In-situ Visualisation of Surface Dynamics of Friction Modifiers

This proposal concerns research into the dynamics and effectiveness of organic friction modifiers (OFMs) in lubricated contacts. In most applications, friction is an expensive nuisance and should be reduced to as low a level as possible. There are, however, also applications, such as friction drives and wet clutches, where high friction is required or friction that increases with speed. Friction must thus be controlled in an informed manner to improve both the efficiency and effectiveness of machines. In the boundary and mixed lubrication regimes, the rubbing surfaces are not fully separated by a fluid film. Contact between rubbing surfaces then occurs and can lead to high friction, wear and scuffing. Therefore surface-active molecules known as organic friction modifiers (OFMs) are commonly added in fuels and lubricants as friction modifiers to improve boundary lubrication.

It is necessary for us to understand how OFMs behave in tribological contacts. Knowledge of the interactions between the surface and these molecules is crucial in the design of effective lubricants. In-situ methods are required to examine these complex processes. This issue is key to this proposed study. The overall aim is to understand the interactions between OFMs, base oil and the rubbing surfaces and how these influence OFM protective films formation and hence lubricating properties. In particular, the effects of the chemistry of rubbing surfaces, degree of OFM saturation and temperature on the formation and dynamics of its self-assembled monolayers (SAMs) will be explored by direct observation of lubricated contacts. In the proposed study, fatty acids are chosen as model OFMs. A fluorescence tracking approach will be developed to explore and understand the lubricating mechanism of OFMs at a single molecule level. A test apparatus for this purpose will be constructed, and advanced, single molecule level fluorescence techniques will be applied to provide direct information of OFM behaviour, and track fatty acid molecules dynamics in-situ. This will enable nanoscopic/molecule level behaviour, in this case, self-assembly and adsorption-desorption kinetics of OFMs measurements, to be correlated of macroscopic observables such as frictional force and wear damage. The proposed work will also provide valuable experimental results that can be used to verify and enhance computer simulations of OFMs in lubricated systems.

This proposal concerns research into the dynamics and effectiveness as boundary lubricants of organic friction modifiers (OFMs) in lubricated contacts by direct in-situ observation at lubricated contacts.

1. Academic Impact
There are two areas of potential impact. Firstly, the proposed studies will provide crucial information on the fundamental lubrication mechanisms by which OFMs act. This can be used to understand the microscopic mechanisms that cause macroscopic phenomena, such as friction and wear. Thus, researchers in tribology, especially those interested in elastohydrodynamic lubrication, boundary lubrication, and additive interactions, will find such knowledge useful. The experimental quantities measured in the proposed experiments, for example, diffusion coefficients of OFMs, will provide direct inputs to theoretical models and simulations research of OFMs and boundary lubrication. The model OFM in this proposed study is fatty acids. Apart from being an additive with important engineering implications and a crucial component in blends of biodiesel with ultra-low-sulphur petroleum-based fuel, fatty acids also form an important group of biopolymers, such as cutin. The study of the interactions between such monomers in a geometrically well defined environment can be extended to the understanding of short-range structure of biopolymers.

Second is methodological. The proposed work will constitute a breakthrough for the observation of molecular interfacial dynamics under real world conditions, in which single molecule dynamics will be captured in-situ at a tribological contact. Both tribologists and researchers in other area, such as fluid dynamics, contact mechanics, engineering, physics, chemistry and materials science can benefit because the fluorescence technique is general, and can be used to study molecules and additives beyond just lubricant systems. The specific experimental platform, and associated software methods being developed are versatile and should be applicable to areas such as, and not limited to, fluid flow, nanofluidics, adsorption-desorption kinetics, and molecular self-assembly. In fact, it should be useful to any research where nanomater scale phenomena are of interests, and where surface interactions dominate.

2. Economic and societal impacts
Friction must be controlled in an informed manner to improve both the efficiency and effectiveness of machines. The lubricant and additive industries, the automobile industry, and engineering components manufacturers, invests billions of dollars in additives and lubrication research and development. They would directly benefit from this research. The proposed work will provide scientific information, as well as a new method to study the fundamental lubrication mechanisms by which OFMs act. An in-depth understanding of lubrication processes helps develop efficient lubricants. An increase in lubricant performance reduces components' wear and tear and improves fuel efficiency. The latter is particularly important given that fossil fuels are an increasingly scarce resource.

At the same time, the personal care industry can also benefit from the proposed work. OFMs are surface active molecules. Many personal care and cleaning products, such as hand cream, shampoo, detergents, contain surface active molecules that can be tracked and observed directly by fluorescence techniques. The success of these products will partly depend on how these particles interact with human skin / cleaning surface. Better understanding of relevant surface active molecules can lead to better product designs.

Finally, the tribology group at Imperial College London is already collaborating with leaders in a multitude of industries such as SFK, Infineum, Ford, Tata Motors and Unilever. These existing relationships will allow any scientific and methodological results from this proposal to disseminate to industry and create meaningful impact in an efficient manner.