Lubrication and Wet Adhesion in the Presence of Draining Channels

Tree frogs are able to adhere remarkably well in flooded environments. This wet adhesion is likely due to their compliant and structured toe pads which facilitate removal of fluid and promotes closer contact with interacting surfaces. We examine the effects of surface structure and elasticity on the adhesion between a smooth and a structured surface submerged in viscous Newtonian fluids. The structured surfaces are designed to mimic the tree frog toe pads and consist either of a rigid polymer or a soft elastomer. The separation before pull out is controlled by putting a weight on top of one of the surface and by controlling the settling time in the viscous fluid. Once the weight is removed, the surfaces are pulled apart by peeling and the force is measured with a load cell. We observe three different regimes of adhesion based on loading conditions. If there is sufficient fluid between the two surfaces we find that surface structures have no role in adhesion. For intermediate surface separations we observe that surface structures lead to a decrease in the pull out force, likely due to drainage through the structured surface. Finally in certain surface separations, the presence of surface structures lead to higher adhesion, likely due to increased elastic deformation and surface compliance. A theoretical analysis based on hydrodynamic and elasticity is presented to explain the onset of these regimes and their behavior which is consistent with our measurements.