Tree frog-inspired micropillar array with nanopits on the surface for enhanced adhesion under wet conditions.

Inspired by nanoconcave top of epidermal cells on tree frog toe pads, array of composite micropillars with nanopits on the surface (CPp) has been designed. Polystyrene (PS) nanoparticles are mixed into polydimethylsiloxane (PDMS) and served as the template for nanopits on the array of PS/PDMS composite micropillars. CPp shows much larger wet adhesion as compared to the arrays of micropillars without nanopits. Under a certain loading force, most of the liquid between CPp and the counterpart surface is squeezed out so that the liquid remained in nanopits forms multiple nanoscale liquid bridges within the contact area of single micropillar. Moreover, a large loading force could squeeze part of the liquid out of nanopits, resulting in the suction effect during the pull-off. The multiple liquid bridges, the suction effect and the solid direct contacts thus contribute to a strong wet adhesion, which could be ~36.5 times of tree frog toe pads. The results suggest the function of nanoconcaves on the toe pad of tree frogs and offer a new design strategy for structured adhesive to gain strong wet adhesion.