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

New fluorine-free low surface energy surfactants and surfaces

Modification and control of surface properties, such as surface tension γ at air-liquid (AL) interfaces and surface energy at solid-liquid (SL) surfaces, are at the heart of colloid and interface science. Certain applications require low or very low surface tensions γAL and surface energies γSL, for example and not limited to: microemulsification, aqueous foams for fire-fighting, hydrophobic, superhydrophobic and oleophobic surfaces to control spreading and wetting of aqueous and oily liquids on solids. In many cases low surface tensions/energies can only be obtained by employing perfluoroalkyl substances (PFAS) as surfactant or polymer additives or surface treatments. Although fluorocarbons and polymers have been employed for over 80 years, with many industrial and commercial benefits, it is now recognized that PFAS are very hazardous to the environment and health. Hence, in the coming years it will be necessary to phase out PFAS entirely, however, at present, there are very few viable alternatives available. This article outlines the chemical principles for designing F-free low surface energy (LSE) additives, and also covers the most recent advances in the quest for non-fluorinated surfactants and polymeric surface coatings.

Minimizing the Use of Per- and Polyfluoroalkyl Substances for Textured Wetting-Resistant Surfaces

Per- and polyfluoroalkyl substances (PFAS) have been used as synthetic chemicals to create textured wetting-resistant surfaces, which have a broad range of applications including omniphobic membranes, self-cleaning textiles, and anticorrosion coatings. However, the high persistence, toxicity, and bioaccumulation potential of PFAS have led to rising public concerns and stringent regulations, especially after the U.S. Environmental Protection Agency (USEPA) announced legally enforceable maximum contamination levels for six PFAS species in April 2024. In this paper, we provide our perspective that the use of PFAS can be avoided in the fabrication of textured omniphobic and superomniphobic surfaces, which display high wetting resistance against not only high surface tension liquids but also more importantly low surface tension liquids. We first discuss the role of PFAS in the design of conventional wetting-resistant surfaces. We then discuss the state-of-the-art strategies for creating PFAS-free textured omniphobic and superomniphobic surfaces with high wetting resistance while elucidating the underlying mechanism. Further, we emphasize that PFAS are indeed not always needed for textured surfaces with a sufficiently high wetting resistance in specific environmental applications such as desalination and wastewater treatment. We envision that this paper will motivate the scientific community to rethink and revolutionize the design framework toward more sustainable wetting-resistant surfaces, thereby circumventing the use of PFAS and the consequent health and environmental risks.

Scalable Multistep Roll-to-Roll Printing of Multifunctional and Robust Reentrant Microcavity Surfaces via a Wetting-Induced Process

Owing to their unique structural robustness, interconnected reentrant structures offer multifunctionality for various applications. a scalable multistep roll-to-roll printing method is proposed for fabricating reentrant microcavity surfaces, coined as wetting-induced interconnected reentrant geometry (WING) process. The key to the proposed WING process is a highly reproducible reentrant structure formation controlled by the capillary action during contact between prefabricated microcavity structure and spray-coated ultraviolet-curable resins. It demonstrates the superior liquid repellency of the WING structures, which maintain large contact angles even with low-surface-tension liquids, and their robust capability to retain solid particles and liquids under external forces. In addition, the scalable and continuous fabrication approach addresses the limitations of existing methods, providing a cost-effective and high-throughput solution for creating multifunctional reentrant surfaces for anti-icing, biofouling prevention, and particle capture.

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.

Nature-inspired adhesive systems

Many organisms in nature thrive in intricate habitats through their unique bio-adhesive surfaces, facilitating tasks such as capturing prey and reproduction. It's important to note that the remarkable adhesion properties found in these natural biological surfaces primarily arise from their distinct micro- and nanostructures and/or chemical compositions. To create artificial surfaces with superior adhesion capabilities, researchers delve deeper into the underlying mechanisms of these captivating adhesion phenomena to draw inspiration. This article provides a systematic overview of various biological surfaces with different adhesion mechanisms, focusing on surface micro- and nanostructures and/or chemistry, offering design principles for their artificial counterparts. Here, the basic interactions and adhesion models of natural biological surfaces are introduced first. This will be followed by an exploration of research advancements in natural and artificial adhesive surfaces including both dry adhesive surfaces and wet/underwater adhesive surfaces, along with relevant adhesion characterization techniques. Special attention is paid to stimulus-responsive smart artificial adhesive surfaces with tunable adhesive properties. The goal is to spotlight recent advancements, identify common themes, and explore fundamental distinctions to pinpoint the present challenges and prospects in this field.

