Fabrication of Flexible and Re-entrant Liquid-Superrepellent Surface Using Proximity and PNIPAM-Assisted Soft Lithography.

The nature-inspired flexible and re-entrant liquid-superrepellent surface has attracted significant attention due to its excellent superomniphobic performance against low-surface-tension liquids. Although conventional photolithography and molding methods offer the advantage of large-area manufacturing, they often involve multiple double-sided alignment and exposure steps, resulting in complex procedures with long processing cycles. In this study, we proposed a straightforward single-exposure ultraviolet proximity lithography method for re-entrant liquid-superrepellent surface fabrication using a photomask with a coaxial circular aperture and ring. A theoretical calculation model for the three-dimensional light intensity distribution in proximity lithography was developed for the prediction of feature sizes for both singly and doubly re-entrant microstructures. Soft lithography techniques, which rely on surface modification and the modulation of the transfer material's flexibility, efficiently optimized the fabrication of flexible re-entrant molds and patterns. By incorporating nanoclay-modified poly(N-isopropylacrylamide) (PNIPAM) into the molding process, we fabricated a three-layer hierarchical structure featuring micrometer-scale wrinkles, re-entrant microstructures, and nanoscale fluorinated silica particles, significantly enhancing the surface's robustness and pressure resistance. The resulting large-area flexible and re-entrant liquid-superrepellent surface demonstrated excellent superomniphobic self-cleaning performance and satisfactory optical transparency, as evidenced by reflection and transmission experiments, showcasing its potential applications in self-cleaning, membrane distillation, and digital microfluidics.

One-Step Fabrication of Flexible Bioinspired Superomniphobic Surfaces.

Flexible superomniphobic doubly re-entrant (Dual-T) microstructures inspired by springtails have attracted growing attention due to their excellent liquid-repellent properties. However, the simple and practical manufacturing processes of the flexible Dual-T microstructures are urgently needed. Here, we proposed a one-step molding process coupled with the lithography technique to fabricate the elastomeric polydimethylsiloxane (PDMS) Dual-T microstructure surfaces with high uniformity. The angle between the downward overhang and the horizontal direction could reach 90° (vertical overhang). The flexible superomniphobic Dual-T microstructure surfaces, without fluorination treatment and physical treatments, could repel liquids with a surface tension lower than 20 mN m-1 in the Cassie-Baxter state. Owing to the excellent robustness of the one-step molding downward overhanging, the max breakthrough pressure of this surface could reach up to 164.3 Pa for ethanol droplets. Furthermore, the flexible superomniphobic Dual-T surface allowed impinging ethanol droplets to completely rebound at the Weber number up to 7.1 with an impact velocity of ∼0.32 m s-1. The Dual-T microstructure surface maintained excellent superomniphobicity even after surface oxygen plasma treatment and exhibited excellent structural robustness and recoverability to various large mechanical deformations.

Superhydrophobic self-similar nonwoven-titanate nanostructured materials.

HYPOTHESIS: Self-similarity is a scale-invariant irregularity that can assist in designing a robust superhydrophobic material. A combinatorial design strategy involving self-similarity and dual-length scale can be employed to create a new library of a doubly re-entrant, disordered, and porous network of superhydrophobic materials. Asymmetric wettability can be engineered in nonwoven materials by rendering them with superhydrophobic characteristics on one side. EXPERIMENTS: A facile, scalable, and inexpensive spray-coating technique was used to decorate the weakly hydrophobicstearate-treatedtitanate nanowires (TiONWs)over the self-similar nonwoven material. Laser scanning confocal microscopy was employed to image the impalement dynamics in three dimensions. With the aid of X-ray microcomputed tomography analysis, the three-dimensional (3D) nonwoven structural parameters were obtained and analyzed. The underwater superhydrophobic behavior of the prepared samples was investigated. FINDINGS: A classic 'lotus effect' has been successfully endowed in self-similar nonwoven-titanate nanostructured materials (SS-Ti-NMs) from a nonwoven material that housed the air pockets in bulk and water repellent TiONWs on the surface. The finer fiber-based SS-Ti-NMs exhibited lower roll-off angles and a thinner layer of water on its surface. An asymmetric wettability and the unusual display of underwater superhydrophobic behavior of SS-Ti-NMs have been uncovered.

