Research progress on eco-friendly superhydrophobic materials in environment, energy and biology

Modeling of Shear Flows over Superhydrophobic Surfaces: From Newtonian to Non-Newtonian Fluids

The design and use of superhydrophobic surfaces have gained special attentions due to their superior performances and advantages in many flow systems, e.g., in achieving specific goals including drag reduction and flow/droplet handling and manipulation. In this work, we conduct a brief review of shear flows over superhydrophobic surfaces, covering the classic and recent studies/trends for both Newtonian and non-Newtonian fluids. The aim is to mainly review the relevant mathematical and numerical modeling approaches developed during the past 20 years. Considering the wide ranges of applications of superhydrophobic surfaces in Newtonian fluid flows, we attempt to show how the developed studies for the Newtonian shear flows over superhydrophobic surfaces have been evolved, through highlighting the major breakthroughs. Despite the fact that, in many practical applications, flows over superhydrophobic surfaces may show complex non-Newtonian rheology, interactions between the non-Newtonian rheology and superhydrophobicity have not yet been well understood. Therefore, in this Review, we also highlight emerging recent studies addressing the shear flows of shear-thinning and yield stress fluids in superhydrophobic channels. We focus on reviewing the models developed to handle the intricate interaction between the formed liquid/air interface on superhydrophobic surfaces and the overlying flow. Such an intricate interaction will be more complex when the overlying flow shows nonlinear non-Newtonian rheology. We conclude that, although our understanding on the Newtonian shear flows over superhydrophobic surfaces has been well expanded via analyzing various aspects of such flows, the non-Newtonian counterpart is in its early stages. This could be associated with either the early applications mainly concerning Newtonian fluids or new complexities added to an already complex problem by the nonlinear non-Newtonian rheology. Finally, we discuss the possible directions for development of models that can address complex non-Newtonian shear flows over superhydrophobic surfaces.

Engineered Sustainable Omniphobic Coatings to Control Liquid Spreading on Food-Contact Materials

The adhesion of sticky liquid foods to a contacting surface can cause many technical challenges. The food manufacturing sector is confronted with many critical issues that can be overcome with long-lasting and highly nonwettable coatings. Nanoengineered biomimetic surfaces with distinct wettability and tunable interfaces have elicited increasing interest for their potential use in addressing a broad variety of scientific and technological applications, such as antifogging, anti-icing, antifouling, antiadhesion, and anticorrosion. Although a large number of nature-inspired surfaces have emerged, food-safe nonwetted surfaces are still in their infancy, and numerous structural design aspects remain unexplored. This Review summarizes the latest scientific research regarding the key principles, fabrication methods, and applications of three important categories of nonwettable surfaces: superhydrophobic, liquid-infused slippery, and re-entrant structured surfaces. The Review is particularly focused on new insights into the antiwetting mechanisms of these nanopatterned structures and discovering efficient platform methodologies to guide their rational design when in contact with food materials. A detailed description of the current opportunities, challenges, and future scale-up possibilities of these nanoengineered surfaces in the food industry is also provided.

Effective separation of water-in-oil emulsions using an under-medium superlyophilic membrane with hierarchical pores

Separating water-in-oil emulsions is important in terms of environmental protection and resource recovery. To address the challenges posed by the water-oil interface, superwetting materials have been designed to accomplish separation through filtration and adsorption. Superhydrophobic membranes prevent the permeation of water droplets owing to extreme repellence and their size-sieving abilities. However, their use in remediating water-contaminated oil is limited by high oil viscosities. Meanwhile, in-air superhydrophilic sorbents are rarely employed for the separation of water-in-oil emulsions due to the thermodynamic and kinetic limitations of water adsorption in oil. Herein, the integration of an under-medium superlyophilic membrane with the hierarchical porous structure of wood is presented for filtration-driven selective adsorption of water from surfactant-stabilized (10 g/L) water-in-oil emulsions. Compared to filtration through a natural wood membrane or direct adsorption using an under-oil superhydrophilic wood membrane, the under-medium superlyophilic wood membrane demonstrated high separation efficiencies of > 99.95% even when applied to the regeneration of high-viscosity lubricating (6.3 mPa s) and edible (50.5 mPa s) oils, exhibiting viscosity-dependent fluxes and excellent stability. Moreover, the cost of purifying 200 mL of lubricating oil using the modified wood membrane was much lower than the oil's market price and required a low energy consumption of ca. 1.72 kWh. ENVIRONMENTAL IMPLICATION: The ever-growing use of petroleum and industrial/domestic oil products has led to excessive (estimated at a million tons per year) output of waste oils. Because direct discharge of waste oils into the environment causes serious pollution problems, separating water-in-oil emulsions is important in terms of environmental protection and resource recovery. Here filtration-driven water adsorption has been demonstrated to be a feasible method for the remediation of water-contaminated waste oils, even those that are highly viscous.

