Lubricants are traditionally used to reduce friction and wear and to provide smooth running and a satisfactory life for machine elements. Lubricants’ physical states include liquid, gaseous, and solid states. Different regimes of a lubricating system were recognized starting in the middle of the nineteenth century. The understanding of hydrodynamic lubrication began with classical experiments and celebrated analysis in the 1880s.1-3 The understanding of boundary lubrication is attributed to Hardy and Doubleday,4,5 who found that extremely thin ﬁlms adhering to surfaces were often sufﬁcient to assist relative sliding. In later years, a better understanding and deﬁnition of other lubrication regimes between these two extremes were obtained, such as elastohydrodynamic (EHD) and mixed lubrications.