Connecting molecular-scale and macroscopic tribology

Recent simulations and experiments have probed dry and lubricated friction between atomically flat surfaces. The behavior in these molecular-scale bearings is often strikingly different than that in their macroscopic counterparts. For example, dry contacts usually do not exhibit static friction, while ''lubricated'' contacts may. These results are summarized, and possible origins for the differences between microscopic and macroscopic behavior are suggested. For example, static friction in dry sliding may result either from disorder, or from the presence of ''third bodies'' between the two surfaces. Both effects are generaly minimized in molecular-scale studies. The unusual behavior in molecularly thin lubricant films is related to confinement-induced phase transitions. These may be closely related to pressure-induced transitions in macroscopic lubricant films.