Friction of Ionic Liquid–Glycol Ether Mixtures at Titanium Interfaces: Negative Load Dependence

Abstract

Our atomic force microscopy (AFM) experiments and non-equilibrium molecular dynamics (NEMD) simulations have demonstrated a negative friction-load dependence to ionic liquid-glycol ether mixtures, that is, the friction decreases as the normal load increases. NEMD simulations revealed a structural reorientation of the studied ionic liquid (IL): as the normal load increases, the cation alkyl chains of ILs change the orientation to preferentially parallel to the tip scanning path. The flat-oriented IL structures, similar to the 'blooming lotus leaf', produce a new sliding interface and reduce the friction. A further molecular dynamics simulation was carried out by adopting slit pore models to mimic the tip approaching process to confirm the dynamics of ILs. We observed a faster diffusion of ILs in the smaller slit pore. The faster diffusion of ILs in the more confined slit pore, facilitates the structural reorientation of ILs. The resulted new sliding surface is responsible for the observed smaller friction at higher loads, also known as the negative friction-load dependence.These findings provide a fundamental explanation to the role of ILs in interfacial lubrications. They help to understand liquid flow properties under confinement, with implications for the development of better nanofluidic devices.

Negative friction coefficient in microscale graphite/mica layered heterojunctions

“…Similar results are also found for tip on suspended graphene (12,13,16). In addition, a negative friction-load dependence is observed during the loading process for Si 3 N 4 tip sliding on titanium substrate with ionic liquidglycol ether mixtures in between as lubricant, which is resulted from the structural reorientation of the ionic liquid (9).…”

“…This general trend has been confirmed by previous studies on various tribological systems (5,6). However, a few exceptions were observed recently, where counterintuitive examples were reported that the friction exhibited a complex dependence on normal load (7)(8)(9)(10)(11)(12)(13)(14)(15). In experiment, the friction of a nanoscale contact between an atomic force microscope (AFM) tip and chemically modified graphite or polymer-coated surfaces shows discrepancy for increasing (loading) and decreasing (unloading) normal loads (14,15).…”

A review of recent advances in tribology

“…The results of Shi et al [85] show that only the ionic liquid C6MIMBF4 with the long alkyl chain can obtain stable ionic liquids-based magnetic nanofluids which can be used as lubricant. An et al [86] found that the ionic liquid, glycol ether mixtures, at titanium interfaces had a negative friction load dependence, i.e., the friction force decreased with the increase of normal load. Han et al [87] discussed the activated slip and flow of ionic liquid lubricating molecules.…”

Graphene tailored by Fe3O4 nanoparticles: low-adhesive and durable superhydrophobic coatings

“…1a). An appearance of the a-Ti phase 48 in 0.1-1.0 Fe 3 O 4 + 0.1 FLG hybrid coatings was also observed in Fig. 2, attributed to the underlying Ti substrates, which is easily detected on thin-prepared coatings.…”