Molecular dynamics characterization of thin film viscosity for EHL simulation

Abstract

Molecular simulations were used to characterize changes in lubricant viscosity that may occur during thin film elastohydrodynamic lubrication (EHL). Molecular dynamics simulations were performed at variable wall speed and film thickness such that the effects of both parameters could be evaluated. Using this approach it was found that the viscosity of thin films under large shear is subject to both shear thinning and oscillation with film thickness. A composite model was developed that incorporated both effects. The expected impact that this model might have on an EHL interface was evaluated using a continuum simulation. An overall decrease in viscosity with some oscillation near the interface edges was predicted due to the molecularly modeled thin film effects.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…For example, Martini et al [160] used NEMD simulations to extract the film thickness and velocity dependence of the viscosity of n-decane confined and sheared between atomically-smooth gold surfaces. This information was then fed into a full numerical solution for EHL problems [161] to assess the impact of these viscosity changes on the predicted film thickness.…”

“…This method is relatively straightforward to implement, but requires careful planning to establish which regions of phase space can be reliably probed with MD to provide useful inputs for the continuum models. For example, Martini et al [160] and Savio et al [168,182] respectively used local viscosity and slip information obtained using MD simulations to improve continuum predictions of film thickness and friction for strongly confined systems. These are examples of phenomena that can be studied at a single relevant scale and integrated at a larger scale in a hierarchical manner.…”

From Continuous to Molecular Scale in Modelling Elastohydrodynamic Lubrication: Nanoscale Surface Slip Effects on Film Thickness and Friction

“…2 can then be modified to take into account nanoscale slip, by integrating this expression. Several recent examples in the literature [31,32] propose to integrate into continuous models lubricant viscosity laws which have been established via MD simulations. Because the shear rate identified in Sect.…”

Hybrid Diffusion: An Efficient Method for Kinetic Temperature Calculation in Molecular Dynamics Simulations of Confined Lubricant Films

“…The Lennard Jones coefficients were gold taken from Ref. [24] whereas AMBER96 was used for modeling all other interactions as previously. The simulation time step was also chosen as 1 fs and the total duration was 15 ns (15,000,000 time steps).…”