Effects of capillarity on the mechanical stability of small-scale interfaces
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Interfacial adhesion and friction are significant factors in determining the reliability of small-scale mechanical devices such as with MEMS and the computer head/disk interface (HDI). As the interface spacing becomes smaller, operational failure via stiction has become a growing concern in these systems. Fundamentally, interface failure is related to mechanical instability of the interface caused by capillary effects. When liquid is present in a small-scale interface, large concave meniscus curvatures often develop at the liquid-vapor interface, leading to negative pressures in the liquid film and large tensile forces on the surfaces. When the elastic restoring force cannot balance the capillary force, the interface will lose its stability and collapse into intimate contact (jump-on). In addition, when the elastic bodies are then pulled away from contact, separation may occur suddenly and is related to another form of instability (jump-off). The jump-on and jump-off behaviors determine the strength of interfacial adhesion. In this study, the interaction between two elastic bodies coupled via a small liquid bridge was investigated. Geometries of two half-spaces and two sphere contact were considered. Stable equilibrium configurations were determined, and the mechanical stability of the interface was examined. Jump-on and jump-off conditions were given out. Then the theory was applied to study the approach and detachment processes of two elastic spheres in the presence of a liquid bridge. Critical values of the control variables at jump-on and jump-off were found. The pull-off force was calculated as a measure of interfacial adhesion. The results provide insight on some experimental data in the literature.