Switchable Wettability Research Articles

Preparation of temperature-responsive smart sponge and its application in oil-water separation and dye adsorption

At present, researchers are committed to studying new intelligent response materials to deal with severe water pollution problems. Using PU sponge as the substrate, dopamine was first modified to construct a rough surface while introducing active groups. The monomer N-isopropylacrylamide (NIPAM) and itaconic acid (IA) were grafted and polymerized onto the polydopamine coating on the sponge surface by vinyltrimethoxysilane (VTMS). The obtained DINA-PU sponge has good temperature response and switchable wettability characteristics, and has good adsorption-desorption cycle function. Moreover, a micro-filtration device was designed to separate heavy (light) oil-water mixtures. The separation efficiency can reach more than 95% and 99% for heavy and light oil-water, respectively. In addition, the adsorption capacity of the material for methylene blue (MB) can reach 660mg·g-1. The separation efficiency in the oil-water separation-dye adsorption integrated separation experiment can reach more than 99%.

PEEK-PANI bi-functional oil-water separation membrane based on structural switchable wettability

PEEK-PANI bi-functional oil-water separation membrane based on structural switchable wettability

Water drop transportation on wettability switchable surface via anisotropic molecules

Active control of the transportation of liquid drops on a horizontal surface is achieved using surfaces with switchable wettability via remote stimuli. However, the mechanism how the dynamic wettability influences drop dynamics is rarely reported. In this paper, we demonstrate that a surface with switchable wettability induces depinning of the contact line through re-orientation of anisotropic molecules. We investigated the dynamics of contact lines and contact angles during the initiation of drop movement by the advancing and receding angles of the surface. We found that imbalance between advancing and receding angles with respect to the dynamic contact angle provides the force needed to overcome the energy barrier due to contact angle hysteresis on the surface. We discovered that the driving energy is accumulated with oscillations in contact angle until it breaks the pinning energy barrier. Understanding the role of dynamic contact angles in drop movement on switchable surfaces paves the way for designing effective fluid manipulation devices, such as water harvesters, biosensors, and oil–water separators.

Candle soot-modified rPET electrospun nanofibrous membrane for separating on-demand oil-water mixture and emulsions

The CS-rPET electrospun nanofibrous membrane is fabricated from recycled polyethylene terephthalate (rPET) through electrospinning and enhanced with candle soot (CS) to separate oil-water mixtures and emulsions when pre-wetted by oil or water. Using rPET polymers and CS waste reduces the environmental impact of plastic bottle waste and improves its value. The CS-rPET electrospun nanofibrous membrane showed excellent separation performance in oil-water mixtures, achieving over 81.18 % and 71.38 % of separation efficiency through 40 separation cycles after pre-wetting by oil and after washing with ethanol and pre-wetting by water, respectively. The membrane maintained high separation performance after being pre-wetted by oil for water-in-oil emulsions with efficiencies above 99 % and flux exceeding 12,200 L m−2 h−1. Similarly, the efficiencies remained above 98 % for oil-in-water emulsions after being pre-wetted by water, with a flux over 8000 L m−2 h−1. Additionally, the CS-rPET electrospun nanofibrous membrane exhibited high separation efficiencies above 97 % and flux over 14,000 L m−2 h−1 after pre-wetting by oil and 7700 L m−2 h−1 after pre-wetting by water in harsh environmental conditions. Its adaptability of switchable wettability on-demand after pre-wetting by oil or water highlights its potential for a wide range of challenging oil-water separation applications. However, multiple separation cycles, separation efficiency and flux were reduced, indicating the necessity to improve the membrane's efficiency and reduce the chance of water accumulation in multicycle separation.

Photo-induced time-dependent controllable wettability of dual-responsive multi-functional electrospun MXene/polymer fibers

