Micro–Nano Hierarchical Structure Enhanced Strong Wet Friction Surface Inspired by Tree Frogs

• Published in: Advanced science Vol. 7; no. 20; pp. 2001125 - n/a
• Main Authors: Zhang, Liwen, Chen, Huawei, Guo, Yurun, Wang, Yan, Jiang, Yonggang, Zhang, Deyuan, Ma, Liran, Luo, Jianbin, Jiang, Lei
• Format: Journal Article
• Language: English
• Published: Weinheim John Wiley & Sons, Inc 01.10.2020; John Wiley and Sons Inc; Wiley
• Subjects: Adhesives; bio‐interfaces; Friction; Frogs; interfacial liquid adjustment; tree frogs; wearable sensors; wet friction
• ISSN: 2198-3844; 2198-3844
• DOI: 10.1002/advs.202001125

Superior wet attachment and friction performance without the need of special external or preloaded normal force, similar to the tree frog's toe pad, is highly essential for biomedical engineering, wearable flexible electronics, etc. Although various pillar surfaces are proposed to enhance wet adhesion or friction, their mechanisms remain on micropillar arrays to extrude interfacial liquid via an external force. Here, two‐level micropillar arrays with nanocavities on top are discovered on the toe pads of a tree frog, and they exhibit strong boundary friction ≈20 times higher than dry and wet friction without the need of a special external or preloaded normal force. Microscale in situ observations show that the specific micro–nano hierarchical pillars in turn trigger three‐level liquid adjusting phenomena, including two‐level liquid self‐splitting and liquid self‐sucking effects. Under these effects, uniform nanometer‐thick liquid bridges form spontaneously on all pillars to generate strong boundary friction, which can be ≈2 times higher than for single‐level pillar surfaces and ≈3.5 times higher than for smooth surfaces. Finally, theoretical models of boundary friction in terms of self‐splitting and self‐sucking are built to reveal the importance of liquid behavior induced by micro–nano hierarchical structure. A strong wet attachment bioinspired surface with hierarchical pillars and nanocavities is introduced based on the unique interfacial liquid adjusting effects on the tree frog toe pad, where robust interfacial capillarity from nanometer‐thick liquid film generates a boundary friction ≈20 times its wet and dry friction. Such bioinspired surfaces demonstrate potential applications in fields including medical devices and wearable sensors.