Superwetting Patterned Membranes with an Anisotropy/Isotropy Transition: Towards Signal Expression and Liquid Permeation

A Wax-Elastomer Superwetting Membrane with Controllable Permeability: Toward Separating a Crude Oil-in-Water Emulsion from an Oil Field

Smart superwetting membranes with finely tunable properties have attracted increased attention recently. However, they mostly focus on controllable wettability rather than controllable permeability. Also, the oil/water separation performance is usually tested with laboratory-simulated samples, making it hard for the materials to meet practical applications. Herein, we fabricate thermally responsive superwetting membranes with wax, polystyrene-B-poly(ethylene-ran-butylene)-B-polys (SEBS, a kind of elastomer), and polydopamine (PDA) to realize emulsion separation with controllable permeability. Benefiting from the elasticity of SEBS and the fluidity difference of wax at different temperatures, the pore size of the membrane could be readily tuned, resulting in different permeability. The separation flux is 0 at ambient temperature (pore size 0.394 μm) and is over 100 L m-2 h-1 at a high temperature (pore size 0.477 μm). The membrane could realize the separation of simulated oil-in-water emulsions with efficiency above 99.4%. Furthermore, it successfully achieved crude oil-in-water emulsion separation from the oil field with oil residues of less than 300 mg L-1 in the temperature range of 60-80 °C, which is the actual working temperature adopted in industrial production. Such a polydopamine/wax-SEBS modified membrane with unprecedented controllable permeability can promote the development of the emulsion treatment field and provide a new direction for designing smart superwetting materials.

Overflow Control for Sustainable Development by Superwetting Surface with Biomimetic Structure

Liquid flowing around a solid edge, i.e., overflow, is a commonly observed flow behavior. Recent research into surface wetting properties and microstructure-controlled overflow behavior has attracted much attention. Achieving controllable macroscale liquid dynamics by manipulating the micro-nanoscale liquid overflow has stimulated diverse scientific interest and fostered widespread use in practical applications. In this review, we outline the evolution of overflow and present a critical survey of the mechanism of surface wetting properties and microstructure-controlled liquid overflow in multilength scales ranging from centimeter to micro and even nanoscale. We summarize the latest progress in utilizing the mechanisms to manipulate liquid overflow and achieve macroscale liquid dynamics and in emerging applications to manipulate overflow for sustainable development in various fields, along with challenges and perspectives.

Ultrastable Super-Hydrophobic Surface with an Ordered Scaly Structure for Decompression and Guiding Liquid Manipulation

Directional droplet manipulation is very crucial in microfluidics, intelligent liquid management, etc. However, excessive liquid pressure tends to destroy the solid-gas-liquid (SAL) composite interface, creating a highly adhesive surface, which is not conducive to liquid transport. Herein, we propose a strategy to enhance the surface durability, in which the surface cannot withstand liquid pressure only by "blocking" but must instead guide liquid transport for "decompression". Learning from the water resistance of water strider legs and the drag reduction of shark skin, we present a continuous integrated system to obtain an ultrastable super-hydrophobic surface with a highly ordered scaly structure via a liquid flow-induced alignment method for lossless unidirectional liquid transport. The nonwetting scaly structure can both buffer liquid pressure and drive droplet motion to further reduce the vertical pressure of the liquid. Moreover, droplets can be manipulated unidirectionally using a voice. This work could aid in manufacturing scalable anisotropic micro-nanostructure surfaces, which inspires efforts in realizing lossless continuous liquid control on demand and related microfluidic applications.

Facile preparation of melamine foam with superhydrophobic performance and its system integration with prototype equipment for the clean-up of oil spills on water surface

A simple and efficient method was developed to rectify the surface properties of commercial melamine foam. The process was based on the siloxane coating originated from the silanization of methyltrimethoxysilane and tetraethoxysilane. The foam can be easily scaled up by employing low-cost chemicals and devices. The studies on the properties of the material showed that the wettability of melamine foam was changed to superhydrophobic with a water contact angle of 156° due to the presence of alkylsilane. The modified foam exhibited excellent oil/water selectivity and high oil absorption capacities of 77-163 times its own weight. The retention of absorption capacity was greater than 97% after 5000 cycles compression. These fascinating characteristics made the modified foam exceptional recyclability for commonly organic solvents and oils without obvious dissolution/swelling. Based on these inspiring results, the material can be employed for the continuous separation of various oil compounds floating on water surface with the assistance of a vacuum pump. Moreover, the prepared material was integrated with an apparatus to develop a prototype oil collector for the remediation of oil spills in a larger open-air environment. The devices could be readily used in a range of real-world applications, including industrial oil spill clean-up.

Multiple strategies to control the hydrophilic-hydrophobic balance of P(DMA-co-DMAEMA-co-QDMAEMA) coatings

The regulation of the hydrophilic-hydrophobic balance of polymers has an important influence not only on their aggregation behavior in aqueous solution, but also on their adhesion properties on the surface of substrates and the applications of the modified surfaces. Based on this, a random copolymer poly(dopamine methacrylamide-co-2-(dimethylamino)ethyl methacrylate) (P(DMA-co-DMAEMA)) was synthesized as a starting polymer to generate P(DMA-co-DMAEMA-co-QDMAEMA) (PDDQ) derivatives by a programmable quaternization of the DMAEMA precursor. By adjusting the pH or temperature, both the aggregation behavior in aqueous solutions and the surface adhesive behavior on the substrate surfaces of PDDQ copolymers were regulated due to the hydrophilic-hydrophobic balance. Specifically, the surface adsorption of PDDQ copolymers on surfaces was enhanced by the increased hydrophobicity of PDDQ. Stainless steel meshes (SSM) modified with the PDDQ0 copolymer without quaternization showed a superoleophobicity in acidic aqueous media, which endowed it with improved oil-water separation performance. In addition, the hydrophilic-hydrophobic balance of PDDQs and their coatings could also be tuned by changing the ratio of DMAEMA to QDMAEMA in the copolymer. From PDDQ0 to PDDQ100, by increasing the hydrophilic QDMAEMA component of PDDQ copolymers, anti-protein properties and oil/water separation efficiency of the modified surfaces were also enhanced gradually. The results provided a reference for designing P(DMA-co-DMAEMA-co-QDMAEMA) coatings in different application environments.

