"Evolutionary tribology: reducing oil pollution by employing water-based lubricants for marine energy generation applications
The replacement of traditional mineral oil lubricants with water-based bio-compatible fluids has long been a desirable, if unrealised, ambition in many applications. This is particularly relevant in marine-based energy generation systems, where oil-based lubricants create a high risk of environmental pollution. The use of bio-lubricants has been explored in several previous studies [1], however no significant technological advances have been achieved. Most of the work has focussed on traditional lubrication mechanisms, with bio-molecules being employed to form an adsorbed surface film which reduces friction. However, due to their inherent biological, thermal and/or oxidative instability, bio-molecules are unsuited to long-term industrial applications. The alternative approach is to use stable, bio-friendly molecules, designed to exploit the lubrication mechanisms found in nature. These mechanisms have evolved to be far more diverse than those found in traditional "mineral oil" tribology and are, as yet, poorly understood [2,3].
In earlier work, we have shown that a molecular shear-aggregation mechanism occurs in synovial fluid which lubricates articular joints, as well as occurring during blood-clotting [4]. In this project we propose to utilise the shear-aggregation mechanisms found in bio-systems [2] to substitute oil-based lubrication. A high-viscosity lubricating film is formed in the sliding contact as the result of proteins denaturing in the shear field. It is this mechanism which will be adapted in this project: the bulk lubricant remains low-viscosity so that energy losses due to fluid shear or churning are minimised. Further, the high viscosity phase is only formed where it is needed, locally in the rubbing contact. Using water as the base-fluid means that problems with corrosion must be addressed and materials suitable to use in these cases include engineering plastics, metal alloys and ceramics. Potential uses include low to moderate contact pressure components operating at intermediate speeds. Examples are hydrodynamic bearings, turbines and compressors, particularly where the use of mineral oils risks environmental contamination, such as in marine, food and medicine applications.
Biological systems use globular proteins to form the high-viscosity phase, however, these are not suitable for long-term lubrication outside of the body. We will explore the use of cationic surfactant micelles [5] and polymer-colloids, as these form super-molecular aggregated structures under shear flow. The project will comprise the following:
Identification of suitable chemical systems (surfactants, polymer-colloids) which form shear-aggregated structures in water
Measurement of rheological properties to confirm shear-thickening behaviour
Measurement of lubricated film thickness, friction and wear in tribology test devices [2]
Visualisation of micelle-aggregation under shear and within a lubricated contact using fluorescence imaging [3]
Review of application in renewable energy generation applications and preliminary tests with a variety of material combinations to include engineering plastics, stainless steels, cobalt chrome alloys and ceramics
References
1. Holmberg, Proc. IMechE Part J (2011) 225:1013-1022 DOI: 10.1177/1350650111406635
2. Cann, Proc. IMechE Part H (2011) 225:696-709. DOI: 10.1177/0954411911401306
3. Masen, JMBBM (2019), DOI: 10.1016/j.jmbbm.2018.09.048
4. Chen, Nature Comms (2012), 4:1333 DOI: 10.1038/ncomms2326
5. Wunderlich, Rheol Acta (1987) 26:532-542 DOI: 10.1007/BF01333737
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