A hybrid bioinspired fiber trichome with special wettability for water collection, friction reduction and self-cleaning

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

Green Superlubricity Enabled by Only One Water Droplet on Plant Oil-Infused Surfaces

The increase in energy loss due to friction and the use of large amounts of lubricants to improve it are major challenges we face from a global environmental perspective. Pitcher-plant-inspired liquid-infused surfaces (LISs) are emerging super-repellent surfaces against liquids. However, their coefficient of friction (CoF) against solids is higher than that of conventional lubricant surfaces. Herein, we demonstrate superlubricity with a single water droplet placed on a LIS holding oleic acid, a component of plant oil. When a water droplet is placed on the fluid layer, the CoF under reciprocating and rotating friction is 0.012 and 0.0098, respectively. A force in the direction opposite to the loading due to the Laplace pressure on the droplet and an autonomous positional movement of the water accompanied by the optimization of surface energy maintain the fluid-lubrication state and prevent direct contact between the surface and the friction material, resulting in a decrease of the dependence of the CoF on the friction velocity. The key technology here will not only present a novel strategy for preparing LISs against solids but also serve as a step toward a sustainable green strategy for friction reduction and lubrication, which would greatly reduce energy loss and environmental degradation.

Anisotropic Janus materials: from micro-/nanostructures to applications

Janus materials have led to great achievements in recent years owing to their unique asymmetric structures and properties. In this review, recent advances of Janus materials including Janus particles and Janus membranes are summarized, and then the microstructures and applications of Janus materials are emphasized. The asymmetric wettability of Janus materials is related to their microstructures; hence, the microstructures of Janus materials were analyzed, compared and summarized. Also presented are current and potential applications in sensing, drug delivery, oil-water separation and so on. Finally, a perspective on the research prospects and development of Janus materials in more fields is given.

Interfacial solar vapor generation for desalination and brine treatment: Evaluating current strategies of solving scaling

Interfacial solar vapor generation, an efficient, sustainable, and low-cost method for producing clean water, has attracted great interest for application in solar desalination and wastewater treatment. Although recent studies indicated significant enhancement of overall performance by developing photothermal materials and constructing different dimensional systems, stable evaporation performance and long-term operation of the evaporator are hindered by severe scaling issues. In this critical review, we present the latest strategies in reducing salt accumulation on the evaporator for solar desalination and brine treatment. We first demonstrate the consequences of salt accumulation, and then discuss various self-cleaning methods based on bio-inspired concepts and other strategies such as physical cleaning, ion rejection and exchange, fast ion diffusion, and controlled crystallization, etc. Importantly, we discuss and address the rational design of the evaporator via establishing a relationship model between its porosity, thickness, and thermal conductivity. Lastly, we evaluate salt-resistance strategies, evaporation performance, and possibilities of real application in different evaporation systems with scaling-resistant abilities.

Cellulose acetate/fiber paper composite membrane for separation of an oil-in-water emulsion

The cellulose composite membrane combines the advantages of cellulose acetate and cellulose filter paper with good antifouling performance and excellent mechanical properties.

Recent advances in biomimetic surfaces inspired by creatures for fog harvesting

In this review, the recent advances in artificial surfaces for fog harvesting are introduced with emphasis on the surfaces and their mechanisms used to enhance water capture and transportation, providing prospects for coping with water shortages.

Biomimetic Fabrication of Janus Fabric with Asymmetric Wettability for Water Purification and Hydrophobic/Hydrophilic Patterned Surfaces for Fog Harvesting

