The Challenge of Superhydrophobicity: Environmentally Facilitated Cassie-Wenzel Transitions and Structural Design

Preparation and properties of anti-weathering and superhydrophobic wood based on palm wax in polydimethylsilane nanocomposite coating

The superhydrophobic coating on wood surface is an effective method to improve the durability and service life of wood. In this paper, dodecyl modified gas-phase nano-SiO2 particles (M-SiO2), polydimethylsilane-trimethoxysilane ends (PDMS-Ts), palm wax, γ-glycidoxypropyl trimethoxysilane (KH560), and isopropyl titanate (TTIP) were blended and sprayed on the surface of wood by a simple one-step method at room temperature. The superhydrophobic modified wood has a water contact angle (WCA) of 161.2° and a sliding angle (SA) of less than 3°, demonstrating excellent anti-fouling, self-cleaning, and durability properties. Specifically, after undergoing various durability tests, including sandpaper abrasion (15 cycles), finger abrasion (30 cycles), sand impact (90 cycles), tape peeling (70 cycles), and chemical stability tests (24 h immersion in acidic and alkaline environments), the wood surface maintained its superhydrophobicity. Importantly, the wood's color remained virtually unchanged after superhydrophobic modification, preserving its inherent aesthetic properties. Moreover, after 768 h of artificial weathering, the color difference (ΔE*) of the modified wood was only 4.35, indicating excellent color stability, with the water contact angle remaining largely unchanged. Notably, the modified wood significantly delayed the combustion time and maintained its superhydrophobic properties after combustion tests. This study presents a novel method to achieve superhydrophobicity, weathering resistance, and flame retardancy in wood, offering new insights for the protection of wood used in outdoor applications.

Femtosecond Laser Fabrication of Wettability-Functional Surfaces: A Review of Materials, Structures, Processing, and Applications

Wettability-functional surfaces are crucial in both theoretical investigation and engineering applications. Compared to traditional micro/nano fabrication methods (such as ion etching, sol-gel, chemical vapor deposition, template techniques, and self-assembly), femtosecond laser processing has unique advantages, such as unmatched precision, flexible controllability, and material adaptability, widely used for the fabrication of wettability-functional surfaces. This paper systematically discusses the principle and advancement of femtosecond laser micro/nano processing in regulating surface wettability and analyzes the laser modulation mechanisms and structural design strategies for wettability-functional surfaces on various materials. Additionally, this paper reviews the practical applications of femtosecond laser-based wettability-functional surfaces in environmental engineering, aerospace, and biomedical fields, while highlighting the challenges and future directions for femtosecond laser processing of wettability-functional surfaces.

Functionalized inorganic hydrogel-based membrane for synergistic oil/water separation and catalytic degradation

Hydrogel-modified superwetting membranes typically exhibit remarkable resistance to oil fouling during oil/water separation but suffer from unfavorable stability due to the inevitable swelling and exfoliation. A functionalized inorganic hydrogel-based membrane (TIH@PVDF) with satisfactory durability was proposed for the first time to ingeniously integrate excellent anti-oil fouling and high flux recovery (FRR) for efficient oil/water separation. The TIH@PVDF membrane exhibited a high separation efficiency of over 99 % for oil-in-water emulsions (including liquid paraffin, isooctane, and hexadecane). Owing to the synergistic effect of hydration and catalytic ability from inorganic hydrogel, a FRR of 97.9 % was achieved by catalytic regeneration after seven cycles of oil/water separation, outperforming hydraulic cleaning (90.6 %). Most importantly, the TIH@PVDF membrane demonstrates outstanding capability in separating actual oil field-produced water, indicating its potential for practical application. Meanwhile, the existence of metallic elements in the inorganic hydrogel endowed the TIH@PVDF membrane with sufficient active sites to produce O2•- and 1O2 via peroxymonosulfate (PMS) activation towards organics decomposition. The TIH@PVDF membrane presented a satisfactory removal efficiency (99.1 %) of sulfamethoxazole during a single-pass catalytic separation process. This research may revolutionize the advancement of inorganic hydrogel-based catalytic membranes for oil/water separation and wastewater decontamination.

