Nonequilibrium molecular dynamics simulations of confined fluids in contact with the bulk

Abstract

A nonequilibrium molecular dynamics ͑MD͒ simulation study is reported of the structural and rheological properties of confined n-decane between two Au͑111͒ surfaces in contact with its bulk under constant normal loads or constant heights. In the constant-load MD simulations, it was observed that fluid molecules were squeezed out of the pore continuously in a single simulation upon compression, whereas fluid molecules in the bulk were soaked into the pore when applied normal load was released. Pore separation depends on bulk pressure under the same normal load and approaches a steady value as normal load increases. In the constant-height simulations, density, velocity, and orientational profiles of the confined film were accumulated along the Z ͑perpendicular to the walls͒ and Y ͑parallel to the walls and finite due to the bulk͒ directions. These distributions are not uniform not only along the Z direction but also along the Y direction, particularly for weak fluid-wall interactions. The shear-thinning behavior and ''slip'' boundary conditions were also studied in this work. Even though the shear-thinning behavior was reported by several previous studies, the number of particles was fixed and the bulk condition was unknown in those simulations. The simulation geometry employed in this work is closer to that of surface-force apparatus ͑SFA͒ experiments and of engine lubricating systems where confined liquid is in contact with its bulk.

Molecular dynamics characterization of thin film viscosity for EHL simulation

“…This method of stress calculation has been found to yield the same results as the Method of Planes [24] if the plane is chosen to be at the position of the walls [22,25]. Another popular method is the Irving-Kirkwood relationship.…”

“…However, only the wall temperature was held constant while the fluid temperature was allowed to increase due to viscous heating. This commonly used approach was employed because it is analogous to the experimentally observed conduction of frictional heat out of a lubricating fluid through solid [12] 0.3930 47.0 14.0 Au [22] 0.2655 529.28 197.0 walls [2,15,16,22]. Simulating the effect of conduction is particularly important in this research because conduction is thought to be the primary mechanism for heat transfer out of a thin EHL film [23].…”

Boundary-controlled barostats for slab geometries in molecular dynamics simulations

“…(1) is referred to as a harmonic oscillator (HO) barostatting scheme. This method has been used in its original form [25] and with the addition of a velocity-dependent damping term to the wall equations of motion [26], where the barostat parallels a damped harmonic oscillator (DHO).…”

Simulation of low speed 3D nanochannel flow

“…This is why the reported velocities of all published papers are more than 5 m/s. For example, in the nanojet problems studied by Eggers (2002) and Moseler (2000), the flow velocity is 400 m/s; the flow velocities are higher than 5 m/s in many other references (Koplik and Banavar 1995;Zhang 2001;Walther et al 2004;Arya et al 2003;Mi et al 2003;Zhang and Todd 2004). However, in reality, the flow velocities in micro-and nanochannels are much smaller than 1 m/s.…”