Electrotunable lubricity with ionic liquids: the influence of nanoscale roughness

Abstract

The properties of ionic liquids can be modified by applying an external electrostatic potential, providing a route to control their performance in nanolubrication applications. Most computational studies to date have focused on the investigation of smooth surfaces. Real surfaces are generally inhomogeneous and feature roughness of different length scales. We report here a study of the possible effects that surface roughness may have on electrotunable lubricity with ionic liquids, performed here by means of non-equilibrium molecular dynamics simulations. In order to advance our understanding of the interplay of friction and substrate structure we investigate coarse grained models of ionic liquids confined in model surfaces with nanometer roughness. The friction is shown to depend on the roughness of the substrate and the direction of shear. For the investigated systems, the friction coefficient is found to increase with roughness. These results are in contrast with previous studies, where roughness induced reduction of friction was reported, and they highlight the strong sensitivity of the friction process to the structure of the surfaces. The friction force features a maximum at a specific surface charge density. This behaviour is reminiscent of the one reported in ionic liquids confined by flat surfaces, showing the generality of this physical effect in confined ionic liquids. We find that an increase of the substrate-liquid dispersion interactions shifts the maximum to lower surface charges. This effect opens a route to control electrotunable friction phenomena by tuning both the electrostatic potential and the composition of the confining surfaces.

Effects of Nanoscale Roughness on the Lubricious Behavior of an Ionic Liquid

“…In the range of small pressures, the friction coefficient was found to be smallest on the 275 µm −2 substrates (see, e.g., Figure 3a,b). MD simulations have shown that friction at contacts lubricated by ILs can both increase [ 20 ] or decrease [ 19 ] with nanoscale roughness, and have associated this change with the enhanced order or disorder of the ions in the confined films, respectively. Our measurements of the interfacial structure revealed a smaller number of well‐organized interfacial layers with an increase in roughness, suggesting the enhanced disorder of the multilayer films.…”

“…Interestingly, MD simulations have used this mechanism to describe the effect of topology on friction at rough contacts lubricated by ILs. Specifically, Mendonça et al [ 19 ] showed that roughness (induced by conical protrusions) led to a decrease in the charge ordering of the IL film and thereby in friction; in contrast, David et al [ 20 ] showed that the presence of periodic ridges and valleys caused an increase in IL ordering and friction.…”

Mechanisms of Electrotunable Friction in Friction Force Microscopy Experiments with Ionic Liquids

“…In this case, we found a previous works by increasing systematically the complexity of the models. 24,27,29,30 No doubt, a comparison between the results of these two approaches, coarse-grained vs.…”

Controlling Friction With External Electric or Magnetic Fields: 25 Examples

“…Example 24: Impact of Surface Roughness on Ionic Lubricant Tribological Performance (David et al, 2017) David et al (2017) studied the impact of surface roughness on electrotuning by means of ionic liquids, noting that most prior studies examined atomically smooth surfaces while in practice most contacting surfaces were rough. Their studies were performed by means of non-equilibrium molecular dynamics for a range of increasingly rough surfaces.…”