The objective of this work is systematic understanding of particle-particle nanorheology based on a single particle-particle contact: two atomically-smooth solid surfaces in molecularly- thin proximity (ultimately 1 nm or less) and the dynamic mechanical properties of the resulting inter-facial film studied directly as they depend on dynamic mechanical frequency and on strain rate.
This was the first year of a 3-year grant. Motivation is provided by the realization, upon surveying the literature, that progress in understanding fine powder applications is impeded by difficulties in separating the overall rheology of a macroscopic-sized sample into various mechanistic subprocesses. Much is known about inter-particle forces: van der Waals, electrostatic, hydrogen bonding, steric. The information obtained in rheology experiments has usually been interpreted in these terms. Yet these attempts have been largely unsuccessful, for systems of realistic makeup, owing to insufficient appreciation that the inter-facial rheology may dominate because it is the weakest link. This is a critical issue because particles are almost always lubricated by condensed films of moisture; films of condensed ambient gases are ubiquitous, whether intentionally or not.
A breakthrough was obtained as concerns rate criteria to observe stick-slip rather than smooth sliding. Complications in the traditional definitions of static and kinetic friction were analyzed as they pertain to lubricated sliding (for example, in the presence of condensed gases). We found that stick-slip motion occurred only when thin films were deformed faster than their intrinsic relaxation time. The observation offers a new strategy to look for methods to avoid stick-slip motion by engineering the relaxation time of a confined film.