Molecular dynamics study on the conversion mechanism between momentum and energy components in gas-surface interaction
Authors: Yu Hang , Zhang Ran , Yang Fan , Li Hua ,   College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, China,   State Key Laboratory of Laser Interaction with Matter, Northwest Institute of Nuclear Technology, Xi’an 710024, China
Published: January 2021

With the rapid development of micro-nano technology and aerospace technology, researches related to rarefied gas flows have received more and more attention. For micro-/nanoscale systems and spacecraft in a rarefied environment, the reduction in the frequency of intermolecular collisions in the flow field makes the interaction between gas molecules and the solid surface develop into a major factor affecting the flow state. However, the mechanism of gas-surface interaction in rarefied flow has not been fully revealed due to its microscopic nature and physical complexity, and the existing simulation methods cannot accurately reflect the effect of this process on the flow state. In this paper, molecular beam method is adopted to simulate the scattering process of argon molecules on platinum surface, and the impacts of incident velocity, angle and wall roughness on the momentum and energy conversion mechanism are explored. By simulating the molecular scattering process under the two incident angles of <inline-formula><tex-math id="M3">\begin{document}$ 5^{\circ} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M3.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M4">\begin{document}$ 75^{\circ}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M4.png"/></alternatives></inline-formula>, the following conclusions are obtained. When colliding with the wall at an angle close to vertical, both components of the momentum of the gas molecules are lost. The normal energy transfers to the tangential direction, and when the molecular velocity is not less than 2.0, the transfer rate is not significantly affected by the incident energy of the molecule and the surface roughness. The total energy loss of gas molecules after scattering becomes significant with the increase of incident velocity, and it is not sensitive to changes of surface roughness. When the gas molecules are incident at <inline-formula><tex-math id="M5">\begin{document}$ 75^{\circ} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M5.png"/></alternatives></inline-formula>, the roughness of the surface has a significant impact on the conversion mechanism of molecular momentum and energy. After colliding with a smooth wall, the momentum and energy values of the gas molecules remain basically unchanged, only the direction of the momentum is reversed. The motion state of molecules is close to the mirror reflection, and the conversion between momentum and energy components is not obvious. The introduction of roughness enhances the degree of accommodation between gas molecules and metal surface, and promotes the transfer of molecular tangential momentum and kinetic energy to the normal direction. When incident at a large polar angle, as opposed to the small-angle cases, the total energy loss of molecules is not sensitive to changes of incident velocity, it goes up significantly with the surface roughness increasing. The research in this article not only explores the gas-surface interaction mechanism, but also provides a useful reference for the high-fidelity simulation of rare gas flow and the development of appropriate gas-surface interaction models.