Newts display superior attachment and climbing abilities under wet conditions due to the distinct pattern of micro- and nanoscale structures (i.e., polygonal epidermal cells with raised boundaries, and dense nanopillar arrays) on their foot pad surfaces. Inspired by the surface features of newt foot pads, a microstructured surface consisting of structural elements of round pillars surrounded by a closed hexagonal ridge is produced on the polydimethylsiloxane (PDMS) elastomer, and its wet adhesion properties are studied experimentally. A homebuilt adhesion tester is developed to carry out the pull-off experiments, in which the adhesion force of samples can be measured directly. The PDMS sample is fixed on the substrate, and a flat glass cylinder of 10 mm diameter is adopted as indenter. Different amounts of liquid are added to the area of contact by using a micropipette. Influences of preload, retraction speed, area ratio of round pillars, amount of liquid and approach-retraction cycle on wet adhesion of the patterned surface of PDMS samples are investigated.

Results show that the pull-off forces of all samples increase with preload and retraction speed. However, the pull-off forces increase slowly when the preload is over 3 N for both dry and wet conditions. The area ratios of round pillars increase the pull-off forces for the dry condition. When a small amount of liquid (0.1 μl) is added, the effect of the area ratios of round pillars on the pull-off force is not consistent with that of dry condition. Effects of amount of liquid on pull-off force for different area ratios of round pillars are various. For a certain amount of liquid, it is observed that the pull-off forces in general show a relatively high value for the first approach-retraction cycle, then decrease to a lower level starting from the second cycle, and suddenly increase and maintain a relatively constant value after several times of approach-retraction cycles. Our results can give insights into the repeated sliding actions of newt foot pads when climbing in wet environments as well as the possible functions of dense nanopillar arrays on newt footpad surface.

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