(285c) Friction of Ionic Liquid-Glycol Ether Mixtures at Titanium Interfaces: Negative Load Dependence

Friction is one of the major causes that results in energy and material losses in many technical and economic areas, including nano-fluidic technology, micro/nano electro-mechanical systems, membrane channels, and boundary lubrication. Lubrication is a principal focus to reduce the losses and improve energy efficiency and mechanical durability. Ionic liquids (ILs) are considered to be potential candidates for lubricant applications to reduce friction. A pioneering attempt to understand friction is established that friction force is linearly proportional to the normal load. This theorem, also known as Amontons’ Law, in which the monotonic increase of friction with load has been observed across a range of length scales.

Our atomic force microscopy (AFM) experiments and non-equilibrium molecular dynamics (NEMD) simulations, however, 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 IL: as the normal load increases, the cation alkyl chains of the ionic liquids change the orientation to preferentially stay 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 force. A further molecular dynamics simulation was carried out to confirm the dynamics of the ILs. Slit pore models have been adopted to mimic the tip approaching the surface at different loads, i.e., the AFM tip moves closer to the substrate as the loading increases. We observed a faster diffusion of ILs in the smaller slit-pore model. This enhanced diffusion, has qualitatively supported the NEMD simulation results and the AFM measurements. The faster diffusion of ILs in the more confined slit pore, facilitates the structural reorientation of the IL cations, and the resulted new sliding surface to the tip is responsible for the observed smaller friction forces at higher normal loads, also known as the negative friction-load dependence in the AFM measurements. These findings provide a fundamental explanation to the role of ILs in interfacial lubrications. They also help to understand liquid flow properties under confinement, with implications for the development of better nanofluidic devices.