Self-Compensating Liquid-Repellent Surfaces with Stratified Morphology

Robust and durable liquid-repellent surfaces

This review provides a comprehensive summary of characterization, design, fabrication, and application of robust and durable liquid-repellent surfaces.

Shape-Deformed Mushroom-like Reentrant Structures for Robust Liquid-Repellent Surfaces

Artificial liquid-repellent surfaces inspired by biomimetic structures provide a wide range of functional surfaces for various practical applications, such as self-cleaning, antisticking, oil/water separation, and droplet manipulation. However, functional biomimetic structures cannot be fabricated using conventional techniques. For example, mushroom-like topologies on the skin of springtails, which are referred to as "doubly reentrant structures," have attracted significant attention owing to their extraordinary liquid-repellent properties. Current methods of fabricating these reentrant structures have several limitations, such as complex material systems, processing steps, and additional chemical treatments. This study proposed a simple micro-shape-deformed approach to fabricate mushroom-like reentrant structures by digital light processing, a three-dimensional (3D) printing technique, with volumetric shrinkage. The nonuniform cross-linking process and light propagation during photopolymerization caused the deformation of the topological patterns atop the micropillar arrays, resulting in bent structures for mushroom-like shape-deformed microarchitectures. This 3D-printed shape-deformed microstructure exhibits a highly stable liquid repellency without perfluorinated coatings.

Biomimetic Water-Repelling Surfaces with Robustly Flexible Structures

Biomimetic liquid-repelling surfaces have been the subject of considerable scientific research and technological application. To design such surfaces, a flexibility-based oscillation strategy has been shown to resolve the problem of liquid-surface positioning encountered by the previous, rigidity-based asymmetry strategy; however, its usage is limited by weak mechanical robustness and confined repellency enhancement. Here, we design a flexible surface comprising mesoscale heads and microscale spring sets, in analogy to the mushroomlike geometry discovered on springtail cuticles, and then realize this through three-dimensional projection microstereolithography. Such a surface exhibits strong mechanical robustness against ubiquitous normal and shear compression and even endures tribological friction. Simultaneously, the surface elevates water repellency for impacting droplets by enhancing impalement resistance and reducing contact time, partially reaching an improvement of ∼80% via structural tilting movements. This is the first demonstration of flexible interfacial structures to robustly endure tribological friction as well as to promote water repellency, approaching real-world applications of water repelling. Also, a flexibility gradient is created on the surface to directionally manipulate droplets, paving the way for droplet transport.

Flexibility-Patterned Liquid-Repelling Surfaces

Droplets impacting solid surfaces is ubiquitous in nature and of practical importance in numerous industrial applications. For liquid-repelling applications, rigidity-based asymmetric redistribution and flexibility-based structural oscillation strategies have been proven on artificial surfaces; however, these are limited by strict impacting positioning. Here, we show that the gap between these two strategies can be bridged by a flexibility-patterned design similar to a trampoline park. Such a flexibility-patterned design is realized by three-dimensional projection micro-stereolithography and is shown to enhance liquid repellency in terms of droplet impalement resistance and contact time reduction. This is the first demonstration of the synergistic effect obtained by a hybrid solution that exploits asymmetric redistribution and structural oscillation in liquid-repelling applications, paving the rigidity-flexibility cooperative way of wettability tuning. Also, the flexibility-patterned surface is applied to accelerate liquid evaporation.

Liquid repellency enhancement through flexible microstructures

Artificial liquid-repellent surfaces have attracted substantial scientific and industrial attention with a focus on creating functional topological features; however, the role of the underlying structures has been overlooked. Recent developments in micro-nanofabrication allow us now to construct a skin-muscle type system combining interfacial liquid repellence atop a mechanically functional structure. Specifically, we design surfaces comprising bioinspired, mushroom-like repelling heads and spring-like flexible supports, which are realized by three-dimensional direct laser lithography. The flexible supports elevate liquid repellency by resisting droplet impalement and reducing contact time. This, previously unknown, use of spring-like flexible supports to enhance liquid repellency provides an excellent level of control over droplet manipulation. Moreover, this extends repellent microstructure research from statics to dynamics and is envisioned to yield functionalities and possibilities by linking functional surfaces and mechanical metamaterials.