Abstract

Approximately 20% of the world’s energy consumption is spent overcoming friction within mechanical contacts.[1] Therefore, controlling friction and wear is important to sustainability in a wide range of industrial scenarios, with large social, economic and environmental impacts. This field of study, known as tribology, focuses on understanding and reducing friction and wear with lubrication. The first lubricant additives, isolated in the 1920s, were simple fatty acids that adsorbed onto surfaces as monolayers in a “head-up-tails-down” fashion.[2] Since then, more complex additives, such as modern friction modifiers have been capable to self-assemble into reverse micelles. This work focuses on how molecular-dynamics (MD) simulations offer a complementary approach to traditional tribology testing alongside start-of-the-art neutron scattering experiments and accelerate the design of lubricant formulations.

In a proof-of-principle study, the aggregation and interfacial behaviour an anionic aerosol-OT surfactant are investigated. In non-polar solvent with water, the surfactants form reverse-micelles, consistent with small-angle neutron scattering data. At the iron-oxide/oil interface, MD simulations with water show the formation of hemi-micelles and hemi-cylinders, structures that are highly anomalous for such compounds. Strikingly, these unique features are in excellent agreement with neutron reflectometry measurements, strongly validating the simulation results.

The second project focuses on Duomeen TDO, an additive consisting of a doubly charged ammonium cation and two carboxylate anions, used for the lubrication and dispersion of molybdenum disulfide-coated magnetic iron nanoparticles in magnetorheological fluids (MRFs). Results on the self-assembly as a function of water concentration are presented, alongside simulation-derived rheological responses and the interfacial adsorption on iron oxide and molybdenum disulfide, which differs significantly between the two surfaces.

Acknowledgements

This work was supported jointly by Engineering and Physical Sciences Research Council and Infineum UK Ltd. The authors would like to thank Professor Alex Routh (University of Cambridge) and Dr. Alexander Armstrong (ISIS Neutron and Muon Source) for conducting the neutron reflectometry experiments.

References

[1] K. Holmerg and A. Erdemir, Friction, 5, 263-284 (2017).

[2] H. Spikes, Tribol Lett, 60, 5 (2015).