Switchable wettability potential in smart fibers is of paramount importance in various applications. Light-induced controllable changes in surface wettability have a significant role in this area. Herein, smart waterborne homopolymer, functional copolymer with different polarity and flexibility, and multi-functional terpolymer particles containing a time-dependent dual-responsive acrylated spiropyran, as a polymerizable monomer, were successfully synthesized through eco-friendly single-step emulsifier-free emulsion polymerization. Presence of 10 wt% of butyl acrylate and dimethylaminoethyl methacrylate relative to methylmethacrylate as functional comonomers decreased the Tg of the samples almost 20 ℃ and increased their polarity. The optical properties of the particles were investigated, and the UV–vis and fluorescence spectroscopy results showed that not only polarity and flexibility of the polymer chains may have a positive effect on improving the optical properties, but also the simultaneous presence of functional groups has a synergistic effect. The smart polymer particles with flexibility and polarity features exhibited higher absorption and emission compared to other samples. Inspired by these findings, multi-functional smart polymer fibers were prepared using the electrospinning method. The smart multi-functional electrospun fibers containing few-layer Ti3C2 MXenes were synthesized to improve the fibers’ properties and change the surface wettability due to the hydrophilic functional groups of MXene. Field-emission scanning electron microscopy images displayed the successful preparation of few-layer MXenes. Smooth and bead-free fibers with bright red fluorescence emission under UV irradiation were shown using fluorescence microscopy. The study on the surface wettability of fibers revealed that UV and visible light irradiation induced reversible time-dependent changes in the wettability of the smart multi-functional MXene/polymer electrospun fibers from hydrophobic to hydrophilic, reaching a water contact angle of 10° from an initial water contact angle of 100° under UV light and also changing to superhydrophilic state with passing time. Upon visible light exposure, the fibers returned to their original state. Furthermore, the fibers demonstrated a high stability over five alternating cycles of UV and visible light irradiation. This study shows that the fabrication of time-dependent smart fibers, utilizing the flexibility and polarity in the presence of MXenes, significantly improves and controls surface wettability changes. The outstanding dynamically photo-switchable wettability of these fibers may offer exciting opportunities in various applications, especially in the separation of oil from water contaminants.

Fabricated pH-responsive sponge with switchable wettability for multitasking and controllable oil-water separation

pH-responsive smart materials with switchable wettability have a great potential application in bidirectional oil-water separation. In this work, a simple, low-cost and environmentally friendly strategy is developed to construct a pH-responsive melamine sponge (MS) with reversible wettability via polydopamine (PDA) modification followed by grafting long-chain alkanes with carboxyl (–COOH) and methyl (–CH3) groups. The as-modified sponge exhibits unique pH-responsiveness, that is, the water contact angle (WCA) is measured to be 144.9° under acidic and neutral environments, while the wettability is radically changed from hydrophobic to hydrophilic under alkaline conditions. Moreover, in alkaline environment, the pH-responsive sponge can rapidly release the absorbed oil, demonstrating a controllable desorption performance. Also, the prepared sponge is utilized to accomplish controllable separation of various oil/water mixtures, as well as to effectively separate oil-in-water emulsions. Furthermore, after 50 cycles, the oil-water separation efficiency was still higher than 92 %. This smart wettability sponge has an extensive application prospect in controlled oil-water separation and on-demand oil-spill remediation.

Demulsification of Pickering emulsions: advances in understanding mechanisms to applications.

Pickering emulsions are ultra-stable dispersions of two immiscible fluids stabilized by solid or microgel particles rather than molecular surfactants. Although their ultra-stability is a signature performance indicator, often such high stability hinders their demulsification, i.e., prevents the droplet coalescence that is needed for phase separation on demand, or release of the active ingredients encapsulated within droplets and/or to recover the particles themselves, which may be catalysts, for example. This review aims to provide theoretical and experimental insights on demulsification of Pickering emulsions, in particular identifying the mechanisms of particle dislodgment from the interface in biological and non-biological applications. Even though the adhesion of particles to the interface can appear irreversible, it is possible to detach particles via (1) alteration of particle wettability, and/or (2) particle dissolution, affecting the particle radius by introducing a range of physical conditions: pH, temperature, heat, shear, or magnetic fields; or via treatment with chemical/biochemical additives, including surfactants, enzymes, salts, or bacteria. Many of these changes ultimately influence the interfacial rheology of the particle-laden interface, which is sometimes underestimated. There is increasing momentum to create responsive Pickering particles such that they offer switchable wettability (demulsification and re-emulsification) when these conditions are changed. Demulsification via wettability alteration seems like the modus operandi whilst particle dissolution remains only partially explored, largely dominated by food digestion-related studies where Pickering particles are digested using gastrointestinal enzymes. Overall, this review aims to stimulate new thinking about the control of demulsification of Pickering emulsions for release of active ingredients associated with these ultra-stable emulsions.

