Packing of Simulated Friction Modifier Additives under Confinement

Abstract

Molecular dynamics simulations have been conducted to obtain detailed information regarding molecular structure and packing of surfactant-like "friction modifier" (FM) chains adsorbed to two confining surfaces under sliding conditions. The simulations are interpreted via the density profile, position-dependent fluctuations in the density profile, and parallel, interlayer position correlation functions. Heterogeneous FM structures were obtained. The adsorbed FM chains were found to form semi-ordered monolayers perpendicular to the atomically well-defined surface, with distinct segment positions in the density profile. More fluid layers with broader density profiles and larger fluctuations were formed by some unoriented FM chains oriented preferentially parallel to the surface, between the opposing monolayers. Segment position correlations parallel to the surface in the adsorbed layers were found at separations up to 30 A. These packings persisted for longer than 4 ns under sliding conditions that ranged from 0 to 7.5 m s(-1) (0 to 0.075 A ps(-1)). Monolayer structures were observed to remain unchanged under a wide range of normal pressures, with the corresponding changes in volume occurring in the gap between monolayers or between a monolayer and trapped fluid.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…The normal load, fluid layer thickness, and additive surface concentration dependencies agreed with those measured experimentally using the SFA [230]. Greenfield and Ohtani also performed NEMD simulations of the same systems and found that the OFM film structure remained virtually unchanged under a wide range of normal pressures (3.2−22 MPa) and sliding velocities (0.5− 7.5 m·s −1 ) [231].…”

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

“…All molecules are connected to a heat bath. While this has been shown to affect the rheology of the confined fluids at shear rates aboveγ = 0.2 (in reduced Lennard-Jones units), the low shear rates here, a maximum ofγ(ǫ/mR 2 ) 1/2 ≈ 0.004, suggest that such artifacts should be negligible [8,9]. Each simulation step corresponds to 0.5 fs, and the time integral due to the motion of the atoms and molecules is calculated by the reversible reference system propagation algorithm (rRESPA) [10] method.…”

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

“…All the molecules are connected to a heat bath. While this has been shown to affect the rheology of confined fluids at shear rates above  = 0.2 (in reduced Lennard-Jones units), the low shear rates here, a maximum of  (ε/mR 2 ) 1/2 ≈ 0.004, suggest that such artifacts should be negligible [17,18]. Each simulation step corresponds to 0.5 fs and the time integral due to the motion of the atoms is calculated by the rRESPA (Reversible Reference System Propagator Algorithm) [19] method.…”