Wet-adhesion properties of microstructured surfaces inspired by newt footpads

To understand wet adhesion of newt foot pads, we firstly examined and evaluated the attachment and climbing abilities of newts under three different wetting conditions. Then we characterized the micro and nanostructures of newts' foot pads by using the scanning electron microscopy. Followed by observing the micro and nanoscale structural features of newts' foot pads, four different micropatterns, including round pillars, hexagonal pillars, and two hybrid patterns with closed and semi-closed hexagonal ridges, were designed and fabricated on PDMS. The static friction and adhesion of microstructured surfaces were measured by using a multi-functional surface meter and an adhesion testing equipment, respectively. Effects of micropattern on static friction, and effects of retraction speed, amount of liquid, approach-retraction cycle on adhesion were investigated experimentally. Results suggested that a larger amount of liquid decreases the adhesion and friction, but a little liquid increases the adhesion and friction. It was found that there exists an optimum amount of liquid (about 0.1 mu l) that can enhance the adhesion. Our results can give insights into the repeated shear movements of a newt's foot pads in the stream condition as well as the possible functions of dense nanopillar arrays on newt foot pads.