Boundary lubrication: Toward a continuum theory of sheared fluids under strong confinement

• Applicants: Dr. Kerstin Falk; Professor Dr. Michael Moseler
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Computer-Aided Design of Materials and Simulation of Materials Behaviour from Atomic to Microscopic Scale
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431945604

This project aims at a comprehensive continuum description of complex tribological contacts in the boundary lubrication regime, which is characterized by extreme non-equilibrium states of the lubricant due to very high pressures, shear rates, and ultra-strong confinement. For this purpose, large-scale atomistic reference models of tribological contacts including surface roughness will be subject to complexity reduction towards a continuum model. In order todescribe the systems’ dynamics in a continuum framework, constitutive relations for the relevant properties, e.g. lubricant viscosity and wall slip velocity, are extracted from high-throughput parametric molecular dynamic simulations studies of smaller representative volume elements. The constitutive relations are then implemented in a continuum model based on the Reynolds lubrication equations and validated by comparison with the large-scale reference systems. The challenges are in formulating physics-based constitutive equations for the nonequilibrium states of the lubricant, and in applying a continuum to a lubrication regime where traditional hydrodynamics is expected to fail. Two research directions, which complement each other, are pursued. One direction is to investigate the extent to which a generalized Reynolds approach quantitatively describes the large-scale molecular dynamics when moving from large gap heights towards a lubricant monolayer. Main questions are which physical effects must be taken into account in the continuum description of extremely confined lubricant films and how non-empirical constitutive laws can be derived for this. The second research direction considers the transition from an initially dry contact to a renewed monolayer lubrication from a neighboring reservoir. The molecular dynamics of the lubricant in the reservoir and the associated driving forces for re-lubrication of the dry contact are studied and the extent to which a reduction to a continuum description can be achieved here. It will be essential that the avoidance of stress singularities at the edges of the reservoir and the 2D dynamics in the monolayer that builds up are correctly reproduced. The final goal of this project is to combine results from both research directions to a transient continuum model that captures all essential features in large-scale atomistic simulations of transient starvation and re-lubrication events.

• DFG Programme: Research Units
• Subproject of: FOR 5099: Reducing complexity of nonequilibrium systems
• Co-Investigator: Dr. Steffen Wolf