Viscous friction in mixed and boundary lubrication: fluid dynamics far from equilibrium

• Applicant: Dr. Kerstin Falk
• 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

Lubrication is the most relevant technique to reduce friction and wear in mechanical applications. The drive towards sustainable technologies has increased the demands on the performance of lubricants. Often the traditional empirical selection or design rules for liquid lubricants fail under the extreme conditions that occur in modern machinery. The current trends to down-size engines, gear boxes, turbines etc. lead to loading conditions that force these liquids into extreme nonequilibrium states (e.g., via high shear rates in molecular thin lubrication layers). In this context the predictive quantitative description of viscosities, i.e., the bulk liquid internal friction and dissipation, at various pressures, temperatures and shear rates plays a pivotal role in the choice of lubricants and, related to this, to the construction of mechanical devices.As part of the Research Unit „Reducing complexity of nonequilibrium systems“, the goal of this project is to find constitutive laws to describe the non-linear rheology of nanometer thin lubricant films which are suitable for continuum modeling of macroscopic tribological contacts. To this aim, the lubricant’s non-linear response to shear forces, which can be triggered by confinement, high shear rate or extreme pressure, will be studied by means of classical molecular dynamics simulations. Different nonequilibrium simulation methods will be employed to evaluate lubricant viscosity and inner friction via a combined top-down and bottom-up approach. Simulations of explicit shearing will serve to top-down investigate changes in both rheological and structural properties of lubricants to identify the relevant structure-property relations. The dissipation-corrected targeted molecular dynamics method will be used to extract the time-resolved microscopic friction force exerted on individual lubricant molecules with the goal to find a bottom-up description of inner friction within a lubricant.

• DFG Programme: Research Units
• Subproject of: FOR 5099: Reducing complexity of nonequilibrium systems
• Co-Investigators: Professor Dr. Michael Moseler; Dr. Steffen Wolf