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.

Superhydrophobic cotton fabrics for effective removal of high-density polyethylene and polypropylene microplastics: Insights from surface and colloidal analysis

HYPOTHESIS

The use of superhydrophobic materials to remove particulate pollutants such as microplastics is still in its infancy. In a previous study, we investigated the effectiveness of three different types of superhydrophobic materials - coatings, powdered materials, and meshes - for removing microplastics. In this study, we will explain the removal process by considering microplastics as colloids and taking into account their wetting properties as well as those of a superhydrophobic surface. The process will be explained through the interactions of electrostatic forces, van der Waals forces, and the DLVO theory.

EXPERIMENTS

In order to replicate and verify the previous experimental findings on the removal of microplastics using superhydrophobic surfaces, we have modified non-woven cotton fabrics with polydimethylsiloxane. We then proceeded to remove high-density polyethylene and polypropylene microplastics from water by introducing oil at the microplastics-water interface, and we determined the removal efficiency of the modified cotton fabrics.

FINDINGS

After achieving a superhydrophobic non-woven cotton fabric (159 ± 1°), we confirmed its effectiveness in removing high-density polyethylene and polypropylene microplastics from water with a removal efficiency of 99%. Our findings suggest that the binding energy of microplastics increases and the Hamaker constant becomes positive when they are present in oil instead of water, leading to their aggregation. As a result, electrostatic interactions become negligible in the organic phase, and van der Waals interactions become more important. The use of the DLVO theory allowed us to confirm that solid pollutants can be easily removed from the oil using superhydrophobic materials.

Electrolyte-Wettability Issues and Challenges of Electrode Materials in Electrochemical Energy Storage, Energy Conversion, and Beyond

The electrolyte-wettability of electrode materials in liquid electrolytes plays a crucial role in electrochemical energy storage, conversion systems, and beyond relied on interface electrochemical process. However, most electrode materials do not have satisfactory electrolyte-wettability for possibly electrochemical reaction. In the last 30 years, there are a lot of literature have directed at exploiting methods to improve electrolyte-wettability of electrodes, understanding basic electrolyte-wettability mechanisms of electrode materials, exploring the effect of electrolyte-wettability on its electrochemical energy storage, conversion, and beyond performance. This review systematically and comprehensively evaluates the effect of electrolyte-wettability on electrochemical energy storage performance of the electrode materials used in supercapacitors, metal ion batteries, and metal-based batteries, electrochemical energy conversion performance of the electrode materials used in fuel cells and electrochemical water splitting systems, as well as capacitive deionization performance of the electrode materials used in capacitive deionization systems. Finally, the challenges in approaches for improving electrolyte-wettability of electrode materials, characterization techniques of electrolyte-wettability, as well as electrolyte-wettability of electrode materials applied in special environment and other electrochemical systems with electrodes and liquid electrolytes, which gives future possible directions for constructing interesting electrolyte-wettability to meet the demand of high electrochemical performance, are also discussed.

Anti-mold, self-cleaning superhydrophobic bamboo fiber/polypropylene composites with mechanical durability

Bamboo fiber/polypropylene composites (BPCs) have been widely used in buildings, interior decoration, and automobile components. However, pollutants and fungi can interact with the hydrophilic bamboo fibers on the surface of Bamboo fiber/polypropylene composites, degrading their appearance and mechanical properties. To improve their anti-fouling and anti-mildew properties, a superhydrophobic modified Bamboo fiber/polypropylene composite (BPC-TiO₂-F) was fabricated by introducing titanium dioxide (TiO₂) and poly(DOPAm-co-PFOEA) onto the surface of a Bamboo fiber/polypropylene composite. The morphology of BPC-TiO₂-F was analyzed by XPS, FTIR, and SEM. The results showed that TiO₂ particles covered on Bamboo fiber/polypropylene composite surface via complexation between phenolic hydroxyl groups and Ti atoms. Low-surface-energy fluorine-containing poly(DOPAm-co-PFOEA) was introduced onto the Bamboo fiber/polypropylene composite surface, forming a rough micro/nanostructure that endowed BPC-TiO₂-F with superhydrophobicity (water contact angle = 151.0° ± 0.5°). The modified Bamboo fiber/polypropylene composite exhibited excellent self-cleaning properties, and a model contaminant, Fe₃O₄ powder, was rapidly removed from the surface by water drops. BPC-TiO₂-F showed excellent anti-mold performance, and no mold was on its surface after 28 days. The superhydrophobic BPC-TiO₂-F had good mechanical durability and could withstand sandpaper abrasion with a weight load of 50 g, finger wiping for 20 cycles, and tape adhesion abrasion for 40 cycles. BPC-TiO₂-F showed good self-cleaning properties, mildew resistance, and mechanical resistance, giving it promising applications for automotive upholstery and building decoration.