A Dually Charged Membrane for Seawater Utilization: Combining Marine Pollution Remediation and Desalination by Simultaneous Removal of Polluted Dispersed Oil, Surfactants, and Ions

Shortage of freshwater and deterioration of the marine environment have a serious effect on the human body and ecological environment. Here, we demonstrated a facile way to prepare a multiple-target superwetting porous material to obtain available water without cumbersome steps. Through the facile immersion and hydrothermal method, a charge-enhanced membrane material combining superwettability, electrostatic interaction, and the steric effect is prepared. Such a material breaks through the limitations of single size sieving and has a universal effect on different kinds of contaminants with accurate wettability manipulation and fluid separation control. The protonation and deprotonation of active carboxyl groups at the novel created solid/liquid interface facilitate the surface wettability and flux transition, which will bring out superior continuous separation and surface lubrication control.

Modulation of solid surface with desirable under-liquid wettability based on molecular hydrophilic-lipophilic balance

There has been great interest in the fabrication of solid surfaces with desirable under-liquid wettability, and especially under-liquid dual-lyophobicity, because of their potential for widespread use. However, there remains the lack of a general principle to modulate the under-liquid wettability in terms of surface energy (SE). Herein, we found that the relative proportion between the polar and dispersive components in SE that reflects the competition between hydrophilicity and lipophilicity governs the under-liquid wettability of the solid surface. For the first time, we introduced hydrophilic–lipophilic balance (HLB) calculated solely based on the amount and type of hydrophilic and lipophilic fragments in surface molecules to rapidly predict the under-liquid wettability of a solid surface, thereby guiding the fabrication of solid surfaces with desirable under-liquid wettability. Accordingly, the under-liquid dual superlyophobic surfaces in a nonpolar oil–water-solid system were fabricated by grafting molecules with appropriate HLB values (e.g., 6.341–7.673 in a cyclohexane–water–solid system) onto porous nanofibrous membranes, which were able to achieve continuous separation of oil–water mixtures. This work provides reasonable guidance for the fabrication of solid surfaces with targeted under-liquid wettability, which may lead to advanced applications in oil–water–solid systems.

Hydrophilic–lipophilic balance calculated based on the component of surface molecules is introduced to predict the under-liquid wettability of solid surfaces, thereby guiding the fabrication of solid surface with desirable under-liquid wettabilities.

Advanced Materials with Special Wettability toward Intelligent Oily Wastewater Remediation

Clean water resources are essential to our human society. Oil leakage has caused water contamination, which leads to serious shortage of clean water, environmental deterioration, and even increasing number of deaths. It is of great urgency to solve the oil-polluted water problems worldwide. Efficient oil/water separation, especially emulsified oil/water mixture separation, is widely used to mitigate water pollution issues. Recently, advanced materials with special wettability have been employed for oily wastewater remediation. Moreover, by endowing them with various intelligent functions, smart materials can effectively separate complex oil/water mixtures including extremely stable emulsions. In this review, oil/water separation mechanisms and various fabrication methods of special wettability separation materials are summarized. We highlight the special wettable materials with intelligent functions, including photocatalytic, self-healing, and switchable oil/water separation materials, which can achieve self-cleaning, self-healing, and efficient oily wastewater treatment. In each section, the acting mechanisms, fabricating technologies, representative studies, and separation efficiency are briefly introduced. Lastly, the challenges and outlook for oil/water separation based on the special wettability materials are discussed.

Bioinspired textile with dual-stimuli responsive wettability for body moisture management and signal expression

A smart bioinspired loofah textile with biosafe wettability shows high directional liquid transport capacity and the ability to identify liquids with different pH values.

Programming Multiphase Media Superwetting States in the Oil-Water-Air System: Evolutions in Hydrophobic-Hydrophilic Surface Heterogeneous Chemistry

Studies toward tailoring macroscopic extreme wetting behaviors on a certain well-defined surface in multiphase media are significant but still at an infant stage. Herein, superantiwetting evolutions in the oil-water-air system can be programmed from single to quadruple superrepellence by controlling the surface hydrophobic-hydrophilic heterogeneous chemistry. Ammonia vapor exposure makes the realization of challenging superhydrophilicity-superoleophobicity possible in air medium, causing the transition from quadruple to triple superantiwetting states in the oil-water-air system. Upon UV illumination, only single superrepellence-underwater superoleophobicity is maintained on titanium dioxide (TiO₂, P25)-based coatings. A reversible transition between underoil superhydrophilicity and superhydrophobicity via an alternating UV irradiation and heating process leads to a switching between "water-absorbing" and "size-sieving" effects in water-in-oil emulsion separation. A comparative study for investigating two such effects in emulsion separation is further investigated. The current conceptual insights not only extend superwetting states to multiphase media, but can also deepen the understanding of the relationship between macroscopic extreme wetting behaviors and surface chemistry.