Springtail-inspired superomniphobic surface with extreme pressure resistance

Abstract

Both high static repellency and pressure resistance are critical to achieving a high-performance omniphobic surface. The cuticles of springtails have both of these features, which result from their hierarchical structure composed of primary doubly reentrant nanostructures on secondary microgrooves. Despite intensive efforts, none of the previous studies that were inspired by the springtail were able to simultaneously achieve both high static repellency and pressure resistance because of a general trade-off between these characteristics. We demonstrate for the first time a springtail-inspired superomniphobic surface displaying both features by fabricating a hierarchical system consisting of serif-T-shaped nanostructures on microscale wrinkles, overcoming previous limitations. Our biomimetic strategy yielded a surface showing high repellency to diverse liquids, from water to ethanol, with a contact angle above 150°. Simultaneously, the surface was able to endure extreme pressure resulting from the impacts of drops of water and of ethylene glycol with We >> 200, and of ethanol with We ~ 53, which is the highest pressure resistance ever reported. Overall, the omniphobicity of our springtail-inspired fabricated system was found to be superior to that of the natural springtail cuticle itself.

Fig. 1. Rational design of a hierarchical system inspired by the springtail cuticle.

(A) Photograph (courtesy of B. Valentine) of a springtail displaying liquid repellency and resistance to high-pressure raindrops in a flooded habitat (left). SEM images showing the hierarchical system in a springtail cuticle composed of primary and secondary granules (middle and right panels). (B) Schematic of the steps used to fabricate serif-T–shaped nanostructures. Nanoimprinting and the SSL method were used here. The serif-T–shaped nanostructures were made with ~400-nm-diameter dots and ~ 400-nm spacing between the dots, both dimensions similar to those for the primary granules of springtail cuticles. (C) Scheme to fabricate microscale wrinkles via heat-induced shrinkage after nanostructure fabrication.

Fig. 2. Morphologies of serif-T–shaped nanostructures on a microscale-wrinkled substrate.

(A) SEM image of a fabricated hierarchical system with serif-T–shaped nanostructures on a microscale-wrinkled substrate. Inset SEM image shows details of a few serif-T–shaped nanostructures. (B) Cross-sectional view of the hierarchical serif-T system imaged by using a FIB-SEM. The inset in the red box shows a magnified view of a serif-T–shaped structure with a well-made pillar. The inset in the blue box shows only the head of this structure, with the head made hollow to provide a doubly reentrant shape.

Fig. 3. Morphological control of nano- and microstructures and corresponding static repellency.

(A) Hierarchical systems with different kinds of nanostructures. (B) Static contact angles of ethanol on hierarchical systems with control nanostructures. (C) Static contact angles of test liquids (water, ethylene glycol, and ethanol) on the hierarchical serif-T–shaped nanostructures system.

Fig. 4. Wetting resistance levels of the hierarchical systems to water drop impact.

(A) Schematic figures of the hierarchical systems with different nanostructures, and plots of maximum spreading as a function of We for each of these systems. (B) Snapshots of water droplets interacting with two of the systems in (A) using We ~ 110. (C) Schematics of the serif-T system with different microstructure morphologies and plots of maximum spreading as a function of We for each of these systems. (D) Snapshots of water droplets interacting with two of the systems in (C) using We ~ 110. (E) Contact time analysis for various systems (We ~ 220).

Fig. 5. Wetting resistance of hierarchical system to impacts by low-surface-tension liquids.

Comparison of bouncing-off behaviors of ethylene glycol for serif-T nanostructure system on (A) flat and (B) highly wrinkled surfaces. Images of the interactions of ethylene glycol drops with these surfaces are shown. (C) Bouncing behaviors on the serif-T hierarchical system on different levels of microstructures subjected to various applied strains with ethanol impact (We from ~13 to 65). (D) Images of ethanol drops interacting with serif-T hierarchical systems with different microscale morphologies at We ~ 53.