Zhiguang Guo

Superhydrophobic sand: a hope for desert water storage and transportation projects

Sand, an abundant natural resource, is the cause behind the harsh environmental conditions of the desert, such as water shortages and sand storms. Because of the strong hydrophilicity of sand itself, water can be quickly absorbed by sand, which greatly impedes desert greening, water storage and transportation projects. In contrast to this conventional understanding of sand (i.e., superhydrophilicity), we propose the design of “superhydrophobic sand”, aimed to address issues associated with the desert environment and sand resource utilization. In our experiments, three kinds of hydrophobic sands with different surface structures and wettability properties were successfully prepared by cladding nonmetal (SiO2) and metal (Ag and Cu) inorganic materials on sand grain surfaces and then modifying them with low-surface-energy chemicals. Combining superhydrophobicity with desert sand, superhydrophobic sand (PFDS-sand@SiO2) is shown to have excellent water repellency, allowing water to stably remain and flow on such a sand surface without any wetting or permeation. Furthermore, the superhydrophobic sand demonstrates a great water-holding capacity, such that a sand layer with a thickness of 2 cm can sustain a water column height of 35 cm. Very significantly, PFDS-sand@SiO2 exhibits extremely high thermal stability up to 400 °C when used for water storage. This result is unprecedented and sufficient for facing the high-temperature conditions of the desert environment and some others. In addition to reliable water storage, such superhydrophobic sand also demonstrates a great anti-flow-dragging effect during water transportation, whereby a water droplet can smoothly and quickly roll down a simulated sand channel (13 cm length) within 0.3 s (∼0.45 m s−1). All of these manifestations imply the significant potential of such “superhydrophobic sand” in its application to desert water storage and transportation.

Simple one-pot approach toward robust and boiling-water resistant superhydrophobic cotton fabric and the application in oil/water separation

Inspired by the strong adhesion of mussel, taking advantage of a simple one-pot approach, we designed a robust and boiling-water resistant superhydrophobic polydopamine@SiO2(PDA@SiO2) coated cotton fabric through copolymerization reaction and trimethyl silyl modified process at room temperature. The as-prepared fabric not only shows great resistance to mechanical abrasion, wear and ultrasonic treatment but also has excellent superhydrophobicity stability towards UV irradiation, high temperature and organic solvents immersion. More importantly, the superhydrophobic fabric also exhibits boiling water (99 °C) repelling properties, while several superhydrophobic surfaces lose their superhydrophobicity when exposed to hot water with temperature more than 50 °C. It is also noticeable that the purity of all the collected oil is as high as 99.9% when superhydrophobic fabric is used to separate oil/water mixture for 20 cycles, indicating the high oil/water separation efficiency of the fabric. We believe that the modified fabric is environmentally friendly, low cost and easy to fabricate, and thus exhibits great potential applications in solving the serious problems of oily waste water and scald-protection clothes.

Inorganic adhesives for robust, self-healing, superhydrophobic surfaces

Superhydrophobic surfaces demonstrate remarkable advantages involving interfacial issues but limited practical applications due to their poor mechanical robustness and environmental durability. The required micro-/nano-hierarchical structures and low-surface-energy nanocomponents are very vulnerable to physical and chemical destruction. Moreover, harsh conditions fundamentally weaken the mechanical strength of already-constructed robust superhydrophobic surfaces. In this work, inorganic adhesives are proposed to strengthen the bonding force between superhydrophobic coatings and various substrates. A simple spray-coating method is adopted to fabricate superhydrophobic surfaces using an all-in-one suspension that contains an aluminum phosphate binder, titanium dioxide nanoparticles, and alkylsilane. The surfaces benefitting from inorganic adhesives still extremely repel water after physical abrasion, and greatly endure harsh conditions including hot oil (80 °C), hot water (80 °C), and hot acetone (50 °C) for 24 h to preserve their high mechanical strength. The prepared coatings also have a self-healing ability against boiling-water treatment, O2-plasma etching, and amphiphilic pollution. Superhydrophobicity can be rapidly regenerated after multiple cycles of high-temperature repairing for 5 min. In addition, the robust interfacial materials exhibit a very reliable performance in oil–water separation after 100 abrasion cycles. Benefiting from the distinctive advantages of inorganic adhesives, interfacial materials will be broadly developed for practical applications in related fields.

