Dynamics of viscous adhesion : From fingering instability to capillary bridges

The adhesion between solid surfaces coated with a viscous liquid is limited by the propagation of a capillary bridge through the narrow gap separating both surfaces. Although the viscous adhering liquid pushes air of low viscosity, the adhesion front undergoes, in some situations, a fingering instability. What are the ingredients involved in the adhesion dynamics and in this unexpected instability? To address these questions, we consider a simplified model experiment where a solid beam is placed horizontally above a bath of liquid at a small distance in comparison with the capillary length. As a liquid bridge is created at one end of the beam, the bridge spontaneously expands towards the opposite extremity. We first show the strong dependence of the meniscus velocity with the liquid properties, the separation gap and the thickness of the reservoir. We then investigate the interactions between bridges propagating along parallel beams.