Molecular dynamics Simulation (Nanoscience And Nanotechnology)
Molecular dynamics simulations of shear-driven gas flows in nano-channels
Microfluidics and Nanofluidics May 2011
doi: 10.1007/s10404-011-0827-0
Keywords Wall force field effects, Tangential momentum accommodation coefficient, Shear stress, Rarefied gas flow
Using the recently developed smart wall molecular dynamics algorithm, shear-driven gas flows in nano-scale channels... more Using the recently developed smart wall molecular dynamics algorithm, shear-driven gas flows in nano-scale channels are investigated to reveal the surface– gas interaction effects for flows in the transition and free molecular flow regimes. For the specified surface properties and gas–surface pair interactions, density and stress profiles exhibit a universal behavior inside the wall force penetration region at different flow conditions. Shear stress results are utilized to calculate the tangential momentum accommodation coefficient (TMAC) between argon gas and FCC walls. The TMAC value is shown to be independent of the flow properties and Knudsen number in all simulations. Velocity profiles show distinct deviations from the kinetic theory based solutions inside the wall force penetration depth, while they match the linearized Boltzmann equation solution outside these zones. Results indicate emergence of the wall force field penetration depth as an additional length scale for gas flows in nano-channels, breaking the dynamic similarity between rarefied and nano-scale gas flows solely based on the Knudsen and Mach numbers.
Smart Wall Model for Molecular Dynamics Simulations of Nanoscale Gas Flows
Communications in Computational Physics May 2010
doi:10.4208/cicp.2009.09.118
Keywords: Rarefied gas flow, kinetic theory, surface effects, shear driven flow.
Three-dimensional molecular dynamics (MD) simulations of gas flows confined within nano-scale channels are... more Three-dimensional molecular dynamics (MD) simulations of gas flows confined within nano-scale channels are investigated by introduction of a smart wall model that drastically reduces the memory requirements of MD simulations for gas flows. The smart wall molecular dynamics (SWMD) represents three-dimensional FCC walls using only 74 wall molecules. This structure is kept in the memory and utilized for each gas molecule surface collision. Linear Couette flow of argon at Knudsen number 10 is investigated using the SWMD utilizing Lennard-Jones potential interactions. Effects of the domain size on the periodicity boundary conditions are investigated using three-dimensional simulations. Domain sizes that are one mean-free-path long in the periodic dimensions are sufficient to obtain domain-size independent MD solutions of nano-scale confined gas flows. Comparisons between the two- and three-dimensional simulations show the inadequacy of two-dimensional MD results. Three-dimensional SWMD simulations have shown significant deviations of the velocity profile and gas density from the kinetic theory based predictions within the force penetration region of the walls.
Equilibrium molecular dynamics studies on nanoscale-confined fluids
Microfluid Nanofluid April 2011
doi: 10.1007/s10404-011-0794-5
Keywords: Wall-force field, Pressure, Normal stresses, Surface and particle virial, rarefied gas, dense gas, liquid, Thermodynamic state
Fluid behavior within nanoscale confinements is studied for argon in dilute gas, dense gas, and liquid states.... more Fluid behavior within nanoscale confinements is studied for argon in dilute gas, dense gas, and liquid states. Molecular dynamics simulations are used to resolve the density and stress variations within the static fluid. Normal stress calculations are based on the Irving–Kirkwood method, which divides the stress tensor into its kinetic and virial parts. The kinetic component recovers pressure based on the ideal-gas law. The particle– particle virial increases with increased density, whereas the surface– particle virial develops because of the surface-force field effects. Normal stresses within nanoscale confinements show anisotropy primarily induced by the surface force field and local variations in the fluid density near the surfaces. For dilute and dense gas cases, surface-force field that extends typically 1 nm from each wall induces anisotropic normal stress. For liquid case, this effect is further amplified by the density fluctuations that extend beyond the force field penetration region. Outside the wall force field penetration and density fluctuation regions, the normal stress becomes isotropic and recovers the thermodynamic pressure, provided that sufficiently large force cut-off distances are used in the computations.

