Structure and dynamics of ionic liquids under shear flow

TL;DR

This study explores how ionic liquids behave under shear flow, emphasizing the importance of electrostatic treatment for accurate dynamic predictions and identifying a critical shear rate affecting structural and dynamic properties.

Contribution

It provides a detailed comparison of electrostatic treatments in simulations and identifies a shear rate threshold where ionic liquid behavior changes significantly.

Findings

Electrostatic treatment affects dynamics more than structure.

A critical shear rate marks the onset of non-equilibrium behavior.

Dynamic heterogeneity decreases with increasing shear rate.

Abstract

We investigate the intrinsic behavior of ionic liquids under shear flow, using a coarse-grained model of C4mim-PF6 as a prototypical example. The importance of long-ranged electrostatics is assessed as a function of shear rate by comparing Ewald and reaction field treatments. An appropriate comparison is achieved through the implementation of the proper Lees-Edwards boundary conditions within the ESPResSo++ simulation software. Our results demonstrate that while structural properties are relatively insensitive to the electrostatic treatment, the more accurate treatment via the Ewald approach is essential for studies of dynamics, in particular, at lower shear rates. Furthermore, we identify a critical shear rate beyond which structural and dynamical properties begin to deviate from equilibrium behavior, while remaining largely unchanged below this threshold. Finally, we demonstrate that the dynamic heterogeneity of the liquid decreases as a function of increasing shear rate, which can be primarily explained by the faster dynamics induced by the shear flow. These results hold relevance for investigations of process-dependent properties of ionic-liquid-based materials.