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

Durable, Photostable Omniphobic Synthetic Leather Surfaces with Anti-Biofouling Properties for Hygienic Applications

Globally, the public health domain is increasingly emphasizing the need for surfaces that can resist bacterial contamination, as the consumption of bacteria-infected substance may cause illnesses. Thus, this study aimed to modify polyurethane (PU) synthetic leather surfaces by coating their upper layer with fluorine-functionalized nano-silica particles (FNPs). This simple modification imparted omniphobic characteristics, realizing anti-biofouling and self-cleaning properties. The effectiveness in preventing bacterial adhesion was confirmed by the dip-inoculation method using Escherichia coli O157:H7 and Staphylococcus epidermidis. Bacterial adhesion was evaluated based on bacterial counts using the pour plate method and by directly enumerating from scanning electron microscopy images. The attachment of bacteria to the modified omniphobic FNPs-coated PU leather surface decreased by over 98.2% compared to that on the bare surface. We expect that the method developed in this study will significantly reduce or even eliminate the potential risks associated with various biological cross-contamination scenarios, thereby enhancing hygiene standards.

Bioinspired Slippery Surfaces for Liquid Manipulation from Tiny Droplet to Bulk Fluid

Slippery surfaces, which originate in nature with special wettability, have attracted considerable attention in both fundamental research and practical applications in a variety of fields due to their unique characteristics of superlow liquid friction and adhesion. Although research on bioinspired slippery surfaces is still in its infancy, it is a rapidly growing and enormously promising field. Herein, a systematic review of recent progress in bioinspired slippery surfaces, beginning with a brief introduction of several typical creatures with slippery property in nature, is presented. Subsequently,this review gives a detailed discussion on the basic concepts of the wetting, friction, and drag from micro- and macro-aspects and focuses on the underlying slippery mechanism. Next, the state-of-the-art developments in three categories of slippery surfaces of air-trapped, liquid-infused, and liquid-like slippery surfaces, including materials, design principles, and preparation methods, are summarized and the emerging applications are highlighted. Finally, the current challenges and future prospects of various slippery surfaces are addressed.

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.

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.

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.

Improving superamphiphobicity by mimicking tree-branch topography

when a droplet impacts on a superhydrophobic structured surface below a certain impact velocity, the droplet can bounce off completely from the surface. However, above such velocity a fraction of the droplet will pin on the surface. Surfaces capable of repelling water droplets are ubiquitous in nature or have been artificially fabricated. However, as the surface tension of the liquid is reduced, the capability of the surface to remain non-wetting gets hindered. Despite progress in previous research, the understanding and development of superamphiphobic surface to impacting low surface tension droplets remains elusive. It is proposed that multi-layer re-entrant like roughness can further enhance the anti-wetting properties also for low surface tension fluids. In this work, we produce patterned conical micro-structures with lateral nano-sized roughness. Furthermore, the droplet impact experiments are conducted on various surfaces with variable surface tensions (27 mN/m - 72 mN/m) by using droplets with different Weber numbers (2-170). We show that conical microstructures with lateral roughness mimicking tree-branches provides a surface topology capable of absorbing the force exerted by the droplet during the impact which prevents the droplet from pinning on the surface at higher impact velocity even for low surface tension droplets. Our study has significance for understanding the liquid interaction mechanism with the surface during the impact process and for the associated surface design considerations.

Superhydrophobic μ-pillars via simple and scalable SLA 3D-printing: the stair-case effect and their wetting models

In nature, superhydrophobic surfaces (SHSs) exhibit microstructures with several roughness scales. Scalable fabrication and build-up along the X-Y plane represent the promise of 3D printing technology. Herein we report 3D printed microstructures with a dual roughness scale that achieves SHS using a readily available Formlabs stereolithography (SLA) printer. Pillar-like structure (PLS) arrangements with a wide range of geometrical shapes were 3D printed at three resolutions and two printing orientations. We discovered that a tilted printing direction enables a stair-case pattern on the μ-PLS surfaces, conferring them a μ-roughness that reduces the solid-liquid contact area. The programmed resolution governs the number of polymerized layers that give rise to the stepped pattern on the μ-PLS surfaces. However, this is reduced as the printing resolution increases. Also, all samples' experimental contact angles were consistent with theoretical predictions from Cassie-Baxter, Wenzel, and Nagayama wettability models. The underlying mechanisms and governing parameters were also discussed. It is believed that this work will enable scalable and high throughput roughness design in augmenting future 3D printing object applications.