A multifunctional coating with switchable wettability for efficient oil-water and emulsions separation

Superwettable materials with switchable wettability have been attracting tremendous attention in the field of oil-water separation. In this work, a multifunctional coating with switchable wettability was designed for efficient oil-water and emulsions separation. Fe3O4 was combined with TiO2 nanoparticles and hydrophobically modified by low surface energy stearic acid (SA) to form the Fe3O4/TiO2@SA coating with uniform micro-nanostructures. The introduction or removal of amino group via ammonia treatment or heating treatment endowed the coating with reversible conversion between hydrophilicity and hydrophobicity. By switching hydrophilicity and hydrophobicity, the coating could separate oil/water/oil ternary mixtures in sequence with high separation efficiency of more than 99.9 %. More importantly, it displayed excellent separation efficiency with oil rejection above 99.5 % for oil-in-water (O/W) emulsions and water rejection above 98.7 % for water-in-oil (W/O) emulsions. Moreover, this coating showed reliable reusability and mechanical stability after 10 cycles of O/W and W/O emulsions separation and 20 cycles of sandpaper abrasion. Fe3O4 and TiO2 imparted magnetic and photocatalytic properties to the coating, allowing it to be used for magnetic navigation of oil absorption and photocatalytic degradation of organic dyes. The unique features of multifunctional coating with switchable wettability provide the blueprint for treating oily wastewater and marine oil spills.

Triboelectrically active and wettability-swichable magnetic textile for self-powered intelligent marine conservation

Intelligent materials that are built with reversibly switchable wettability between superhydrophilicity-hydrophobicity are crucial for combating environmental challenges such as spill oil cleanup and fuel purification. In this work, inspired by mussel, a robust magnetic textile was prepared by anchoring titanium dioxide and magnetic carbon nanotubes onto textile through Schiff condensation between the amine groups of dopamine and the carboxyl groups of lauric acid. Reversibly switchable wettability was presented on the as-fabricated textile, where superhydrophobic/oleophilic could be achieved at pH 3–7 and hydrophilic/underwater oleophobic were achieved at pH 13. With a maximum saturation magnetization of 17.99 emu/g, it achieves non-contact oil spill recovery through its response to pH and magnetic fields. Interestingly, the asymmetric electric field of M−TENG accelerate the coalescence of water on fabric and oil–water mixture separation. Furthermore, the output performance of M−TENG was dielectrically and magnetically enhanced, with a power density of 0.837 W·m−2. Additionally, a self-powered marine protection system was developed by integrating cathodic protection principles with M−TENG, where the three-dimensionally accommodated triboelectric unit enhanced the efficiency for collecting wave energy, thereby protecting marine vessel components effectively. This work provides guiding strategies for developing self-powered devices for marine conservation and protection based on smart materials and TENG.

Wettability regulation of membranes based on biodegradable aliphatic polyester

Biodegradable aliphatic polyesters are intensively applied in food packaging, regenerative medicine, tissue engineering, water purification and flexible electronics. However, the performance of these polymers is largely hindered by their inherent hydrophobicity. Here we report a facile method for producing hydrophilic fibers based on biodegradable aliphatic polyesters. Specifically, poly (lactide caprolactone) (PLCL) was dissolved in solvent systems containing trifluoroacetic acid (TFA) for electrospinning. Increasing TFA proportion or dissolving time successfully decreased the water contact angle (WCA) of PLCL fibrous membranes from 124° to 67°. The mechanism of wettability alteration was elucidated and ascribed to the generation of hydrophilic carboxyl acid and hydroxyl groups vis the acid hydrolysis reaction that cleaved PLCL chains. The exponential relationship between molecular weight and WCA confirmed the mechanism, pointing out an in-situ way for accurately controlling the wettability of PLCL. The efficacy of using hydrophilic PLCL membranes in biomedical engineering and environmental protection was preliminarily verified. Janus membranes with unidirectional water transfer were also fabricated. The versatility of our method was further proven by the switchable wettability of fibers spun from various biodegradable polyesters, including polylactide, polycaprolactone and poly (lactic-glycolic acid). We anticipate this work will broaden the application fields of biodegradable aliphatic polyesters.

Adhesion switchable transition on bio-inspired Janus surface for multi-directional droplet manipulation

Achieving precise and rapid droplets manipulation in versatile directions transport remains a considerable challenge, however adhesion switchable transitions deemed to be rather serviceable. Here, a double-sided microplate array (DMPA) with bio-inspired Janus surfaces, is proposed to fulfill adjustable adhesion through surface wettability switching and microplate dynamic deformation, thereby executing the droplet multi-dimensional manipulation in response to dynamic magnetic field. The triple-coupled bio-inspired Janus surfaces are newly designed with a convex hull and striped structure respectively, inspired by the respiratory ciliary motion and surface properties of lotus and rice leaves. Leveraging variation in superhydrophobicity and low adhesion of convex hull side on Janus surface, diverse droplet directional transports are achieved in plane, such as high-speed horizontal transport (120 mm/s and 500 µL), anti-gravitational climbing (inclined by 15°), and selective droplet orientation. The adhesion switchable transition of surface striped structure on other side is predominated by solid–liquid contact lines evolution resulting from microplate bending deformation, realizing droplet capture/release for long-distance multi-dimensional transport. Concurrently, a magnetic-responsive droplet switch is developed based on multi-directional droplet manipulation, demonstrating enormous potential for practical application.