Preparation of durable and multifunctional superhydrophobic cellulose paper-based materials using the phase change properties of carnauba waxes

Amid growing environmental awareness, hydrophobic paper-based materials demonstrate pronounced advantages in sustainable packaging. However, current superhydrophobic modification techniques exhibit inherent limitations in durability and moisture resistance, rendering them inadequate for addressing diverse environmental challenges. This study presents a protection-and-release strategy, employing a hierarchical curing process to fabricate a graded rough surface featuring a honeycomb-like primary structure and nano-fumed silica (NFS) as the secondary microstructure. The formation mechanism of continuous, hierarchical, and durable rough structures in superhydrophobic coated paper-based materials (SCPBM) was investigated. SCPBM exhibited remarkable durability under mechanical abrasion (200 g, 50 cycles), tape peeling (30 cycles), ultrasonic treatment (300 W, 21 min), and various extreme conditions. Moreover, SCPBM has been demonstrated to exhibit exceptional resistance to water flow impact, immersion (68.5 % reduction), and bending-induced moisture resistance, Shows particular promise in Anti-icing performance and Multiphase repellence applications. Building upon the protection and release strategy, this study proposes a novel hierarchical curing transition model termed 'lotus seed head-honeycomb', which offers a versatile design blueprint for the broad-scale fabrication of durable superhydrophobic materials.

Fabrication of Superhydrophobic Ultra-Fine Brass Wire by Laser Processing

Superhydrophobic metal wires have shown great application prospects in oil-water separation, anti-corrosion, anti-icing, and other fields due to their excellent water repellency. However, how to fabricate a superhydrophobic surface on ultra-fine metal wire remains a challenge. Here, we proposed a method using laser processing to efficiently fabricate superhydrophobic ultra-fine brass wire. Firstly, we analyzed the mechanism of the laser processing of curved surfaces and designed a controllable angle rotation fixture to avoid the machining error caused by secondary positioning in the machining process. Then, we investigated the influences of the laser power, scanning speed, and scanning times on the surface morphology and wettability of the ultra-fine brass wire. The optimal laser processing parameters were obtained: laser power of 6 W, scanning speed of 500 mm/s, and scanning time of 1. After low surface energy modification, the water contact angle and surface roughness Sa of the ultra-fine brass wire were 156° and 1.107 μm, respectively. This work is expected to enrich the theory and technology for fabricating superhydrophobic ultra-fine brass wire.

Constructing Robust and Multifunctional Superamphiphobic Surfaces by Using Two Different Nanostructures of Silicon Dioxide

Superamphiphobic surfaces, combining both superhydrophobic and superoleophobic properties, show tremendous potential applications in various fields. However, the complicated procedures, expensive equipment, and poor mechanical robustness and durability seriously limit their commercialization. In this study, superamphiphobic surfaces have been fabricated by a simple spraying method using two different nanostructures of silicon dioxide. The nanosheet silica (about 80 nm) constructed a large-scale layer structure, and the nanospherical silica (about 30 nm) interweaved with nanosheet silica to form a double-scale "reentrant" micronano structure. The contact angle (CA) and sliding angle (SA) were estimated to be 158° and 1° for a water droplet, while the CA and SA were measured to be 154° and 3° for olive oil, meeting the superamphiphobic requirement. On the other hand, control surfaces that only adopted nanosheet or nanospherical silica just satisfy the superhydrophobic requirement. Owing to the similar chemical components of nanosheet and nanospherical silica, the two mixed very well and were uniformly dispersed, endowing the whole surface with the same superamphiphobic behavior. More importantly, the mixture of nanosheet and nanospherical silica was sprayed on the wet poly(amide-imide) (PAI) substrate, making them easily enter into the PAI film. This unique structure is helpful for improving mechanical robustness and durability. Thus, the surface wettability was nearly unaffected even though it underwent lots of tests, including sandpaper abrasion, ultrasonic treatment, acid-base immersion, UV irradiation, and boiling water jet impact. As expected, the superamphiphobic surfaces show antifouling, self-cleaning, and icing delay performances, and the icing time was prolonged from 55 to 342 s. It is believed that robust superamphiphobic surfaces with multifunctions of antiliquid-adhesion, self-cleaning, anticorrosion, and anti-icing have enormous potential applications in the industrial environment.

