Molecular dynamics simulations on scattering of single Ar, N2, and CO2 molecules on realistic surfaces

Abstract

The understanding of rarefied gas flows is of great interest since microelectromechanical systems are becoming ever smaller and intensification of all gas-related tasks in process engineering takes place on the microscale. With increasing rarefaction the influence of gas-surface interactions on momentum and energy transport becomes more significant producing rarefaction effects, such as wall slip. These gas-surface interactions are commonly described by means of accommodation coefficients that are input parameters for scattering kernels in rarefied gas flow simulations. In this context the tangential momentum accommodation coefficient (TMAC) is the most important input parameter since it allows to define the boundary conditions.In this work we present an approach in molecular dynamics simulations to determine the TMAC of a three-dimensional system on the basis of a few thousand single gas-surface collisions using a comparatively small super cell instead of relying on a large(r) system with a specific flow geometry which is the typical approach in literature. We demonstrate that TMAC results obtained by this method are in good agreement with results in relevant literature. The advantage of this method is the small number of atoms in the cell reducing the computational effort. Thus, the approach allows to analyze the interactions of realistic gas molecules (Ar, N2, and CO2) with realistic surfaces (Pt-, Si-, and hydroxylated Si-surfaces). This includes gas molecules with rotational degrees of freedom as well as surfaces with moving surface groups, whereby partial charges of the different atoms are taken into account. Furthermore, the presented approach allows for systematic investigation of each influencing parameter, such as temperature, electrostatic interactions, moving surface atoms, and interactions with chemical surface groups. Additionally, this approach is used to analyze the dependencies of angular scattering distributions on temperature and the incident angle of the impinging molecule.