The fabrication and research of pH-responsive cement-based materials with switchable wettability

Recently, intelligent responsive materials have received increasing attention due to the controllability of their wettability. However, research has primarily focused on the surface properties of materials such as sponges and copper meshes, with a notable lack of investigation into cement-based materials in this regard. Currently, multi-functionality has become an inevitable trend in the development of cement-based materials. The controllable and adjustable wettability of cement-based materials can further expand their application scope. In this article, saturated fatty acids and dicarboxylic acids were selected to modify cement-based materials, fully utilizing their rich hydroxyl characteristics. Firstly, the feasibility of developing pH-stimuli-responsive surfaces on cement-based substrates with chosen raw materials was rapidly verified using the immersion method. When the mass fraction of dicarboxylic acid in the modifier is between 60 % and 80 %, the modified substrate surface exhibits hydrophobicity towards neutral liquids and hydrophilicity towards alkaline liquids. Based on this, the corresponding sodium salts of organic acid were synthesized and used as additives, which were then mixed with cement to prepare modified cement-based materials with switchable wettability. Through acid solution treatment, the modified hardened cement paste can transform from initially hydrophilic to hydrophobic, and the conversion angle can reach 90°. This study broadens the application field of intelligent response surfaces and enriches the functionality of cement-based materials.

Hierarchical Fusiform ZnO-Constructed Nanoflowers Electrodeposited on Stainless-Steel Mesh with Switchable Wettability for On-Demand Separation of Oil–Water Emulsion and Photocatalytic Degradation for Water-Soluble Pollutants

Hierarchical Fusiform ZnO-Constructed Nanoflowers Electrodeposited on Stainless-Steel Mesh with Switchable Wettability for On-Demand Separation of Oil–Water Emulsion and Photocatalytic Degradation for Water-Soluble Pollutants

Amino‐modified banana nanocellulose aerogels with quick pH‐responsive switchable wettability for oil–water separation

AbstractWith the rapid development of the maritime industry, oily wastewater discharges and oil spills cause acute environmental challenges. Currently, cellulosic materials with switchable wettability have received increasing interest for oil–water separation. Moreover, work efficiency needs to be improved with a shorter response time. To realize these, dimethylaminoethyl methacrylate (DMAEMA) and methyltrimethoxysilane (MTMS) synthesized pH‐responsive copolymer (DMAEMA‐co‐MTMS) which grafted banana nanocellulose fiber made from wasted pseudostem to achieve the pH‐responsive aerogel (BCNA‐DcoM). BCNA‐DcoM exhibited hydrophilic/submerged lipophobicity under acidic conditions and hydrophobic/lipophilic under alkaline conditions. A response time of 3 s and a contact angle of 135° were shown when the mass ratio of banana nanocellulose suspension, MTMS, and DMAEMA‐co‐MTMS was 5:3:9. Moreover, BCNA‐DcoM possessed favorable mechanical friction properties, salt resistance (25% NaCl), compression resilience (50 cycles of compression at 60% strain), switchable wettability (>30 cycles), and highly efficient separation performance (>96%). The adsorption capacity of BCNA‐DcoM for oil is 80–184 times the initial weight of the aerogel based on the different oil densities. For practical applications, BCNA‐DcoM could continuously separate oil–water mixtures with the constant‐flow pump. The present work provides more possibilities for rapidly treating industrial oily wastewater with low‐cost and ecological banana pseudostem materials.

Polymer System Based on Polyethylene Glycol and TFE Telomers for Producing Films with Switchable Wettability.

Today a lot of attention is paid to the formation of thermosensitive systems for biomedical and industrial applications. The development of new methods for synthesis of such systems is a dynamically developing direction in chemistry and materials science. In this regard, this paper presents results of the studies of a new synthesized supramolecular polymer system based on polyethylene glycol and tetrafluoroethylene telomers. The films formed from the polymer substance have the property of switching wettability depending on temperature after heating activation. It has been established that the wettability changes at 60 °C. The contact angle of activated hydrophobic polymer film reaches 143°. Additionally, the system exhibits its properties regardless of the pH of the environment. Based on data obtained by the methods of infrared and x-ray photoelectron spectroscopy, differential thermal analysis and thermal analysis in conjunction with wettability and morphology, a model of the behavior of molecules in a polymer system was built that ensures switching of the hydrophilic/hydrophobic surface state. The resulting polymer system, as well as films based on it, can be used in targeted drug delivery, implantation surgery, as sensors, etc.

