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.

Fog harvesting is an important method to solve the water shortage in arid and semi-arid areas by collecting water from air. Improving fog harvesting efficiency is still a big challenge to be overcome. Herein, under the inspiration of natural creatures, a novel harvesting structure that couples a hierarchical microchannel (HMC) needle with the Janus membrane by taking a conical pore as their junction is proposed. Such an HMC-conical pore-Janus membrane system can improve the harvesting efficiency by regulation of liquid behavior in the whole fog harvesting process involving droplet capture from air, high speed transport on the microchannel, and droplet detachment from Janus. The synergistic effects of the hierarchical channel-conical pore-Janus structure are exploited in terms of capture, transport, and detachment capabilities, and their underlying mechanism to enhance fog harvesting efficiency is built. Compared with the traditional harvesting structure, the proposed hierarchical channel-conical-Janus coupling mode was demonstrated to improve fog harvesting efficiency by 90%. Such a coupled system has potential applications in efficient fog harvesting systems, microfluidic devices, and liquid manipulation.

No abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202103423.

Robust and controllable wet attachment like tree frog toe pads attracts worldwide attention owing to potential applications in wet climbing robots, medical devices, and wearable sensors. Instead of conventional uniform pillars, nonuniform pillar arrays with features of inclination and gradients are discovered as typical structures on tree frog toe pads, whereas their effects on wet friction have been ignored. Micro‐nano in situ observation demonstrates that such a nonuniform pillar surface brings about unique multi‐dimensional self‐splitting behaviors in interfacial liquid films and contact stress distribution to enhance the wet attachment. The self‐splitting of the interfacial liquid film breaks the thick large area liquid film into an immense number of more uniform and robust tiny thin liquid bridges. Furthermore, the contact stress is redistributed by the inclined and gradient pillar array with contact stress self‐splitting, where the peak normal separating stress decreases ≈91% and lateral stress transmission increases ≈63%. Such contact stress self‐splitting further improves the liquid film self‐splitting by forming sturdy thin liquid films even under a larger load, which generates more robust capillarity with enhanced strong friction. Finally, theoretical models are built for the multi‐dimensional self‐splitting enhanced wet attachment, and applications in robotic and medical fields are performed to validate its feasibility.