Effect of gas adsorption on momentum accommodation coefficients in microgas flows using molecular dynamic simulations

Abstract

The tangential momentum accommodation coefficient (TMAC), usually used in slip boundary conditions in micro-gas flows, is reported to be always less than unity and greatly influenced by temperature and the strength of gas-wall interactions. According to the definitions of accommodation coefficients, a proper statistical algorithm in non-equilibrium molecular dynamics method was described and verified. In planar Poiseuille gas flow in a smooth microchannel, the TMAC were calculated considering both the effects of temperature and gas-wall interaction. In the simulation processes, more gas molecules began to be adsorbed near walls under the condition of stronger gas-wall interaction and lower temperature. The gas adsorption resulted in a longer gas-wall interaction time so that the TMAC increased. While the gas-wall interaction became much stronger, more and more gas molecules were adsorbed to form an explicit layer above the wall. The full coverage of gas molecules on the wall prevented further adsorption; therefore the TMAC did not keep on increasing as the interaction strength continued to increase. Meanwhile, the normal momentum accommodation coefficient (NMAC) was also calculated according to the definition. In the isothermal flow, the average gas momentum normal to the wall was in complete accommodation with the wall, and the NMAC was almost unity in smooth micro channels.

Molecular Momentum Transport at Fluid-Solid Interfaces in MEMS/NEMS: A Review

“…The nonlinear flows inside the Knudsen layer are still difficult to describe, whether for the constitutive relations or for the effective mean free path and viscosity. The constitutive relations from the numerical results of the Boltzmann equations are often lack of physical explanations, and the relation between the effective mean free path and viscosity are not valid when the density (pressure) are not constant in the microchannels [ 116 , 117 ]. Therefore, researches on nonlinear slip models, especially for large Kn gas flows, are still challenging.…”

“…The gas molecules are easier to be adsorbed near the wall under conditions where the temperature is low [ 186 ] or the gas-wall interactions are strong [ 117 ]. When more and more molecules are adsorbed and molecular layers are formed near the walls, the molecular behaviors are consequently changed, and the momentum transport at gas-solid interfaces is dominated by both the gas-gas and gas-wall interactions.…”

Three-Dimensional Molecular Dynamic Study on Accommodation Coefficients in Rough Nanochannels

“…According to the Einstein model, wall atoms vibrate harmonically at the FCC lattice sites in x, y, and z directions, and the spring constant [20,21] is (8) in which m w is the atomic mass of the wall and is the reduced Planck constant. The Einstein temperature of platinum is T E = 180 K so that k = 179.5 N/m.…”

“…From the preceding definitions, the key point to calculate the accommodation coefficients is to distinguish between the incident and reflected molecules during the gas-wall interactions, which are detected and determined in the computational processes [20,21]. In MD simulations of gas flows, the short range force dominates in both the gas-gas and gas-wall interactions so that the intermolecular forces are cut off.…”

Molecular Dynamics Simulation of Nanoscale Channel Flows with Rough Wall Using the Virtual-Wall Model

“…e masses of argon and platinum atoms are 6.64 × 10 −26 kg and 3.24 × 10 −25 kg, respectively. ese parameters have been validated in previous studies [25,26]. e simulation box is set to be 40.9 nm × 17.1 nm × 40.9 nm in x, y, and z directions.…”