Nonfluorinated Superomniphobic Surfaces through Shape-Tunable Mushroom-like Polymeric Micropillar Arrays.

Superomniphobic surfaces showing extremely liquid-repellent properties have received a great amount of attention as they can be used in various industrial and biomedical applications. However, so far, the fabrication processes of these materials mostly have involved the coating of perfluorocarbons onto micro- and nanohierarchical structures of these surfaces, which inevitably causes environmental pollution, leading to health concerns. Herein, we developed a facile method to obtain flexible superomniphobic surfaces without perfluorocarbon coatings that have shape-tunable mushroom-like micropillars (MPs). Inspired by the unique structures on the skin of springtails, we fabricated mushroom-like structures with downward facing edges (i.e., a doubly re-entrant structure) on a surface. The flexible MP structures were fabricated using a conventional micromolding technique, and the shapes of the mushroom caps were made highly tunable via the deposition of a thin aluminum (Al) layer. Due to the compressive residual stress of the Al, the mushroom caps were observed to bend toward the polymer upon forming doubly re-entrant-MP structures. The obtained surface was found to repel most low-surface-tension liquids such as oils, alcohols, and even fluorinated solvents. The developed flexible superomniphobic surface showed liquid repellency even upon mechanical stretching and after surface energy modification. We envision that the developed superomniphobic surface with high flexibility and wetting resistance after surface energy modification will be used in a wide range of applications such as self-cleaning clothes and gloves.

Superoleophobic Slippery Lubricant-Infused Surfaces: Combining Two Extremes in the Same Surface.

The ability to create superoleophobic surfaces repellent toward low-surface-tension liquids is important for various applications, and has been recently demonstrated using re-entrant or doubly re-entrant microtopography. Liquid droplets on such surfaces feature composite liquid-solid-air interfaces, whereas composite liquid-lubricant-air interfaces would have potential for additional repellency. Here, the development of a novel slippery superoleophobic surface with low adhesion is demonstrated via combining doubly re-entrant microtopography with slippery lubricant-infused porous surfaces. This is realized by using 3D direct laser writing to fabricate doubly re-entrant micropillars with dedicated nanostructures on top of each pillar. The top nanostructures stabilize the impregnated slippery lubricant, while the re-entrant geometry of the micropillars prevents lubricant from spreading. The slippery layer reduces the adhesion of liquid to the pillars, as proved using scanning droplet adhesion microscopy (SDAM), while the doubly re-entrant micropillars make the surface superoleophobic. This novel interface combining two extremes, superoleophobicity and slippery lubricant-infused surface, is of importance for designing superoleophobic and superhydrophobic surfaces with advanced liquid repellent, anti-icing, or anti-fouling properties.

Flexible and Robust Superomniphobic Surfaces Created by Localized Photofluidization of Azopolymer Pillars.

Springtails, insects which breathe through their skins, possess mushroom-shaped nanostructures. As doubly re-entrant geometry in the mushroom head enhances the resistance against liquid invasion, the springtails have robust, liquid-free omniphobic skins. Although omniphobic surfaces are promising for various applications, it remains an important challenge to mimic the structural feature of springtails. This paper presents a pragmatic method to create doubly re-entrant nanostructures and robust superomniphobic surfaces by exploiting localized photofluidization of azopolymers. Irradiation of circularly polarized light reconfigures azopolymer micropillars to have a mushroom-like head with a doubly re-entrant nanogeometry through protrusion and inward bending of polymer film from the top edge. The light-driven reconfigured micropillars facilitate the pining of triple line as the springtails do. In particular, the unique geometry exhibits superomniphobicity even for liquids whose equilibrium contact angles are almost zero in the presence of a practical level of external pressure. In addition, the simple fabrication process is highly reproducible, scalable, and compatible with various substrate materials including flexible polymeric film. Our results suggest that our photofluidization technology will provide a practical route to develop robust superomniphobic surfaces.