Inspired by the nanoconcave top of epidermal cells on tree frogs' toe pads, an array of composite micropillars with nanopits on the surface (CPp) has been designed. Polystyrene (PS) nanoparticles are mixed with polydimethylsiloxane (PDMS) and serve as the template for nanopits on the PS/PDMS composite micropillars. CPp shows much larger wet adhesion 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 the liquid that remained in nanopits forms multiple nanoscale liquid bridges within the contact area of a 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 contact thus contribute to strong wet adhesion, which could be similar to 36.5 times that of tree frogs' toe pads. The results suggest the function of nanoconcaves on the toe pad of tree frogs and offer a new design strategy for structured adhesives to gain strong wet adhesion.
