Direct Imaging of Superwetting Behavior on Solid-Liquid-Vapor Triphase Interfaces

Through-drop imaging of moving contact lines and contact areas on opaque water-repellent surfaces

A myriad of natural surfaces such as plant leaves and insect wings can repel water and remain unwetted inspiring scientists and engineers to develop water-repellent surfaces for various applications. Those natural and artificial water-repellent surfaces are typically opaque, containing micro- and nano-roughness, and their wetting properties are determined by the details at the actual liquid-solid interface. However, a generally applicable way to directly observe moving contact lines on opaque water-repellent surfaces is missing. Here, we show that the advancing and receding contact lines and corresponding contact area on micro- and nano-rough water-repellent surfaces can be readily and reproducibly quantified using a transparent droplet probe. Combined with a conventional optical microscope, we quantify the progression of the apparent contact area and apparent contact line irregularity in different types of superhydrophobic silicon nanograss surfaces. Contact angles near 180° can be determined with an uncertainty as low as 0.2°, that a conventional contact angle goniometer cannot distinguish. We also identify the pinning/depinning sequences of a pillared model surface with excellent repeatability and quantify the progression of the apparent contact interface and contact angle of natural plant leaves with irregular surface topography.

All-organic Superhydrophobic Coating Comprising Raspberry-like Particles and Fluorinated Polyurethane Prepared via Thiol-click Reaction

Mechanically robust superhydrophobic coatings have been extensively reported using chemically susceptible inorganic fillers like SiO2, TiO2, ZnO, etc. for constructing micro-nano structures. Organic particles are good candidates for improving chemical resistance, whereas the synthesis of organic particles with well-defined and stable micro-nano structures remains exclusive. Here, an all-organic, cross-linked superhydrophobic coating comprising raspberry-like fluorinated micro particles (RLFMP) and fluorinated polyurethane (FPU) was prepared via thiol-click reaction. Benefiting from the robust micro-nano structure of RLFMP and the excellent flexibility of FPU, the coating could maintain superhydrophobic after severe alkali corrosion or mechanical damage, while the superhydrophobicity could be repaired readily by the fast recovery of micro-nano roughness and migration of branched fluoroalkyl chains to the coating surface. Our design strategy is expected to provide a good application of thiol-click chemistry. This article is protected by copyright. All rights reserved.

Solid-Liquid-Vapor Triphase Gel

Gels are soft functional materials with solid networks and open pores filled with solvents (for wet gels) or air (for aerogels), displaying broad applications in tissue engineering, catalysis, environmental remediation, energy storage, etc. However, currently known gels feature only a single (either solid-liquid or solid-vapor) interface, largely limiting their application territories. Therefore, it is both fundamentally intriguing and practically significant to develop conceptually new gel materials that possess solid-liquid-vapor multiple interfaces. Herein, we demonstrate a unique solid-liquid-vapor triphase gel, named as aerohydrogel, by gelling of a poly(vinyl alcohol) aqueous solution with glutaraldehyde in the presence of superhydrophobic silica aerogel microparticles. Owing to its continuous solid, liquid, and vapor phases, the resultant aerohydrogel simultaneously displays solid-liquid, solid-vapor, and liquid-vapor interfaces, leading to excellent properties including tunable density (down to 0.43 g·cm^-3), considerable hydrophobicity, and excellent elasticity (compressive ratio of up to 80%). As a proof-of-concept application, the aerohydrogel exhibits a higher evaporative cooling efficiency than its hydrogel counterpart and a better cooling capability than the commercial phase change cooling film, respectively, showing promising performance in cooling various devices. Moreover, the resulting aerohydrogel could be facilely tailored with specific (e.g., magnetic) properties for emerging applications such as solar steam generation. This work extends biphase gel (hydrogel or aerogel) to solid-liquid-vapor triphase gel, as well as provides a promising strategy for designing more aerohydrogels serving as soft functional materials for applications in various emerging fields.

Magnetic-Sensitive Nanoparticle Self-Assembled Superhydrophobic Biopolymer-Coated Slow-Release Fertilizer: Fabrication, Enhanced Performance, and Mechanism

Although commercialized slow-release fertilizers coated with petrochemical polymers have revolutionarily promoted agricultural production, more research should be devoted to developing superhydrophobic biopolymer coatings with superb slow-release ability from sustainable and ecofriendly biomaterials. To inform the development of the superhydrophobic biopolymer-coated slow-release fertilizers (SBSF), the slow-release mechanism of SBSF needs to be clarified. Here, the SBSF with superior slow-release performance, water tolerance, and good feasibility for large-scale production was self-assembly fabricated using a simple, solvent-free process. The superhydrophobic surfaces of SBSF with uniformly dispersed Fe₃O₄ superhydrophobic magnetic-sensitive nanoparticles (SMNs) were self-assembly constructed with the spontaneous migration of Fe₃O₄ SMNs toward the outermost surface of the liquid coating materials ( i.e., pig fat based polyol and polymethylene polyphenylene isocyanate in a mass ratio 1.2:1) in a magnetic field during the reaction-curing process. The results revealed that SBSF showed longer slow-release longevity (more than 100 days) than those of unmodified biopolymer-coated slow-release fertilizers and excellent durable properties under various external environment conditions. The governing slow-release mechanism of SBSF was clarified by directly observing the atmosphere cushion on the superhydrophobic biopolymer coating using the synchrotron radiation-based X-ray phase-contrast imaging technique. Liquid water only contacts the top of the bulges of the solid surface (10.9%), and air pockets are trapped underneath the liquid (89.1%). The atmosphere cushion allows the slow diffusion of water vapor into the internal urea core of SBSF, which can decrease the nutrient release and enhance the slow-release ability. This self-assembly synthesis of SBSF through the magnetic interaction provides a strategy to fabricate not only ecofriendly biobased slow-release fertilizers but also other superhydrophobic materials for various applications.

Bioinspired Designs of Superhydrophobic and Superhydrophilic Materials

Bioinspired designs of superhydrophobic and superhydrophilic materials have been an important and fascinating area of research in recent years for their extensive potential application prospects from industry to our daily life. Despite extensive progress, existing research achievements are far from real applications. From biomimetic performance to service life, the related research has faced serious problems at present. A timely outlook is therefore necessary to summarize the existing research, to discuss the challenges faced, and to propose constructive advice for the ongoing scientific trend. Here, we comb the process of development of bioinspired superhydrophobic and superhydrophilic materials at first. Then, we also describe how to design artificial superhydrophobic and superhydrophilic materials. Furthermore, current challenges faced by bioinspired designs of superhydrophobic and superhydrophilic materials are pointed out, separately, and the possible solutions are discussed. Emerging applications in this field are also briefly considered. Finally, the development trend within this field is highlighted to lead future research.