(213g) Role of Draining Channels in Hydrodynamic Interactions

The adhesion and locomotion mechanisms employed by tree frogs for flooded conditions offer the ultimate solution for the need of strong, reversible, reusable, tunable, and water tolerant adhesives. The presence of structured toe-pads (hexagonal epithelial cells separated by channels) has been proposed to facilitate drainage of fluid and reduce hydrodynamic repulsion during approach. These channels may remain closed during retraction for hydrodynamic adhesion and open for pull-off, both of which require active/passive control of toe-pad deformation.  

We present results from our investigation of the role of draining channels on normal hydrodynamic interactions in the absence of elastohydrodynamics (rigid surfaces). The surface force apparatus is employed to measure hydrodynamic forces between a smooth silver film and a structured SU-8 surface driven towards or away from each other at a constant drive velocity. The structured SU-8 comprises of hcp array of cylindrical posts. The hydrodynamic forces are analyzed within the framework of Reynolds’ continuum approach in the lubrication limit for smooth surfaces in cross-cylinder geometry.

 Reduction in repulsion upon approach with respect to that predicted by Reynolds’ approach in the lubrication limit is analyzed using a scaling argument based on the geometric parameters associated with the structures (diameter of posts, channel width and depth, and surface coverage), and the range of hydrodynamic interactions, which depends on the fluid viscosity and drive velocity. Viscosity-dominated drainage of fluid in the presence of a network of channels intrinsically results in competition between drainage from in contact region and drainage through the network of channels. The scaling argument can, then, be used to predict regimes of preferential drainage of fluid through network of channels vs drainage of fluid from in contact region for a single post or the cross-cylinders as a whole as the surface separation changes. These predicted regimes are compared with those observed experimentally for different combinations of diameter of posts, channel width and depth, and surface coverage as a function of drive velocity to highlight the universal nature of the scaling argument employed.