Meniscus and viscous forces during normal separation of liquid-mediated contacts

Abstract

Menisci form between two solid surfaces with the presence of an ultra-thin liquid film.Meniscus and viscous forces contribute to an adhesive force when two surfaces areseparated. The adhesive force can be very large and can result in high friction, stictionand possibly high wear. The situation may become more pronounced when thecontacting surfaces are ultra-smooth and the normal load is small, as is common formicro-/nanodevices. In this study, equations for meniscus and viscous forces duringseparation of two flat surfaces, and a sphere and a flat surface, are developed, and thecorresponding adhesive forces contributed by these two types of forces are examined. Thegeometric meniscus curvatures and break point are theoretically determined, and the role ofmeniscus and viscous forces is evaluated during separation. The influence of separationdistance, liquid thickness, meniscus area, separation time, liquid properties andcontact angles are analyzed. Critical meniscus areas at which transition in thedominance of meniscus to viscous forces occurs for different given conditions,i.e. various initial liquid thicknesses, contact angles and designated separationtime, are identified. The analysis provides a fundamental understanding of thephysics of separation process, and insights into the relationships between meniscusand viscous forces. It is also valuable for the design of the interface for variousdevices.