Molecular Origin of Driving-Dependent Friction in Fluids

The friction coefficient offluids may become afunction of the velocity at increased external driving. This non-Newtonian behavior is of general theoretical interest and of greatpractical importance, for example, for the design of lubricants.Although the effect has been observed in large-scale atomisticsimulations of bulk liquids, itstheoretical formulation andmicroscopic origin are not well understood. Here, we usedissipation-corrected targeted molecular dynamics, which pullsapart two tagged liquid molecules in the presence of surroundingmolecules, and analyze this nonequilibrium process via ageneralized Langevin equation. The approach is based on asecond-order cumulant expansion of Jarzynski's identity, which isshown to be valid forfluids and therefore allows for an exactcomputation of the friction profile as well of the underlying memory kernel. We show that velocity-dependent friction influidsresults from an intricate interplay of near-order structural effects and the non-Markovian behavior of the friction memory kernel. Forcomplexfluids such as the model lubricant C40H82, the memory kernel exhibits a stretched-exponential long-time decay, which reflects the multitude of timescales of the system.