The frictional force on sliding drops

TL;DR

This study uses molecular dynamics simulations to investigate the frictional forces on sliding liquid drops, revealing that these forces are generally constant over time, contrasting with the static and dynamic friction distinction in solids.

Contribution

It provides direct measurement of frictional forces on liquid drops via simulations, challenging previous assumptions and experimental limitations.

Findings

Frictional forces are mostly constant over time during drop sliding.

Inertial effects influence force measurements at motion initiation.

Aging effects may occur for unconfined drops.

Abstract

The dynamic frictional force between solid surfaces in relative motion differs from the static force needed to initiate motion, but this distinction is not usually thought to occur for liquid drops moving on a solid. Recent experiments [Gao, et al., Nature Phys. {\bf 114}, 191 (2018)] have challenged this view, and claim to observe an analog of solid-on-solid friction for sliding drops. We use molecular dynamic simulations to investigate the forces that moving liquids exert on solids in several situations. In contrast to the indirect techniques required in laboratory experiments, the forces involved in friction are directly accessible in these calculations. We find that, aside from possible inertial effects due to the abrupt initiation of motion and aging effects for unconfined drops, the frictional forces are constant in time.