materials and Systems for Sustainability, Nagoya University, Furo-cho, Chikusa, Nagoya, Aichi, 464-8603, Japan (Received 11 April 2017; revised manuscript received 15 June 2017; published 25 July 2017) 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. DOI: 10.1103/PhysRevE.96.013116 I. INTRODUCTION Along with the recent development of microscale and nanoscale technologies, microscale flows are gaining signifi- cance. In such microscale flows, especially in gaseous flows, the Knudsen number, defined by the ratio of the molecular mean free path to the characteristic length of a system, is an important parameter. Because of the size of the flow field, the Knudsen number becomes large; the flow is called a high Knudsen number flow. In high Knudsen number flows, the collision number of molecules to wall surfaces cannot be neglected compared to that between molecules in the fluid. In addition, the surface-to-volume ratio of the fluid is large due to the small size. Therefore, the gas-surface interaction plays an important role in microscale gaseous flows. The amounts of the velocity slip and temperature jump phenomena in the slip flow regime are determined by the gas-surface interaction [1]. To represent the gas-surface interaction, the accommo- dation coefficient is often employed. The accommodation coefficient is defined as α = ζ i − ζ r ζ i − ζ s , (1) where ζ is a physical property of molecules and the overline represents the averaging over all concerned molecules. The subscripts i, r , and s denote the incident condition, the reflected condition, and the condition fully accommodated to a surface. This coefficient represents the mean degree of accommodation of each physical property of molecules to a surface. The size of * hiroki@nagoya-u.jp the accommodation coefficient is known to differ by physical properties of molecules, like energy and momentums. The accommodation coefficient on the energy is called the energy accommodation coefficient (EAC) [2,3], and it is defined as α E = E i − E r E i − E s , (2) where E denotes the energy of molecules. This coefficient is related to heat transfer between gas and surface. If there is no flow, namely the static condition, the energy of the molecules can be expressed by using temperature T as E = 2kT , where k is the Boltzmann constant, and this coefficient can be expressed by T instead of E, which is called the thermal accommodation coefficient. EAC has been experimentally measured from the heat transfer through gas confined between two surfaces with different temperatures [2–7]. A typical measurement technique is the low-pressure method: The accommodation coefficient is deduced from the measurement of the heat flux through gas in the free-molecular flow regime as a function of pressure, because the heat flux is proportional to the number density of molecules, namely pressure, and the accommodation coefficients. It is important to note that the heat flux is measured without flow in the measurement system. The tangential momentum accommodation coefficient (TMAC) [8,9] is also often employed to understand resistance to flow by surfaces. Since the condition fully accommodated to a surface p t,s = 0, it is defined as α t = p t,i − p t,r p t,i , (3) where p t denotes the tangential momentum in the flow direc- tion of molecules. TMAC has been experimentally measured 2470-0045/2017/96(1)/013116(8) 013116-1 ©2017 American Physical Society