The long-term shortage of freshwater resources has drawn increasing research attention for water purification and collection. This work reports a facile method to prepare Janus fabrics with asymmetric wettability for on-demand oil/water separation and hydrophobic/hydrophilic patterned fabrics for efficient fog harvesting. Here, the superhydrophobic fabric was prepared by in situ polymerization of polydivinylbenzene (PDVB) on cotton fabric. By regulating the polymerization time, the PDVB polymer content was changed, thereby achieving the regulation of the surface structure and wettability of the prepared fabric. Meanwhile, the superhydrophobic fabric exhibited excellent self-cleaning and antifouling performance, mechanical abrasion and chemical resistance, and environmental durability. Moreover, the photocatalytic degradation properties of PDVB were utilized to prepare the Janus fabric with asymmetric wettability. Water droplets could spontaneously penetrate from the hydrophobic side to the hydrophilic side, while not vice versa, achieving unidirectional transport of water. In addition, the prepared Janus fabric could be used for on-demand oil/water separation, including the heavy oil/water mixture and light oil/water mixture. The separation efficiency and collected oil purity of each mixture were higher than 99.00 and 99.94%, respectively. Furthermore, the hydrophobic/hydrophilic patterned fabrics were prepared by using the lithographic masks with different apertures under UV light irradiation. Based on the fog-capturing ability of the hydrophilic areas and the water transport performance of the hydrophobic regions, efficient fog harvesting was achieved. For the patterned fabric with larger hydrophobic/hydrophilic areas, the water collection rate reached 224.7 mg cm^-2 h^-1. Therefore, this simple strategy to achieve controllable gradient wettability by adjusting the surface structure and chemical composition of the fabric shows great potential in the filtration of purification of oily sewage and the efficient condensed collection of water.

Bioinspired Free-Standing One-Dimensional Photonic Crystals with Janus Wettability for Water Quality Monitoring

Materials with specific wettability properties have aroused enormous interest and research for their broad application prospects in chemical reaction, medical diagnosis, biological analysis, etc. Here, inspired by the unique Janus wettability of lotus leaf and Bragg stacks of beetles, we present a free-standing film with Janus wettability and tunable structural color for water quality monitoring. This film is constructed by using a flexible polymer polyurethane (PU) to pack poly(N-isopropyl acrylamide-bis-acrylamide-acrylic acid) (P(NiPAAm-bis-AA))/TiO₂ one-dimensional photonic crystals (1DPCs) into a free-standing state with Janus wettability and tunable structural color. The outer top surface of the film could achieve vivid structural color and a superhydrophobic ability; meanwhile, the outer lower surface could achieve a superhydrophilic ability. Owing to the outstanding pH-sensitive property of the P(NiPAAm-bis-AA), the Janus films could switch its structural color under different pH conditions. This imparts the free-standing film with stability and an antirotation property on the air-water interface. Based on this phenomenon, we have demonstrated a Janus wettability film, together with tunable structural color for water quality monitoring, which gives the bioinspired materials high potential applications in environmental protection.

Preparation of a Flexible Superhydrophobic Surface and Its Wetting Mechanism Based on Fractal Theory

Substrates of the superhydrophobic surface are important for their application. Preparation of a flexible superhydrophobic surface has drawn more and more attention. In this work, a flexible substrate was made using a semicuring spray method to obtain a flexible superhydrophobic surface with excellent abrasion resistance on the surface of a room temperature vulcanized silicone rubber. Results show that under a bending condition, excellent superhydrophobic properties are still maintained. The Cassie-Baxter model and Wenzel model can be used to estimate the static water contact angle for regular roughness surfaces. There are few numerical theoretical models to predict contact angle or wetting mode for irregular micronanostructures superhydrophobic surfaces. The fractal theory can be used to transform the equation of the Wenzel model and obtain the fractal wetting theory suitable for fractal structures on irregular rough surfaces. However, this fractal-wetting model cannot be applied to the Cassie-Baxter state, which is always suitable for superhydrophobic surfaces. A new method was developed to calculate the static water contact angle of water droplets in the Cassie-Baxter model state. Using image identification and the splitting surface method, a new model is constructed based on the fractal theory. Experimental data for water contact angles on the flexible superhydrophobic surface with SiC/CNTs micronanostructures is in agreement with the simulated values.

Beetle-like droplet-jumping superamphiphobic coatings for enhancing fog collection of sheet arrays

Fog collection from atmosphere is an effective way to solve the water resource crisis in arid or semi-arid areas. Inspired by the bumpy surface of the desert beetle, this work provides a beetle-like superamphiphobic coating by adding silicon carbide particles to nano-SiO2 superamphiphobic coating in proportion, which shows excellent superamphiphobic performance, high nucleation rate, efficient drop removal efficiency and recommendable fog collection effect. In this work, drop removal is facilitated by the collisions of water droplets between the array sheets, and when the as-prepared samples are placed parallel to each other and with a space of ∼2 mm, the jumping drop collisions between two sample surfaces could promote the departure of droplets, and the water collection rate of the collision surface increased by ∼217% compared to that of the non-collision surface, which provides a new idea to promote water droplet removal. This work findings are instrumental in water collection and have wide application prospects in desalination, heat transfer, anti-fogging and other fields.