Molecular Dynamics Simulation of Adsorbent Layer Effect on Tangential Momentum Accommodation Coefficient

Abstract

The tangential momentum accommodation coefficient (TMAC) is used to improve the accuracy of fluid flow calculations in the slip flow regime where the continuum assumption of zero fluid velocity at the surface is inaccurate because fluid “slip” occurs. Molecular dynamics techniques are used to study impacts of individual gas atoms upon solid surfaces to understand how approach velocity, crystal geometry, interatomic forces, and adsorbed layers affect the scattering of gas atoms, and their tangential momentum. It is a logical step in development of techniques estimating total TMAC values for investigating flows in micro- and nano-channels or orbital spacecraft where slip flow occurs. TMAC can also help analysis in transitional or free molecular flow regimes. The impacts were modeled using Lennard-Jones potentials. Solid surfaces were modeled approximately three atoms wide by three atoms deep by 40 or more atoms long face centered cubic (100) crystals. The gas was modeled as individual free atoms. Gas approach angles were varied from 10 to 70deg from normal. Gas speed was either specified directly or using a ratio relationship with the Lennard-Jones energy potential (energy ratio). To adequately model the trajectories and maintain conservation of energy, very small time steps (approximately 0.0005 of the natural time unit) were used. For each impact the initial and final tangential momenta were determined and after many atoms, TMAC was calculated. The modeling was validated with available experimental data for He gas atoms at 1770m∕s impacting Cu at the given angles. The model agreed within 3% of experimental values and correctly predicted that TMAC changes with angle. Molecular Dynamics results estimate TMAC values from high of 1.2 to low of 0.25, generally estimating higher coefficients at the smaller angles. TMAC values above 1.0 indicate backscattering, which numerous experiments have observed. The ratio of final to initial momentum, when plotted for a gas atom sequence spaced across a lattice cycle typically follows a discontinuous curve, with continuous portions forward and backscattering and discontinuous portions indicating multiple bounces. Increasing the energy ratio above a value of 5 tends to decrease TMAC at all angles. Adsorbed layers atop a surface influence the TMAC in accordance with their energy ratio. Even a single adsorbed layer can have a substantial effect, changing TMAC +∕−20%. The results provide encouragement to continue model development and next evaluate gas flows with Maxwell temperature distributions involving numerous impact angles simultaneously.