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

Abstract

Abstract. 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 localised 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 show 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 more complex mix of species. If applicable, this could also be an alternative (potentially time saving) approach to Monte-Carlo simulations for molecular dynamics.

Frictional characteristics of molecular length ultra-thin boundary adsorbed films

“…They include the works reported by Matsuoka and Kato [10], Al-Samieh and Rahnejat [11,12,13] and Chong et al [14] for the study of lightly loaded conjunctions with diminutively thin films, entrained through by hydrodynamic viscous action. To describe solvation, Chong et al [15,16] also adopted the use of Ornstein-Zernike (OZ) equation as expounded by Mitchel et al [17], Henderson and Lozada-Cassou [18] and Attard and Parker [19].…”

Formation of ultra-thin bi-molecular boundary adsorbed films

“…In a recent paper, Chong et al [27] showed that near surface effects dominate in gaps of the order of several molecular diameters. They also showed that spherical molecules tend to solvate near assumed smooth surfaces of asperities at nano-scale.…”

“…In an evolutionary process to extend the work of Chong et al [27], it is necessary to develop a realistic physio-chemical hydrodynamic model, where intermolecular and surface interactions account for a mix of lubricant molecular species. The current contribution supplements the solution of OZ equation with an attractive narrow well potential to describe the characteristics of a simplified real physical fluid system through PY approximation.…”

“…In the solution of the OZ equation with PY approximation, adhesion energy of hard spherical molecular species with the solid planar boundaries are allowed. Thus, the work of Chong et al [27] is not only extended from an idealized fluid to a physical one, but also includes the adsorption/adhesion of molecules to the solid boundaries. This is the prelude to the formation of tribo-films of interest in boundary lubrication.…”

“…However, as a further increase in concentration takes place, the density variation near the surfaces reaches a saturation point, where the solvation effect seems to exacerbate again. With the solvation phenomenon, rows of molecules are ejected at discrete energy levels as shown by Chong et al [27]. It is, therefore, natural that with an increasing concentration, there would be a limit where the advantages gained through reduced shear strength of a layer is lost by the energy required to eject a larger number of molecules out of a conjunction of a mutually approaching solid barriers.…”