Transient fluid removal at soft interfaces: Contact-time-controlled squeeze-out in a cylinder-on-flat contact

Abstract

We study transient fluid removal in cylinder-on-flat contacts between stiff PMMA cylinders and a soft PDMS substrate. The cylinder geometry eliminates the edge-scraping mechanism that can occur for deformable rubber blocks, allowing the influence of sliding on fluid squeeze-out to be examined more directly. Experiments were performed mainly in glycerol at the low sliding speed $v=3 \ {\rm μm/s}$ using cylinders with different surface roughness. After the initial elastic-loading stage, we find that the friction during sliding depends primarily on the total time elapsed since application of the normal load rather than on the preceding sliding distance. The subsequent friction evolution follows approximately the same dependence on total contact time. Stationary squeeze-out calculations predict the evolution of the mean surface separation and real contact area, in reasonable agreement with that inferred from the measured friction. These results show that essentially the same squeeze-out process governs fluid removal during stationary contact and low-speed sliding, and that sliding-induced elastohydrodynamic effects have only a minor influence under these conditions.