Macroscopic No-Slip Boundary Condition Confirmed in Full Atomistic Simulation of Oil Film

Abstract

The no-slip boundary condition widely used in the macroscopic fluid mechanics has not been explained from the molecular level. This letter describes all atom molecular dynamics simulation to study boundary slip of hydrocarbon oil film under shear of a submicron thickness confined between solid walls. The large time-space scale simulation under the realistic interactions of fluid atoms, solid-fluid interaction and sliding speed has shown the no-slip of the oil film. The difference between the nanoscale film and the submicron thick film is explained from the viewpoint of the anisotropic viscosity rise in the vicinity of the solid wall and the correlation length of the momentum up to several tenths of nanometers. The results of the present simulation coincide with the experiments of fluid flow through nanometer-scale channels.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…This final observation is critical since most experimental surfaces will contain fractal roughness, even down to the atomic scale [178], which is likely to discourage boundary slip on conventional surfaces. Moreover, large-scale NEMD simulations of > 100 nm n-hexane films between atomically-smooth iron surfaces under realistic EHL conditions (F N = 1 GPa,  = 10 5 s −1 ) confirmed the suitability of a macroscopic no-slip boundary condition for this particular system [170]. NEMD simulations of n-alkanes (C 3 −C 10 ) confined between surfaces with patterned stick and slip regions by Savio et al [179] showed that such heterogeneous surfaces can lead to cavitation under sufficient confinement.…”

“…These simulations have shown that the slip length generally increases with increasing sliding velocity and pressure [168][169][170][171]. At very high sliding velocity, the slip length asymptotes towards a constant value [57].…”

“…Inflexible molecules, which can conform to the surface only with a greater expense of conformational energy generally exhibit more slip [177]. Conversely, the slip length decreases with increasing surface-fluid attraction strength and film thickness [169][170][171], and is virtually eliminated in the presence of atomic-scale surface roughness, which frustrates molecular layering [153,154]. This final observation is critical since most experimental surfaces will contain fractal roughness, even down to the atomic scale [178], which is likely to discourage boundary slip on conventional surfaces.…”

“…This can lead to large reductions in friction compared to the Couette case [146]. Slip has been commonly observed in NEMD simulations of thin fluid films confined between atomically-smooth surfaces [57,[168][169][170][171]. Many of these studies have quantified the slip length, which is defined as an extrapolated distance into the wall where the tangential velocity component vanishes (see Fig.…”

Molecular origin of limiting shear stress of elastohydrodynamic lubrication oil film studied by molecular dynamics

“…The film thickness is calculated from the average of the difference between the highest and lowest fluid molecules in the fluid film. The simulation code is parallelized and tested on a massively parallel computer, which was described in our previous work [2,3].…”