Models are proposed for thick-film-coverage adsorption on a general fractal surface. The basic idea is that long-range dispersion forces dominate the thermodynamic behavior. The analysis predicts the coverage to be proportional to [ln($P_0$/P${\left)\right]}^{\left(D\mathrm{-}3\right)/3}$ where P/$P_0$ is the relative vapor pressure and D is the fractal dimension of the surface. Experimental examples for this behavior are presented. They are rough Ag surfaces, giving D=2.30. .AE

A quartz-crystal microbalance has been utilized to probe sliding-friction effects in Kr monolayers undergoing solidification on Au and Ag surfaces. Solid layers are observed to be far more sensitive to surface morphology than liquids. The behavior of liquid layers is consistent with a frictional force whose magnitude is a nonlinear function of the sliding velocity.

We describe a straightforward method for obtaining a precise value for the fractal dimension (self-affine scaling exponent) of a surface, via scanning tunneling microscopy. It is applied to investigate ion-beam erosion of an iron film surface, provoding strong support for the applicability of scaling theory to submicron erosion processes. A self-affine surface with scaling exponent H=0.53±0.02 is observed to develop.

The acoustic impedance of a monolayer adsorbed on the surface of a resonating quartz microbalance produces a shift in both the frequency and vibrational amplitude of the oscillator. The relation between acoustic impedance and fluctuations in film momentum is rigorously derived via the fluctuation-dissipation theorem within the context of two-dimensional fluid mechanics. The calculations reveal that the film-substrate interfacial viscosity can be deduced from measurements of acoustic impedance.

This review provides an overview of recent advances that have been achieved in understanding the basic physics of friction and energy dissipation in molecularly thin adsorbed films and the associated impact on friction at microscopic and macroscopic length scales. Topics covered include a historical overview of the fundamental understanding of macroscopic friction, theoretical treatments of phononic and electronic energy dissipation mechanisms in thin films, and current experimental methods capable of probing such phenomena. Measurements performed on adsorbates sliding in unconfined geometries with the quartz crystal microbalance technique receive particular attention. The final sections review the experimental literature of how measurements of sliding friction in thin films reveal energy dissipation mechanisms and how the results can be linked to film-spreading behavior, lubrication, film phase transitions, superconductivity-dependent friction, and microelectromechanical systems applications. Materials systems reported on include adsorbed films comprised of helium, neon, argon, krypton, xenon, water, oxygen, nitrogen, carbon monoxide, ethane, ethanol, trifluoroethanol, methanol, cyclohexane, ethylene, pentanol, toluene, tricresylphosphate, t-butylphenyl phosphate, benzene, and iodobenzene. Substrates reported on include silver, gold, aluminum, copper, nickel, lead, silicon, graphite, graphene, fullerenes, C60, diamond, carbon, diamond-like carbon, and YBa₂Cu₃O₇, and self-assembled monolayers consisting of tethered polymeric molecules.

We have employed a quartz crystal microbalance technique to measure the friction associated with sliding of solid nitrogen along a lead surface, above and below the lead's superconducting transition temperature. The friction is observed to drop abruptly at the transition as the substrate enters the superconducting state. The observation signifies a new phenomenon, most likely attributable to electronic contributions to friction. While such contributions have been discussed and debated theoretically for a number of years, they have heretofore not been observed in any direct experimental fashion.