Dear Colleagues,

Lubrication at small-scale contact is a complex matter given the prominent size effect and atomistic nature. For example, liquids under high confinement often exhibit a layering-like structure, with in-plane molecules arranged in a disordered manner. Because of their material nature, confining geometry, etc., such a confinement-induced molecular ordering can significantly extend the structural relaxation time and thereby result in a much-enhanced shear viscosity. When first-order or second-order phase transformation takes place, collective and solid-like interfacial behavior can be observed, which in turn can affect the tribological performance of the system, as well as its stability. Despite that, anisotropic stress tensor, essentially a natural consequence of liquid’s wavenumber dependent compressibility, is able to allow the confined film to be load-bearing. Do other roles come into play?

In some cases, friction modifiers are used as lubricant additives to lower friction and wear in the boundary lubrication regime. The combining polar head groups and long hydrocarbon chains enable them to adsorb onto a metal surface and self-assemble into a protecting monolayer, to prevent direct solid–solid contact. The viscoelastic nature of such an intermediate substance, however, complicates the interfacial behavior, given that its short-ranged deformation differs from typical elastic materials, and the so-called plowing mechanism renders classical models, e.g., JKR and DMT, inadequate. In this regard, further exploring size and molecular structural effects, alongside how energy dissipates during tip sliding, may be of interest, both scientifically and practically.

This Special Issue promotes the improved understanding of interfacial (tribological) behavior of liquid lubricants and additives in the boundary lubrication regime, and origins of the potential effects. Insights can be gained from investigations at different length scales and by means of experimental, modeling, and theoretical approaches.

Dr. Hongyu (Larry) Gao
Guest Editor

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• boundary lubrication
• friction modifiers
• lubricants and additives
• experiments and simulations at crossed scales
• rheology
• friction and wear
• liquid confinement