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

Abstract

Using molecular dynamics simulations and a coarse-grained model of ionic liquids, we study mechanisms of electrotunable friction measured in friction force microscopy experiments, where only one layer of ionic liquid (IL) is present between the tip and electrode (substrate). We show that the variation of the friction force with the electrode surface charge density is determined by the regime of motion of the confined IL relative to the substrate and tip. The latter depends on the strengths of the ion-substrate and iontip interactions and on the commensurability between the characteristic ion dimensions and lattice spacings of the substrate and tip surfaces. Related with those factors, our simulations predict two strictly different scenarios for the variation of the friction force with the electrode surface charge. Revealing mechanisms of frictional energy dissipation in nanoscale IL films offers a way for controlling friction by tuning ionsubstrate interactions and electrical polarization of sliding surfaces. ''switched'' on and off in situ, by polarizing its surface relative to the reference electrode. Friction phenomena in RTILs have also been studied through computer simulations performed at various levels of system idealization.

Structural Forces in Mixtures of Ionic Liquids with Organic Solvents

“…This idea has been demonstrated in recent experiments and simulations. 28,29,30,32,33,34 Although investigations of the role of solvents on the properties of ionic liquids are currently attracting a lot of attention, 9,35,36 we still lack a fundamental understanding of the most basic question, namely, how the structure and composition of confined nanoscale RTIL layers change with addition of organic solvent. Until now, only few studies addressed this problem focusing on the impact of the solvent on structural and frictional forces.…”

Investigation of the tribological performance of ionic liquids in non-conformal EHL contacts under electric field activation

“…Moreover, as stated in Ref. [30], the ionic liquid evidently acquires a solid-like structure due to a strong attraction at a charged electrode surface, what results in an increased friction force caused by bonding forces of excited and arranged ion-pair layers, what is typical for ionic liquids between charged surfaces [31]. In contrast, the unexcited state of the ionic liquid has a liquid-like structure, and therefore the bonding forces are significantly smaller.…”

Controlling Friction With External Electric or Magnetic Fields: 25 Examples

“…Example 23: Molecular Dynamics Simulations of Friction Force Microscopy of Ionic Liquid Lubricated Contacts (Fajardo et al, 2015;Pivnic et al, 2018) Fajardo et al employed molecular dynamics simulations and a coarse-grained model of ionic liquids, to study mechanisms of electrotunable friction for ionic liquids confined between planar substrates. Their model employed parameters to simulate the ionic liquid 1-butyl-3-methylimidazo-lium hexafluoro phosphate, BMIM+PF6-confined between mica surfaces.…”

“…(2) "The nanoscale film will exhibit the electrostriction/ electroswelling effect, which will be asymmetric with respect to the sign of the surface charge, as long as the cations and anions have different molecular shapes, chemical structures, and intramolecular charge distributions." Pivnic et al (2018) also employed molecular dynamics simulations and a course-grained model to examine a tipsubstrate geometry lubricated by one layer of an ionic liquid (Pivnic et al, 2018). Their work revealed the dependence of the friction force on the electrode surface charge density to be dependent on the motion of the confined liquid relative to the substrate and tip.…”