Capillary-pressure driven adhesion of rigid-planar surfaces

A thin film of viscous-Newtonian fluid sandwiched between parallel-plane walls, is examined both experimentally and theoretically for gap spacings initially much smaller than the capillary length to determine the conditions for adhesion. The problem is parameterized by the variables F, which is the ratio of an external load generated by one of the parallel-plane surfaces, to the product of surface tension and a characteristic length scale, and the static contact angle α. An analytical solution for the change in gap height as a function of elapsed time is derived in the limit of small Reynolds and zero capillary numbers. The load is suspended for a long but finite elapsed time as the gap spacing approaches a critical value, and for gap spacing values less than the critical one the load is suspended indefinitely. Experiments are performed with typical elapsed times of O(100–1000 s) using fluids with viscosity, O(1000 cSt), but different surface tension and contact angle for gap spacings O(10–100 μm) with loads of either 2.7 N or 4.9 N. There is good agreement between the theory and experiments.

Top view of a liquid bridge initial sandwiched between a stationary wall and a load generating a force oriented into the page. After an elapsed time the liquid either continues to spread generating adhesion (1) or de-wets (2).Figure optionsDownload high-quality image (72 K)Download as PowerPoint slideResearch highlights
► Capillary-pressure driven adhesion of a load is studied using viscous fluids.
► Conditions for adhesion depend on surface tension, contact angle and the magnitude of the external load.
► Adhesion occurs at an unstable critical point.
► De-wetting leads to a fingering instability.