Nature-inspired reentrant surfaces

Abstract

Reentrant feature widely exists in natural structures ranging from mushroom, overhang, undercut, trapezoid, sphere, spatula and taper to sharp edge, and has triggered a biomimetic design revolution of functional surfaces for extreme repellency to fluid foulants, directional fluid navigation, and strong adaptive adhesion. In this Review, we discuss how the bionic introduction of reentrant feature on a surface mediates fluid wetting, contact line motion, as well as solid adhesion. We briefly introduce nature-inspired design principles from representative reentrant surfaces on the springtail, gecko, and pitcher plant. Through quantitatively associating microscopic structural parameters of reentrant feature, including angle, shape, diameter, thickness, sharpness and anisotropy, with macroscopically visible wetting and adhesion properties, we evaluate how the design of reentrant feature can enable exceptional superomniphobicity, strong adaptive adhesion, and directional fluidic navigation, promising an effective guidance for the creation of superomniphobic surface, fluid-navigation surface, adhesive surface or their combinations. The realization of the reentrant surfaces benefits from the rapid development of cutting-edge manufacturing technologies, such as 3D printing, lithography, microfluidics, self-assembly, and template-assisted soft replication. We finally provide potential applications of reentrant surfaces, as well as scientific and technological challenges and opportunities.

Introduction

Reentrant structure, where a concave topographic curvature exists from the top to the bottom of the structure, has been elegantly designed into various shapes ranging from mushroom, overhang, trapezoid, undercut, spatula, taper and sphere to sharp edge in the past several decades [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Relying on the rapid development of manufacturing techniques including printing, lithography, self-assembly, soft replication and microfluidics, these complex reentrant structures can be facilely constructed to engineer functional surfaces with super-repellency to virtually all fluids [1], [3], [4], [5], [6], strong and adaptive adhesion to smooth and rough surfaces [7], [8], [9], [10], [12], [13], as well as preferential fluid navigation in a desired direction [14], [15], [16], [17]. Benefiting from these superior capabilities, such reentrant-decorated surfaces have received an enormous amount of interest, from the academic research to the industrial worlds, in diverse applications including self-cleaning, anti-fouling, anti-virus, functional textiles, droplet manipulation, microfluidics, oil–water separation, Raman scattering, solar evaporation, water desalination, adhesives, bioelectronics, sensors, soft grippers and robots [1], [4], [7], [12], [13], [16], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28].

The custom design of reentrant surfaces with different functions is initially inspired by fascinating reentrant surfaces in nature, as representatively exemplified by springtail cuticle [18], [29], [30], [31], gecko toe [32], [33], [34], [35], [36], [37], and pitcher plant peristome [38], [39], [40], [41], [42], [43]. Specifically, overhang structures on the hierarchically arranged and highly textured springtail cuticle can effectively prevent outside fluids from wetting the cuticle cavities so that springtails can freely survive in various fluid environments [18]. Spatula-shaped reentrant structures at the terminal end of seta promise geckos to intimately contact with various rough surfaces, attaching adaptively and climbing freely [36]. Sharp-edge reentrant structures decorated along one side of arch-shaped cavities inside hierarchical microgrooves act as a dam to arrest the spreading of the liquid in one direction, thereby resulting in directional liquid transport from the inner side to the outer side of the peristome on the pitcher plant [39], [43]. Therefore, learning from nature to rationally design reentrant structure can provide an effective route to create surfaces with superomniphobicity (Fig. 1a), directional fluid navigation (Fig. 1b), strong adhesion (Fig. 1c) or their combinations. More importantly, these functions can help each other. For instance, due to the exceptional repellency to ultralow-surface-tension fluids, double reentrant can enable a strong adhesion in the presence of ultralow-surface-tension fluids. The unidirectional fluid transport on the peristome of pitcher plant can greatly facilitate the formation of lubricant-infused omniphobic surfaces.

Nature is so magical that a similar reentrant is able to sculpt multiple functions or surfaces. Despite such similarity, the attainment of a specific function or surface still requires the rational design of structural parameters of reentrant feature including reentrant angle, shape, spacing, diameter, thickness, sharpness and anisotropy, as well as the proper selection of materials depending on the wettability and mechanical properties. For example, superomniphobic and fluid-navigation surfaces prefer sharp reentrants, which can promise steady prevention of liquid penetration into the structures, rather than round reentrants [44], [45]. Differently, adhesive surfaces care more about the contact area, the thickness and the hierarchy of reentrant structures, which is conducive to strongly adhere to various rough surfaces [8], [12]. Thus, it is extremely necessary to distinguish the underlying association between structural parameters of nanoscale and microscale reentrants and macroscopic functions in wetting and adhesion. In turn, the clear revelation of these structure–function associations is extensively beneficial to guide either the optimization of reentrant structures to achieve an optimal desirable function, or the trade-off of reentrant parameters to gain multiple functions on a single surface. To our best knowledge, existing reviews mainly focus on a single specific function of mushroom-shaped reentrant, for example, superomniphobic surfaces inspired by springtail [18] and adhesion inspired by gecko [10]. Although some insightful reviews have shown the regulation of reentrant structures on both adhesion and wetting [2], a comprehensive and timely review on how nature-inspired reentrant structures create superomniphobic surfaces, fluid-navigation surfaces, adhesive surfaces or their combinations is still lacking. Thus, it is necessary to summarize how the optimization of reentrant feature can enable superior superomniphobicity, strong adaptive adhesion, and directional fluidic navigation.

In the Review, we first introduce the fundamentals in wetting and adhesion, and inspired principles on natural reentrant surfaces from springtail, gecko, and pitcher plant. Relying on these principles and relevant functions, superior capabilities of diverse reentrant structures in the attainment of superomniphobicity, adhesion, and fluid navigation are elucidated and compared by their individual criteria. Finally, we provide manufacturing technologies to create reentrant surfaces, potential applications, as well as scientific and technological challenges and opportunities.