Kinetics and Energetics in Nanolubrication

Abstract

In the 19th century, lubrication, one of humankind's oldest engineering disciplines, gained a theoretical base from Reynolds's classical hydrodynamic description that was unmatched by most of the theories developed in tribology to date. In the 20th century, however, increasing demands on lubricants shifted attention from bulk films to ultra-thin film lubrication. Finite-size limitations imposed constraints on the lubrication process that were not considered in the bulk phenomenological treatments introduced by Reynolds. At this point, as is common in many engineering applications, empiricism took over. Functional relationships derived from the classical theories were tweaked to accommodate the new situation of reduced scales by introducing effective or apparent properties. With the inception of nanorheological tools of complementary nature in the later decades of the 20th century (e.g., the surface forces apparatus and scanning force microscopy), tribology entered the realm of nanoscience. Through an increasing confidence in experimental findings on the nanoscale, kinetic and energetic theories incorporated interfacial and molecular constraints. The very fundamentals have been challenged in recent years. Researchers have realized that bulk perceptions, such as solid and liquid are defied on the nanoscale. The reduction in dimensionality of the nanoscale imposes constraints that bring into question the use of classical statistical mechanics of decoupled events. The diffusive description of lubrication is failing in a system that is thermodynamically not well-equilibrated. The challenge any nanotechnological endeavor encounters is the development of a theoretical framework based on an appropriate statistics. In tribology this is met with spectral descriptions of the dynamic sliding process. Statistical kernels are being developed for probability density functions to explain anomalous transport processes that involve long-range spatial or temporal correlations. With such theoretical developments founded in nanorheological experiments, a more realistic foundation will be laid to describe the behavior of lubricants in the confined geometries of the nanometer length scale.