Switchable liquid-mediated soft adhesive contacts

Abstract

In a previous publication, we demonstrate that the inclusion into a soft polymer surface of subsurface micro-fluidic features, whose internal pressure can be varied, can be used to modify the effective radius of curvature of the contact and thus switch the magnitude of the dry adhesion of the surface to a harder counterface from one state to another – in other words to control the ‘stickiness’ of the surface. Under these circumstances, the phenomenon of adhesion is due principally to van der Waals forces operating across the junction between the two solid materials. Macroscopic adhesion can also arise when the junction is surrounded by a concentric droplet of liquid that wets at least one of the surfaces. In this case, an additional attractive force, conventionally known as the meniscus force, arises as a result of the concave meniscus of the interfacial pendular ring of liquid and the associated drop in Laplace pressure within its bulk. Under the action of a sufficient applied tensile load, the junction separates in three stages. Initially, the central ‘dry’ adhesion patch must be detached but this is followed by a period during which a tensile load must be maintained to draw out a filament of viscous liquid between the solid surfaces. The magnitude of this force will therefore be dependent on the speed of detachment. Finally, this filament becomes unstable and breaks driven by the influence of surface tension. As with dry contact, the initiation of the separation process is dependent on the effective radius of curvature of underlying contact and so, as in the previous case, a reduction in adhesion can again be conveniently brought about by varying the gauge air pressure in a network of channels just below the surface of the softer component.