Electrotunable friction with ionic liquid lubricants: how important is the molecular structure of the ions?

TL;DR

This study uses molecular dynamics simulations to explore how ionic liquid ion structures influence electric-field-controlled friction at the nanoscale, revealing significant structural and frictional changes driven by ion shape and charge distribution.

Contribution

It demonstrates the critical role of ion molecular structure in electrotuneable friction, highlighting ion-specific effects in ionic liquid lubrication under electric fields.

Findings

Electric field causes significant structural changes in ionic liquid films.

Ion structure influences electrostriction and electroswelling effects.

Friction force exhibits non-monotonic variation under electric fields.

Abstract

Using non-equilibrium molecular dynamics simulations and a coarse grained model of ionic liquids, we have investigated the impact that the shape and the intramolecular charge distribution of the ions have on the electrotuneable friction with ionic-liquid nanoscale films. We show that the electric-field induces significant structural changes in the film, leading to dramatic modifications of the friction force. Comparison of the present work with previous studies using different models of ionic liquids indicate that the phenomenology presented here applies to a wide range of ionic liquids. In particular, the electric-field-induced shift of the slippage plane from the solid-liquid interface to the interior of the film and the non-monotonic variation of the friction force are common features of ionic lubricants under strong confinement. We also demonstrate that the molecular structure of the ions plays an important role in determining the electrostriction and electroswelling of the confined film, hence showing the importance of ion-specific effects in electrotuneable friction.