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.

Molecular dynamics-based prediction of boundary slip of fluids in nanochannels

“…It is often taken to be unity for most of the practical engineering conditions. Although some investigations have demonstrated that TMAC is sensitive to flow and solid conditions, many factors which affect the TMAC are not well understood (Finger et al 2007). Thus, the TMAC is usually obtained by comparing the experimental or simulation data of velocity profiles at solid boundary to the solutions of N-S equation with the specified velocity-slip boundary conditions (Colin et al 2004;Cao et al 2005).…”

Molecular dynamics simulations of shear-driven gas flows in nano-channels

“…As a result, several experimental and numerical studies were conducted for determination of TMAC. Experimental research ranged from molecular beam experiments to gas flow experiments in the slip and early transition flow regimes (Arkilic et al 2001;Bentz et al 1997Bentz et al , 2001Goodman and Wachman 1976;Gronych et al 2004;Rettner 1998;Sazhin et al 2001), while the numerical research includes MD simulations of single-molecule interactions with crystalline surfaces (Arya et al 2003;Chirita et al 1993Chirita et al , 1997Finger et al 2007), two-and three-dimensional MD simulations for Kn \ 1 flows (Cao et al 2005;Li 2010, 2011), and coupled MD/ DSMC models (Yamamato et al 2006).…”

Nanochannel flow past permeable walls via molecular dynamics

“…For example, comparing with the experimental results, Finger et al found that the TMAC (ς) can be greater than unity because of the backscattering of reflected gas molecules on the solid surface. 51 It can be also seen in Fig. 9 that the slip length on the permeable surface is smaller than that on the smooth surface, and even the negative slip appears on the permeable surface for small K n. Sun et al also reported that the fluid molecules that can be firmly trapped in the concaves due to the rough surface and strong fluid-wall interaction may cause locking boundary in the velocity profile.…”