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 novel unidirectional liquid spreading surface with an inclined arc pitted groove, inspired by the continuous unidirectional liquid spreading mechanism on the peristome surface of N. alata, is explored and fabricated by two‐step UV lithography. Its superior unidirectional liquid spreading capability to that of other surface patterns is demonstrated, and its unidirectional liquid spreading mechanism is investigated.

Uni-directional liquid spreading without energy input has gained much attention due to its potential application in various areas such as microfluidic devices and energy fields. Recently, continuous uni-directional liquid spreading with fast speed was discovered on the peristome of Nepenthes alata, which possesses superhydrophilic hierarchical microgrooves and duck-billed microcavities with arc-shaped edges and gradient wedge corners. Inspired by the surface structure of the peristome, a novel bio-inspired uni-directional liquid spreading surface with various arc curvatures and wedge angles was built via two-step inclined UV exposure photolithography. The effects of the surface wettability and structural features, i.e. the arc-shaped outlines and wedge corners of microcavities, on the anisotropy of liquid spreading were investigated. The underlying mechanisms were made clear by comparing the effects of surface wettability and structural features of microcavities on both liquid spreading ability and liquid pinning ability. Finally, the controlling of anisotropic liquid spreading and thorough uni-directional liquid spreading were realized. This study provides inspiration to design novel uni-directional liquid spreading surfaces without energy input, and can further expand their application in areas such as non-powered delivery systems, microfluidic devices and self-lubrication in mechanical engineering.

Liquid/air accurate regulation has attracted growing attention in recent years for its diverse potential applications in bio-medicines, heat management, green energy, etc. Natural surfaces evolved innumerable hierarchical structures with exceptional functions to govern or regulate the liquid dynamic behaviors for their vital living, which have gradually been discovered as inspirations for creative design, such as fog harvesting, water fast transporting, and strong wet attachment. This review summarizes the current progress of bioinspired liquid/air regulations and their underlying mechanisms, including fast liquid/air spreading, liquid/air directional transport, and the interfacial liquid/air bridge acting forces. A fundamental understanding of both liquid/air dynamic behaviors on liquid–air–solid interfaces and their effects on the surface function has been increased with awareness of the importance of coupling effects from surface structures and material properties. The design principles and fabrication methods for bioinspired surface structure with unique liquid/air regulation are concluded, and several significant applications for electronics heat dissipation and biomedical devices are also presented. Finally, we provide new insights and future perspectives for the liquid/air regulation-based bioinspired functional materials.

Robust and reversible wet attachments are important for medical engineering and wearable electronics. Although ultrastrong capillarity from interfacial nano-thick liquid bridges creates tree frog’s strong wet friction, its unstable nano-liquid characteristic challenges further wet friction enhancement. Here, unique hierarchical micro-nano fibrous pillars have been discovered on Chinese bush crickets exhibiting a robust wet friction ~3.8 times higher than tree frog’s bulk pillar. By introducing a nano-fibrous pillar array covered with thin films (NFPF), the pillar’s separation position switches from the rear to front side compared with bulk pillars, indicating the interfacial contact stress shifting from compressing to stretching. This largely decreases the interfacial separation stress to form more stable and larger nano-liquid bridges. The NFPF array with self-splitting of interfacial liquid and contact stress further guards such interfacial stress shifting to ensure a ~1.9 times friction enhancement. Last, the theories are established, and the applications on wearable electronics are validated.