Effect of lubricant molecular rheology on formation and shear of ultra-thin surface films

Abstract

The physics of molecularly thin fluid films formed between surface features at close range is investigated. It is found that the interplay between discrete lubricant drainage from such contacts and localized contact deflection plays an important role both on the load carrying capacity of these asperity level conjunctions as well as on friction. Small spherical molecules tend to solvate near assumed smooth surfaces of asperities at nano-scale. Their discrete drainage at steadily decreasing gaps adds to the viscous friction of any bulk lubricant film. However, at the same time the generated solvation pressures increase the load carrying capacity. Conversely, long chain molecules tend to inhibit solvation, thus showing a decrease in the load carrying capacity, whilst through their wetting action reduce friction. Consequently, real lubricants should comprise molecular species which promote desired contact characteristics, as indeed is the case for most base lubricants with surmised properties of certain additives. The methodology presented underpins the rather empirical implied action of surface adhered films. This is an initial approach which must be expanded to fluids with a more complex mix of species. If applicable, this could also be an alternative (potentially time saving) approach to Monte Carlo simulations for molecular dynamics.