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.

Effect of Silica Size and Content on Superamphiphobic Properties of Silica-Fluoropolymer Core-Shell Coatings

We present a facile approach to fabricate superamphiphobic surfaces by spray coating silica-fluoropolymer core-shell particles without substrate pretreatment with an additional binder resin. A series of SiO2@poly(1H,1H,2H,2H-heptadecafluorodecyl methacrylate) (SiO2@PFMA) core-shell particles with core particles of different sizes were prepared via thiol-lactam initiated radical polymerization (TLIRP). The surface of each SiO2 particle with an average particle size of 12, 80, 150, and 350 nm was modified with (3-mercaptopropyl) trimethoxysilane and used as a seed for TLIRP. The SiO2@PFMA particles with various SiO2 sizes and contents were coated on aluminum substrates by a spray gun and then thermally treated to form a stable, rough composite layer. During the spray coating, the core-shell particles were aggregated by rapid evaporation of the solvent and then irregularly adhered to the substrate resulting in hierarchical structures. In the case of SiO2@PFMAs with low SiO2 contents, the roughness created mainly by the polymer shell disappeared during heat treatment. However, the substrates coated with SiO2@PFMAs with high SiO2 contents maintained the roughness even after heat treatment. The core-shell particles prepared with 12 nm SiO2 formed a stable superamphiphobic surface. The water/hexadecane contact and sliding angles on an aluminum plate coated with SiO2@PFMA, prepared using 12 nm silica at 46 wt% silica content (12 nm-SiO2(46)@PFMA), were 178.5°/159.2° and 1°/7°, respectively. The cross-cut tape test showed that adhesion between the 12nm-SiO2(46)@PFMA and the aluminum substrate was classified as 5B. A glass surface spray-coated with the core-shell composite particles exhibited transparent superhydrophobicity and translucent superamphiphobicity by controlling the concentration of the coating solution.

Lubricant-infused directly engraved nano-microstructures for mechanically durable endoscope lens with anti-biofouling and anti-fogging properties

While a clear operating field during endoscopy is essential for accurate diagnosis and effective surgery, fogging or biofouling of the lens can cause loss of visibility during these procedures. Conventional cleaning methods such as the use of an irrigation unit, anti-fogging surfactant, or particle-based porous coatings infused with lubricants have been used but proven insufficient to prevent loss of visibility. Herein, a mechanically robust anti-fogging and anti-biofouling endoscope lens was developed by forming a lubricant-infused directly engraved nano-/micro-structured surface (LIDENS) on the lens. This structure was directly engraved onto the lens via line-by-line ablation with a femtosecond laser. This directly engraved nano/microstructure provides LIDENS lenses with superior mechanical robustness compared to lenses with conventional particle-based coatings, enabling the maintenance of clear visibility throughout typical procedures. The LIDENS lens was chemically modified with a fluorinated self-assembled monolayer (F-SAM) followed by infusion of medical-grade perfluorocarbon lubricants. This provides the lens with high transparency (> 70%) along with superior and long-lasting repellency towards various liquids. This excellent liquid repellency was also shown to be maintained during blood dipping, spraying, and droplet condensation experiments. We believe that endoscopic lenses with the LIDENS offer excellent benefits to endoscopic surgery by securing clear visibility for stable operation.

Liquid-Superrepellent Bioinspired Fibrillar Adhesives

Bioinspired elastomeric fibrillar surfaces have significant potential as reversible dry adhesives, but their adhesion performance is sensitive to the presence of liquids at the contact interface. Like their models in nature, many artificial mimics can effectively repel water, but fail when low-surface-tension liquids are introduced at the contact interface. A bioinspired fibrillar adhesive surface that is liquid-superrepellent even toward ultralow-surface-tension liquids while retaining its adhesive properties is proposed herein. This surface combines the effective adhesion principle of mushroom-shaped fibrillar arrays with liquid repellency based on double re-entrant fibril tip geometry. The adhesion performance of the proposed microfibril structures is retained even when low-surface-tension liquids are added to the contact interface. The extreme liquid repellency enables real-world applications of fibrillar adhesives for surfaces covered with water, oil, and other liquids. Moreover, fully elastomeric liquid-superrepellent surfaces are mechanically not brittle, highly robust against physical contact, and highly deformable and stretchable, which can increase the real-world uses of such antiwetting surfaces.