Lubricated friction at the nano and mesoscale

Friction is central to numerous natural processes and technological applications, from the motion of synovial joints to car engines and wind turbines. Despite its ubiquitous relevance, a comprehensive picture is still lacking and current models are largely semi-empirical. Experiments conducted at the atomic scale have shed light on the fundamental origins of friction, but linking findings on the single atom or molecule level to macroscale observations involving countless tribological contacts remains a considerable challenge. To bridge this gap, results are needed at the mesoscale, typically between 1 nm and 1 $\mu$m, where atomistic information is still tractable but macroscopic behaviour begins to emerge. The problem becomes even more complex when considering the presence of a fluid lubricant confined in a nanogap between the two sliding surfaces. This thesis aims at bridging the current gap between atomistic models for lubricated friction and larger scale observations. This is done mainly using atomic force microscopy (AFM) which allows investigations of both the molecular level details and the mesoscale picture within the same experiment. Wherever possible, AFM measurements have been complemented by other experimental and computational techniques. Using a variety of model systems, the thesis studies the organisation and dynamics of lubricants under nanoconfinement at the solid/liquid interface. The investigations lead to some novel insights. First, polar and non-polar lubricants are shown to experience a structural transition under nanoconfinement, with the solid-like characteristics of the boundary layer being responsible for an increase in lubricated friction. Second, the lubricant molecular ordering can be modulated by surface singularities that limit the configurational entropy of the fluid molecules. This suggests surface defects indirectly influence the lubricant properties by inducing local molecular rearrangement. External factors, such as humidity and temperature, are also investigated. Some common threads in the different model systems suggest that atomistic models can be adapted at the mesoscale to describe lubricated friction based on a thermally activated process