Impact of molecular structure on the lubricant squeeze-out between curved surfaces with long range elasticity

TL;DR

This study models how molecular structure influences lubricant squeeze-out between curved elastic surfaces, revealing that linear n-butane forms more ordered layers and is more effective as a boundary lubricant than branched iso-butane at low speeds.

Contribution

It introduces a model accounting for surface curvature and elasticity to compare the squeeze-out behavior of linear and branched butane lubricants.

Findings

Linear n-butane forms well-defined molecular layers.

Branched iso-butane remains liquid-like at smaller separations.

N-butane is more effective as a boundary lubricant at low speeds.

Abstract

The properties of butane (C4H10) lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and elastic properties of the solid surfaces. We consider the linear n-butane and the branched iso-butane. For the linear molecule, well defined molecular layers develop in the lubricant film when the width is of the order of a few atomic diameters. The branched iso-butane forms more disordered structures which permit it to stay liquid-like at smaller surface separations. During squeezing the solvation forces show oscillations corresponding to the width of a molecule. At low speeds (< 0.1 m/s) the last layers of iso-butane are squeezed out before those of n-butane. Since the (interfacial) squeezing velocity in most practical applications is very low when the lubricant layer has molecular thickness, one expects n-butane to be a better boundary lubricant than iso-butane. N-butane possessing a slightly lower viscosity at high pressures, our result refutes the view that squeeze out should be harder for higher viscosities, on the other hand our results are consistent with wear experiments in which n-butane were shown to protect steel surfaces better than iso-butane.