Molecular origin of driving-dependent friction in fluids

TL;DR

This study investigates the microscopic origins of velocity-dependent friction in fluids, revealing that structural effects and non-Markovian memory kernels contribute to non-Newtonian behavior, with implications for lubricant design.

Contribution

The paper introduces a novel molecular dynamics approach using a generalized Langevin equation to exactly compute friction profiles and identify their microscopic origins.

Findings

Velocity-dependent friction arises from structural and memory effects.

Memory kernels in complex fluids decay as stretched exponentials.

The method provides insights into non-Newtonian fluid behavior.

Abstract

The friction coefficient of fluids may become a function of the velocity at increased external driving. This non-Newtonian behavior is of general theoretical interest as well as of great practical importance, e.g., for the design of lubricants. While the effect has been observed in large-scale atomistic simulations of bulk liquids, its theoretical formulation and microscopic origin is not well understood. Here we use dissipation-corrected targeted molecular dynamics, which pulls apart two tagged liquid molecules in the presence of surrounding molecules and analyzes this nonequilibrium process via a generalized Langevin equation. The approach is based on a second-order cumulant expansion of Jarzynski's identity, which is shown to be valid for fluids and therefore allows for an exact computation of the friction profile as well of the underlying memory kernel. We show that velocity-dependent friction in fluids results from an intricate interplay of near-order structural effects and the non-Markovian behavior of the friction memory kernel. For complex fluids such as the model lubricant \alkane, the memory kernel exhibits a stretched-exponential long-time decay, which reflects the multitude of timescales of the system.