Hydrophilic polymer brushes constitute a man-made approach to imitating nature’s lubrication mechanisms. A polymer that has been frequently used to explore such systems is poly(ethylene glycol) (PEG). While PEG may not be the ideal solution for water-lubricated tribosystems for a number of technical reasons, this well-characterized polymer has served as an extremely useful model for the development of other polymer-brush-based lubricant approaches. This review covers the history of PEG brushes used as aqueous lubricants, including the large body of work on electrostatically attached PEG brushes, and ends with a discussion of current and future research directions that build upon the knowledge gained over a decade and a half of PEG-brush research in tribology.
This paper provides a brief overview of investigations that have been carried out on the tribological properties of polysaccharides. Much of the work into long chain carbohydrates focuses on adhesion due to the propensity of these molecules to form hydrogen bonds. Polysaccharides play an important role in bioadhesion, but are also used in the mining industry to assist in the separation of minerals. Despite the high adhesion associated with polysaccharides, investigations showing that they can be used to achieve low friction have also been reported.In order to observe an effect on the friction between sliding surfaces in the boundary regime, the polysaccharide must first adsorb onto the surface. Although hydrogen bonding is the dominant interaction for polysaccharides, they can also interact through hydrophobic interactions in aqueous solution, providing the structure of the carbohydrate allows this. Many polysaccharides are charged and can, therefore, be adsorbed onto charged surfaces via electrostatic interactions. If the sliding contact is symmetric, that is both surfaces are of the same material, then it could be expected that bridging between the surfaces would occur due to the same interactions being formed on both sides. However, friction studies have been carried out that show that this phenomenon does not necessarily lead to high friction coefficients.
Fundamental aspects and mechanisms of acoustic levitation together with governing equations are presented first. Then, the acoustic levitation phenomenon is considered as a new way to design air suspension systems capable of self-levitation. A particular emphasis is on journal bearings and their specific geometrical configuration. A practical feasibility of using acoustic levitation to separate contacting surfaces is supported and illustrated by results of experimental testing of a number of prototype devices.
In this paper, a theoretical study of the combined effects of viscosity variation and surface roughness on the squeeze film performance of journal bearings lubricated with micropolar fluid is made. The modified averaged Reynolds equation for micropolar fluids accounting for the randomized surface roughness structure and variation of viscosity is mathematically derived. The Christensen’s stochastic theory for hydrodynamic lubrication of rough surfaces is used to study the effect of two types of one dimensional surface roughness patterns on the squeeze film characteristics of a journal bearing with micropolar fluid. Closed form expressions for the mean pressure load carrying capacity are obtained for the infinitely short journal bearing. It is observed that, the transverse surface roughness pattern improves the squeeze film characteristics where as the adverse effects are observed for the one-dimensional longitudinal surface roughness pattern. The effect of variation of viscosity in micropolar fluid on the squeeze film characteristic of rough short journal bearings is analyzed.
Slider wear is investigated as a function of D-4OH lubricant film thickness and molecular weight. Slider wear increases with decreasing lubricant film thickness and with increasing molecular weight when compared at the same film thickness in the submonolayer film thickness regime. The two sets of observations are readily interpreted on the basis of the monolayer fraction which corrects the film thickness for the molecular weight-dependent surface coverage.