Wall slip in the molecular dynamics simulation of thin films of hexadecane

Abstract

A molecular dynamics simulation of a thin liquid film as it is sheared between two planar walls is reported. The model liquid is composed of linear chain molecules of hexadecane (C16H34) with intramolecular architecture such as bond stretching, angle bending and dihedral potentials included in the model. Designing a model that can mimic the planar shear flow enables us to study important questions on the effects of the wall properties on the slip between the liquid film and the wall. Different properties of the wall such as wall density, wall stiffness and wall–fluid interaction strength have been studied to determine the slip between the wall and fluid. The slip has been investigated for strong and weak adsorbing surfaces at various shear rates. The results emphasize the importance of adsorption on the degree of slip. The dependence of slip on the film thickness is also demonstrated.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…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].…”

“…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.…”

“…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.…”

The unique properties of the solid-like confined liquid films: A large scale molecular dynamics simulation approach

“…The boundary slip is dependent on various factors, such as the wettability of the solid surface [32] , surface roughness [33] , the nanoscale vapor bubbles generated at the liquid-solid interface [34] , shear rate [35] , the electric double layer [36] , the shape and the arrangement of the liquid molecules [37] , variation of viscosity near the solid walls [38] , as summarized in Fig.1. For the slip of the confined liquid, Jabbarzadeh et al used a united atoms model in MD simulations to study the influence of wall properties on the slip between hexadecane film and the walls [39,40] . Pit et al probed the velocity of hexadecane within 80 nm from the solid wall and gave the direct experimental evidence of slip [41] .…”

Effect of surface roughness on rate-dependent slip in simple fluids

“…On the other hand, an excessive penetration of the wall atoms into the fluid phase reduces the slip velocity for soft thermal walls [21]. Nevertheless, the effect of thermal surface roughness on the slip length in the shear-rate-dependent regime was not systematically explored even for atomically smooth walls.…”