Outmatching superhydrophobicity: bio-inspired re-entrant curvature for mighty superamphiphobicity in air

Understanding the complementary roles of surface roughness and energy in natural super-non-wetting surfaces has greatly promoted the development of biomimetic superhydrophobic surfaces that repel water at a much greater rate than oils. These surfaces that are highly repellent to low-surface-tension oils and organic liquids, termed superoleophobic surfaces, are poorly understood. Inspired by springtails (collembolan), a third factor, re-entrant surface curvature, has been introduced to the design and fabrication system of superoleophobic surfaces in conjunction with two other factors of surface chemical composition and roughness. Over the past decade, superoleophobic surfaces have attracted tremendous attention with respect to their design, fabrication and applications due to their extraordinary properties. This review focuses on these aspects and thus summarizes recent research progress in superoleophobic surfaces. Starting from the origin, features of natural oil-resistant creatures have been introduced, and fundamental theories for surface design have been discussed. Calculations suggest that creation of these surfaces requires specific re-entrant structures and fluoride modifiers. Based on this principle, various fabrication methods, from top-down to bottom-up approaches, have been used, and some derivative structures with desirable properties have been produced. A precise and detailed classification has been provided in this review that includes representative methods and structures as well as functions (i.e., transparence and self-healing). Significantly, superoleophobic materials have many valuable applications, including oil pollution resistance, oil transportation, and synthesis of mesoporous supraparticles. However, their complicated manufacturing techniques, poor physical–chemical properties and environmentally unfriendly surface chemicals jointly impede their real-life applications. Therefore, it is highly necessary to optimize the craft and performance of theses surfaces for industrial operation and practical applications. To this end, some challenges and perspectives will be provided regarding the future research and development of superoleophobic surfaces.

Nonflammable superhydrophobic paper with biomimetic layered structure exhibiting boiling-water resistance and repairable properties for emulsion separation

Commercial paper as an indispensable material in our daily life is extremely easily destroyed by water and fire. And obtaining paper with the writable, non-flammable and superhydrophobic properties is still a challenging issue. Inspired from nature, biomimetic nanowires made from hydroxyapatite (HAP) can be used as raw materials to fabricate functional paper. In this study, we present a superior, fire-resistant and repairable superhydrophobic PFDS-paper@ZnO exhibiting remarkable oil absorption–combustion performance. In particular, the layered structure of the paper might be the reason of its excellent superhydrophobicity even after 20 abrasion cycles with sandpaper (400 cW). In contrast, the common paper was destroyed after 5 abrasion cycles with sandpaper under identical conditions. It was observed that the intrinsic fire-resistant nature of the paper was expected to reduce the risk of fire and might be used as an absorbent for flammable oil. On the one hand, such burnt paper can recover its original superhydrophobicity by facile modification after multiple cycles, achieving the repairable performance of its superhydrophobic surface. On the other hand, the burnt paper without subsequent modification exhibits superhydrophilicity in air and also underwater superoleophobicity, which can be used for efficient surfactant-stabilized oil-in-water emulsion separation. This study expands the potential applications of functional paper, which might be a breakthrough for traditional papermaking industries.

Dual superlyophobic surfaces with superhydrophobicity and underwater superoleophobicity

Lotus leaf-inspired superhydrophobic, fish scale-inspired underwater superoleophobic, and the switchable superwetting surfaces have been broadly developed by entire modification with water-repellent, water-loving, and smart components, respectively. Inspired by beetles, here a strategy of fractional modification is proposed to construct dual superlyophobic surfaces that have both superhydrophobic and underwater superoleophobic properties. Specifically, copper-based coatings on various substrates are fractionally modified by adjusting the concentration of perfluorinated mercaptan. The obtained dual superlyophobic surfaces display mutual advantages like using either superhydrophobic or underwater superoleophobic materials without any continuous external stimulus, for example on-demand oil–water separation. In theory, the distinctive dual superlyophobic state exists in a narrow range of surface chemistry, and thus needs elaborate surface modification. This discovery will facilitate the extension of the surfaces with completely opposite superwettability to enjoy the superiority in interfacial issues and applications.