Wetting Transition from Wenzel to Cassie States: Thermodynamic Analysis

Superhydrophobicity is closely linked to the chemical composition and geometric characteristics of surface roughness. Building on our structural studies on water and air-water interfaces, this work aims to elucidate the mechanism underlying the wetting transition from the Wenzel to the Cassie state on a hydrophobic surface. In the Wenzel state, the grooves are filled with water, meaning that the surface roughness becomes embedded in the liquid. To evaluate the effects of surface roughness on water structure, a wetting parameter (WRoughness) is proposed, which is closely related to the geometric characteristics of roughness, such as pillar size, width, and height. During the wetting transition from Wenzel to Cassie states, the critical wetting parameter (WRoughness,c) may be expected, which corresponds to the critical pillar size (ac), width (wc), and height (hc). The Cassie state is expected when the WRoughness is less than WRoughness,c (ac), decreasing width (hc). Additionally, molecular dynamic (MD) simulations are conducted to demonstrate the effects of surface roughness on superhydrophobicity.

Light-Responsive Materials in Droplet Manipulation for Biochemical Applications

Miniaturized droplets, characterized by well-controlled microenvironments and capability for parallel processing, have significantly advanced the studies on enzymatic evolution, molecular diagnostics, and single-cell analysis. However, manipulation of small-sized droplets, including moving, merging, and trapping of the targeted droplets for complex biochemical assays and subsequent analysis, is not trivial and remains technically demanding. Among various techniques, light-driven methods stand out as a promising candidate for droplet manipulation in a facile and flexible manner, given the features of contactless interaction, high spatiotemporal resolution, and biocompatibility. This review therefore compiles an in-depth discussion of the governing mechanisms underpinning light-driven droplet manipulation. Besides, light-responsive materials, representing the core of light-matter interaction and the key character converting light into different forms of energy, are particularly assessed in this review. Recent advancements in light-responsive materials and the most notable applications are comprehensively archived and evaluated. Continuous innovations and rational engineering of light-responsive materials are expected to propel the development of light-driven droplet manipulation, equip droplets with enhanced functionality, and broaden the applications of droplets for biochemical studies and routine biochemical investigations.

Ex Situ pH-Induced Reversible Wettability Switching for an Environmentally Robust and High-Efficiency Stain-Proof Coating

Developing superwetting coatings with environmental adaptability is critical for sustainable industrial applications. However, traditional anti-wetting coatings often fall short due to their susceptibility to environmental factors (UV light, temperature, mold growth, and abrasion) and inadequate stain resistance in complex media. Herein, a durable ex situ pH-responsive coating with reversible wettability switching, engineered by integrating hydrophobic polydimethylsiloxane and tertiary amine structures is presented. The resulting hierarchical micro-nano surface structure, combined with a trapped air cushion, ensures low water adhesion and stable superhydrophobicity. Notably, after ex situ pH treatment, the modulation of surface N+ content synergistically interacts with polydimethylsiloxane chains, enabling a controlled transition in surface wettability from 150° to 68.5°, which can spontaneously revert to a hydrophobic state upon heating and drying. This transition enhances stain resistance in both air and underwater environments, resulting in a 17.2% increase in detergency compared to superhydrophobic controls. Moreover, the coating demonstrates remarkable durability, with no staining, peeling, or mildew growth (grade 0) even after 1500 h of UV radiation and 28 days of mildew resistance testing. This work offers a highly adaptable and stain-resistant coating for applications in building and infrastructure protection, as well as in smart textiles designed for multi-media decontamination.