Quasi-seamless stitching for large-area micropatterned surfaces enabled by Fourier spectral analysis of moiré patterns

The main challenge in preparing a flexible mold stamp using roll-to-roll nanoimprint lithography is to simultaneously increase the imprintable area with a minimized perceptible seam. However, the current methods for stitching multiple small molds to fabricate large-area molds and functional surfaces typically rely on the alignment mark, which inevitably produces a clear alignment mark and stitched seam. In this study, we propose a mark-less alignment by the pattern itself method inspired by moiré technique, which uses the Fourier spectral analysis of moiré patterns formed by superposed identical patterns for alignment. This method is capable of fabricating scalable functional surfaces and imprint molds with quasi-seamless and alignment mark-free patterning. By harnessing the rotational invariance property in the Fourier transform, our approach is confirmed to be a simple and efficient method for extracting the rotational and translational offsets in overlapped periodic or nonperiodic patterns with a minimized stitched region, thereby allowing for the large-area and quasi-seamless fabrication of imprinting molds and functional surfaces, such as liquid-repellent film and micro-optical sheets, that surpass the conventional alignment and stitching limits and potentially expand their application in producing large-area metasurfaces.

High-Throughput Metal 3D Printing Pen Enabled by a Continuous Molten Droplet Transfer

In metal additive manufacturing (AM), arc plasma is attracting attention as an alternative heat source to expensive lasers to enable the use of various metal wire materials with a high deposition efficiency. However, the stepwise material deposition and resulting limited number of degrees of freedom limit their potential for high-throughput and large-scale production for industrial applications. Herein, a high-throughput metal 3D printing pen (M3DPen) strategy is proposed based on an arc plasma heat source by harnessing the surface tension of the molten metal for enabling continuous material deposition without a downward flow by gravity. The proposed approach differs from conventional arc-based metal AM in that it controls the solidification and cooling time between interlayers of a point-by-point deposition path, thereby allowing for continuous metal 3D printing of freestanding and overhanging structures at once. The resulting mechanical properties and unique microstructures by continuous metal deposition that occur due to the difference in the thermal conditions of the molten metal under cooling are also investigated. This technology can be applied to a wide range of alloy systems and industrial manufacturing, thereby providing new possibilities for metal 3D printing.

Fresnel Diffraction Strategy Enables the Fabrication of Flexible Superomniphobic Surfaces

Doubly re-entrant surfaces inspired by springtails exhibit excellent repellency to low-surface-tension liquid. However, the flexible doubly re-entrant surfaces are difficult to fabricate, especially for the overhang of the structure. Herein, we demonstrate a simple Fresnel aperture diffraction modulation strategy in microscale lithography coupled with a molding process to obtain the flexible doubly re-entrant superomniphobic surfaces with nanoscale overhangs. The negative nanoscale overhang features were formed in a single-layer photoresist due to the fine-modulation of the optical intensity fluctuation of the Fresnel aperture diffraction. The as-prepared flexible non-fluorinated polydimethylsiloxane (PDMS) doubly re-entrant microstructure based on the Fresnel aperture diffraction (D-BF) surface (without any additional treatments) could repel ethanol droplets (21.8 mN m^-1) in the Cassie-Baxter state. The robust nanoscale overhangs obtained by the molding process enable the maximum breakthrough pressure for the low-surface-tension ethanol droplets on the D-BF surfaces up to about 230 Pa, allowing ethanol liquids with Weber numbers up to 8.7 to fully bounce off. The fabricated non-fluorinated D-BF superomniphobic surface maintains outstanding liquid repellency after the surface wettability modification and deformation test.

Second-Level Microgroove Convexity is Critical for Air Plastron Restoration on Immersed Hierarchical Superhydrophobic Surfaces

Air plastrons trapped on the surfaces of underwater superhydrophobic surfaces are critical for their function. Fibrillar morphologies offer a natural pathway, yet they are limited to a narrow range of liquid-surface systems and are vulnerable to pressure fluctuations that irreversibly destroy the air layer plastron. Inspired by the convexly grooved bases of water fern (Salvinia) leaves that support their fibrous outgrowths, we focus on the effect of such second-level grooved structures or microgrooves on the plastron restoration on immersed three-dimensional (3D)-printed hierarchical surfaces. Elliptical, interconnected microgrooves are fabricated with varying surface curvatures to study the effect of their morphology. Immersion experiments reveal that the convex groove curvature stabilizes a seed gas layer (SGL) that facilitates plastron restoration for all immersed hydrophobic surfaces. Theoretical calculations and atomic-scale computations reveal that the SGL storage capacity that sets the SGL robustness follows from the liquid menisci adaption to the groove geometry and pressure, from micro- to nanoscales, and it can be further tuned using separated grooves. Our study highlights groove convexity as a key morphological feature for the design of second-level architectures for underwater air plastron restoration on hierarchical superhydrophobic surfaces.

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.