Robust superhydrophobic and self-lubricating PTES-TiO2@UHMWPE fabric and its tribological properties

A superhydrophobic/self-lubricating fabric with double hierarchical structure was prepared by first etching pristine ultrahigh molecular weight polyethylene (UHMWPE) fibers with air-plasma, followed by a facile in situ growth method. Scanning electron microscopy (SEM) showed that the air-plasma etching treatment generated pits on the fiber surface and TiO2 was grafted onto the etched fiber, which greatly improved its surface roughness. The fabric was further modified by 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PTES), which possesses a low surface free energy. The results showed the PTES-TiO2@UHMWPE fabric possessed superhydrophobic properties with a water contact angle (WCA) of 157.75°, enhanced thermal resistance, and exhibited excellent antiwear properties with a friction coefficient of 0.05. In addition, the PTES-TiO2@UHMWPE fabric retained its superhydrophobicity without an apparent reduction after 30 cycles of washing or 5 minutes of friction treatment, revealing its good mechanical properties. Meanwhile, the mechanical strength of the fabric had little reduction after UV irradiation, which is helpful in enlarging the fabric applications in industry.

Fundamentals of icing and common strategies for designing biomimetic anti-icing surfaces

Ice accumulation on solid surfaces can incur irretrievable losses and catastrophic affairs in daily life. These issues can be addressed and strategically alleviated by adopting effective methods for remediation. Compared with conventional active deicing strategies, such as electro-impulse, the chemical method, and the mechanical deicing approach, passive anti-icing and ice-phobic surfaces inspired by animals in nature and plants can confer an exceedingly profound advantage. Herein, the kinetics of ice nucleation, ice accretion on solid surfaces, and heat transformation during ice formation were investigated. Then, the reduced attachment pathway of water droplets on substrate surfaces is reviewed for the purpose of achieving the self-removal of water droplets and averting the wettability transition on superhydrophobic surfaces. Furthermore, recent biomimetic anti-icing strategies regarding reduction of ice adhesion, decrease of ice nucleation temperature, and delay of freeze time were considered. Significantly, other external factors affecting ice formation and growth, such as environmental humidity and exoteric gas flow, are comprehensively discussed. Finally, outstanding mechanical and chemical stability are vital for lengthening the durability and longevity of anti-icing/ice-phobic surfaces. This article discusses all of these matters along with outlooks for future research directions for anti-icing and ice-phobic surfaces.

An all-water-based system for robust superhydrophobic surfaces

Superhydrophobic surfaces with micro-/nanohierarchical structures are mechanically weak. Generally, organic solvents are used to dissolve or disperse organic adhesives and modifiers to enhance the mechanical strength of superhydrophobic surfaces. In this work, an all-water-based spraying solution is developed for the preparation of robust superhydrophobic surfaces, which contains ZnO nanoparticles, aluminum phosphate as an inorganic adhesive, and polytetrafluoroethylene with low surface energy. The all-water-based system is appreciated for low price and less pollution. Importantly, the prepared superhydrophobic surfaces are durable enough against various harsh conditions (such as UV irradiation for 12u202fh, pH values from 1 to 13, and temperatures from -10 to 300u202f°C for 12u202fh) and physical damages (including sandpaper abrasion and sand impact tests for 50 cycles). In addition, the obtained interfacial materials show promise for practical applications such as anti-icing and oil-water separation.

An easy preparation of photo-response TiO2@copper wire mesh with quick on/off switchable superwetting for high efficiency oil–water separation

With the increase of peoples awareness of environmental protection and the enhancement of relevant management and regulations, the effective treatment of oily wastewater has gradually attracted peoples attention. In this study, copper wire mesh (CWM) is used as the base. A convertible wettability titanium dioxide @CWM (TiO2@CWM) micro–nanostructured surface of a filter screen was prepared using an easy hydrothermal method for separating different varieties of oily wastewater is presented. In particular, the rapid change of wettability can be achieved because of the photohydrophilic properties of TiO2. A stearic acid modified filter screen has superhydrophobic properties and has the ability to separate a heavy oil–water mixture. However, when the light oil–water mixture needs to be separated, the surface wettability of the filter screen can be converted to superhydrophilicity using ultraviolet irradiation. This method can play a significant role in practical industrial applications because of its easy preparation process, fast wettability conversion and the product has a certain mechanical strength.