Tribology International Vol. 30, No. 12, pp. 889–894, 1997  1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301–679X/97/$17.00 + 0.00 PII: S0301–679X(97)00076–5 Molecular tribology of lubricants and additives Yoon-Kyoung Cho*, Lenore Cai and Steve Granick Knowledge of the bulk viscosity provides little guidance to predict accurately the interfacial shear strength and effective viscosity of a fluid in a lubricated contact. To quantify these differences between bulk and thin-film viscosity, an instrument was developed to measure the shear of parallel single crystal solids separated by molecularly-thin lubricant films. The effective shear viscosity is enhanced compared to the bulk, relaxation times are prolonged, and nonlinear responses set in at lower shear rates. These effects are more prominent, the thinner the liquid film. Studies with lubricant additives cast doubt on the usefulness of the concept of a friction coefficient for lubricated sliding.  1998 Elsevier Science Ltd. All rights reserved. Keywords: nanotribology, friction, SFa, lubricants, nanorheology Introduction In surveying the present situation, one notices that one of the major difficulties in explaining tribology is the paucity of direct, experimental information concerning dynamic events on a microscopic scale. With macro- scopic approaches one can measure, for example, the friction between surfaces (and other global properties) while the surface are actually in contact. Such macro- scopic measurements are often difficult to interpret from a molecular point of view. Most of the questions regarding mechanisms are molecular in nature. With microscopic analytical methods, one generally exam- ines surfaces before and after sliding has occurred. As such, these approaches yield incomplete information concerning the many interfacial situations where the presence of a lubricant is an essential part of the physical situation. A further major problem is to characterize the true contact area and thickness of fluid films during flow. They can be gauged by resistance, capacitance, and other measurements, but of course many distributions of surface roughness are compatible with a given measurement. Furthermore, to the degree that a surface is rough, the surface separation and the thickness of the intervening film must be described by a distribution rather than any single number. Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801, U.S.A. *Corresponding author. Received 12 December 1996; accepted 6 November 1997 Tribology International Volume 30 Number 12 1997 889 Consider now the canonical experiment depicted in Fig 1. Imagine that one takes a droplet of liquid, puts it between a ball and a table, and lets the ball fall. Of course the liquid squirts out, initially rapidly, then slower and slower as the liquid thickness becomes less than the radius of the ball. This problem was solved Fig. 1 Hypothetical experiment showing that a liquid can support a normal force. A liquid droplet is placed between a ball and a flat surface. The graph, in which liquid thickness is ploted schematically against time after the ball has begun to fall, shows that the film thickness remains finite (a few molecular dimensions) even at equilibrium.