Structure and dissipation mechanisms of nano-confined lubricants

• Applicant: Dr. Hongyu Gao
• Subject Area: Computer-Aided Design of Materials and Simulation of Materials Behaviour from Atomic to Microscopic Scale ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 469054794

The proposed research promotes the study of interfacial behavior and tribological response of liquid/solid lubricants and lubricant additives at nanoscale asperity contacts. Molecular dynamics will be primarily used to complement experimental study and deepen our fundamental understanding of the following three sub-projects: (i) Stress or rather stress-anisotropy relaxation of confined liquid lubricants after an expulsion of a molecular layer (from n-1 to n layers) during compression and after its re-emergence (from n to n-1 layers) during decompression. Preliminary simulations indicate that confined-liquid stresses may reach different plateaus in the two cases, of which the origin requires to be further investigated. Here, the load-bearing ability of liquids under confinement and the existence of significant stress anisotropy remaining present, even after molecules have diffused substantially, are of particular interest. (ii) Dissipation mechanisms of self-assembled monolayers, as friction modifiers adsorbed on solid surfaces, in the presence of base oils. This sub-project aims to clarify the roles of molecular length and isomer type in determining sliding friction and to establish correlations between energy dissipation and dynamics of local structural deformation. It is also meant to unravel the reason(s) why conflicting trends of surface coverage dependence of friction have been observed between simulations and experiments. (iii) Contact aging induced friction instability at the onset of rubbing a gold nanoparticle against graphite. The origin of experimentally observed frictional duality will be explored through different models, in which effects of surface contamination on contact strength and its variation under shear will be rationalized.

• DFG Programme: Research Grants
• International Connection: United Kingdom, USA
• Co-Investigators: Professor Dr. Roland Bennewitz; Professor Dr. Martin Müser
• Cooperation Partners: Professor Mehmet Z. Baykara; Professor Dr. Daniele Dini; Dr. James P. Ewen