Advanced strategies for controlling three-phase boundaries in photocatalysis

This review delves into the latest advancements in controlling three-phase boundaries (TPBs) in photocatalytic systems, with a focus on photo(electro)catalytic processes for nitrogen reduction, oxygen reduction, and water reduction. We critically analyze various strategies and advanced materials designed to enhance TPB performance, evaluating their impact on catalytic efficiency and identifying gaps in the existing literature. By examining sophisticated triphasic systems that integrate superwetting materials, we emphasize their essential role in improving light absorption, charge separation, and mass transfer. Key challenges in TPB optimization are discussed, and future research directions are proposed to advance photocatalytic technologies for sustainable energy applications. This review highlights the crucial importance of TPBs in photo(electro)catalysis, aiming to inspire further innovation for more efficient and scalable solutions.

Preparation of Pressure-Resistant and Mechanically Durable Superhydrophobic Coatings via Non-Solvent Induced Phase Separation for Anti-Icing

Inspired by the lotus leaf effect, superhydrophobic coatings have significant potential in various fields, However, their poor pressure resistance, weak mechanical durability, and complex preparation processes severely limit practical applications. Here, a method for preparing pressure-resistant and durable superhydrophobic coatings by simply spray-coating a phase separation suspension containing fluorinated silica nanoparticles and polyolefin adhesive onto substrates is introduced, which forms superhydrophobic coatings with a porous and hierarchical micro-/nanostructure. The resulting superhydrophobic coatings exhibit outstanding pressure resistance, maintaining a Cassie-Baxte state after 18 days of submersion in 1 m of water. Furthermore, the coatings demonstrate remarkable mechanical durability, withstanding 200 cycles of Taber abrasion, 100 cycles of tape-peeling, and 750 g of sand abrasion. The coatings also show excellent chemical stability, enduring long-term immersion in corrosive liquids and 120 d of outdoor exposure. Additionally, the coatings display excellent anti-icing properties and can be applied to various substrate surfaces. This approach improves on the limitations of conventional superhydrophobic coatings and accelerates the application of superhydrophobic coatings in real-world environments.

A review of various dimensional superwetting materials for oil-water separation

In recent years, the application and fabrication technologies of superwetting materials in the field of oil-water separation have become a research hotspot, aiming to address challenges in marine oil spill response and oily wastewater treatment. Simultaneously, the fabrication technologies and related applications of superwetting materials have been increasingly diversified. This paper systematically reviews the sources and hazards of oily wastewater and oil-water emulsions, several traditional oil-water separation methods, and their limitations, thereby highlighting the advantages of superwetting materials. Additionally, this paper provides an overview of the fundamental theories of wetting and conducts a microanalysis of the penetration mechanism based on Laplace pressure at the gas-liquid-solid three-phase interface. Following this, the latest advances in superwetting oil-water separation materials are elucidated, focusing on five categories: (i) superhydrophobic-superoleophilic materials; (ii) superhydrophilic-underwater superoleophobic materials; (iii) superhydrophobic-superoleophobic materials; (iv) "special" superwetting materials; and (v) smart switchable superwetting materials. This paper innovatively discusses these materials from the perspectives of two-dimensional and three-dimensional materials, deeply studying the mechanisms of oil-water separation and using data to quantify the separation efficiency. Comparative discussions are conducted on the materials from various dimensions, including different substrates, innovations in existing technologies, and fabrication methods as discussed in various articles, followed by corresponding summaries. Finally, the existing shortcomings and challenges of current superwetting materials are summarized, and prospects are proposed. We firmly believe that developing low-cost, stable, environmentally friendly, and practical large-scale superwetting oil-water separation materials will have broad application prospects and potential in the future.