Tree Frog-Inspired Micropillar Arrays with Nanopits on the Surface for Enhanced Adhesion under Wet Conditions

Abstract

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 ∼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.

Tree frog-inspired nanopillar arrays for enhancement of adhesion and friction

“…It has been proposed that they are helpful to build up nanoscale liquid bridges at the contact interface and may generate a suction effect under a relatively large loading force. 26 The dopaHexNRA shows a hexagonal micropatterned nanorod array after molding [Fig. 3(e)].…”

“…Mixing the PS nanoparticles into PDMS micropillars, followed by the removal of PS nanoparticles on the surface, composite micropillars with nanopits on the micropillar top were successfully fabricated. 26 The composite micropillars showed ∼36.5 times wet adhesion of a tree frog's toe pad, where the micro-and nano-sized liquid bridges at the interface play a significant role. 26 These previous works suggest the important role of these aligned keratin nanofibrils in the adhering ability of tree frogs in dry and wet environments.…”

“…26 The composite micropillars showed ∼36.5 times wet adhesion of a tree frog's toe pad, where the micro-and nano-sized liquid bridges at the interface play a significant role. 26 These previous works suggest the important role of these aligned keratin nanofibrils in the adhering ability of tree frogs in dry and wet environments. However, the understanding of the functions of these closely packed keratin nanopillars remains far from satisfactory.…”

“…8,20,21 Inspired by the structured surfaces on tree frog toe pads, micro-and nanostructured surfaces have been composed and investigated. 11,15,20,[22][23][24][25][26][27][28] Compared to the flat surface, hexagonal micropillar arrays presented a higher friction force on wet surfaces due to the formation of direct contacts by draining the liquid out of the contact interface and the effect of contact splitting. 22,23,29 For the hexagonal micropillars, the sizes of the micropillar and the channel have a strong influence on the adhesion performances.…”

A spontaneously sustainable bioinspired interfacial evaporation driven power generation strategy

“…Although the photothermal conversion characteristics are still the limiting bottleneck of the interfacial evaporation performance, the functional structure of natural organisms provides us with new inspiration [8][9][10][11] . In view of the interfacial light and heat absorption of solar energy, the lighttrapping structure on the surface of moth eyes gives us much inspiration due to its zero reflection of sunlight [12][13][14] .…”