  Tree frogs are able take advantage of an interconnected network of epithelial
cells in their toe pads to modulate their adhesion to surfaces under dry, wet,
and flooded environments. It has been hypothesized that these interconnected
drainage channels reduce the hydrodynamic repulsion to facilitate contact under
a completely submerged environment (flooded conditions). Using a custom-built
apparatus we investigate the interplay between surface structure and loading
conditions on the peeling force. By combining a normal approach and detachment
by peeling we can isolate the effects of surface structure from the loading
conditions. We investigate three surfaces: two rigid structured surfaces that
consist of arrays of cylindrical posts and a flat surface as a control. We
observe three regimes in the work required to separate the structured surface
that depend on the fluid film thickness prior to pull out. These three regimes
are based on hydrodynamics and our experimental results agree with a simple
scaling argument that relates the surface features to the different regimes
observed. Overall we find that the work of separation of a structured surface
is always less than or equal to the one for a smooth surface when considering
purely viscous contributions.