Starch-Based Polysaccharide Systems with Bioactive Substances: Physicochemical and Wettability Characteristics.

Polysaccharide-based systems have very good emulsifying and stabilizing properties, and starch plays a leading role. Their modifications should add new quality features to the product to such an extent that preserves the structure-forming properties of native starch. The aim of this manuscript was to examine the physicochemical characteristics of the combinations of starch with phospholipids or lysozymes and determine the effect of starch modification (surface hydrophobization or biological additives) and preparation temperature (before and after gelatinization). Changes in electrokinetic potential (zeta), effective diameter, and size distribution as a function of time were analyzed using the dynamic light scattering and microelectrophoresis techniques. The wettability of starch-coated glass plates before and after modification was checked by the advancing and receding contact angle measurements, as well as the angle hysteresis, using the settle drop method as a complement to profilometry and FTIR. It can be generalized that starch dispersions are more stable than analogous n-alkane/starch emulsions at room and physiological temperatures. On the other hand, the contact angle hysteresis values usually decrease with temperature increase, pointing to a more homogeneous surface, and the hydrophobization effect decreases vs. the thickness of the substrate. Surface hydrophobization of starch carried out using an n-alkane film does not change its bulk properties and leads to improvement of its mechanical and functional properties. The obtained specific starch-based hybrid systems, characterized in detail by switchable wettability, give the possibility to determine the energetic state of the starch surface and understand the strength and specificity of interactions with substances of different polarities in biological processes and their applicability for multidirectional use.

Intelligent Wearable Graphene Nano‐Electronics with Switchable Surface Wettability Capabilities for Autonomous Sweat Enrichment‐Purification‐Analysis

AbstractSweat wearable biosensors facilitate continuous monitoring of individuals’ in‐depth body physiological state with real‐time and molecular‐level insight. However, limited detection accuracy and sensitivity resulted from insufficient amount of sweat sampling and impurities interferences still hinder their practical applications. Here, a miniature wearable skin‐interfaced intelligent graphene nano‐electronic (SIGN) patch employing a novel Janus membrane integrated surface wettability switchable microfluidic module with autonomous sweat sampling and purification capabilities is presented for in situ analysis of sweat biomarkers. Due to the asymmetric surface energy distribution characteristics of the microfluidic surfaces, rapid, directional transport of sufficient amount of sweat to the Janus membrane is achieved. The Janus membrane purifies the sweat sample and transport the sample to the sensing surface autonomously, thus eliminating impurities interferences and enhancing the sensing performance. An ultra‐flexible bio‐receptor functionalized graphene transistor for accurately monitoring sweat biomarkers such as lactate, with outstanding signal reproducibility and good long‐term (over 1 month) stability, and a signal processing unit are employed incorporating with the microfluidic module. In practical wearing tests, the SIGN patch enables the continuous measuring of sweat lactate levels for volunteers during exercises and intelligently providing a preliminary diagnostic assessment on their exercise intensity successfully, suggesting its potential commercialization prospects.

Sustainable superwetting membrane for effective separation of oil–water emulsion and dye removal

Water pollution from organic oily solvents and dyes is a major global problem. While some wettable materials exist for oily pollutants, few can effectively separate different oil–water emulsions and dyes. To address this problem, we developed a smart citric acid-crosslinked bamboo powder-coated membrane (CBM) with approprite surface roughness and switchable wettability. The CBM membrane is superhydrophobic in oil and superoleophobic in water, making it highly effective in separating oil–water emulsions and dyes. The membrane can efficiently separate various oil–water emulsions, including crude oil-in-water emulsions, with a separation efficiency of 99.76 % and a high flux of 1756 L/m2/h. It also adsorbs anionic (e.g. Congo red) and cationic (e.g. methylene blue) dyes in wastewater, with an adsorption rate of 99.45 % after multiple cycles. The membrane can simultaneously separate oil-in-water emulsions and remove dyes from water, facilitating high-value utilization of biomass waste and offering great promise for addressing global water pollution from oily organic solvents and dyes.

An Eco-Friendly Manner to Prepare Superwetting Melamine Sponges with Switchable Wettability for the Separation of Oil/Water Mixtures and Emulsions.

Oil/water separation processes have garnered significant global attention due to the quick growth in industrial development, recurring chemical leakages, and oil spills. Hence, there is a significant demand for the development of inexpensive superwetting materials in an eco-friendly manner to separate oil/water mixtures and emulsions. In this study, a superwetting melamine sponge (SMS) with switchable wettabilities was prepared by modifying melamine sponge (MS) with sodium dodecanoate. The as-prepared SMS exhibited superhydrophobicity, superoleophilicity, underwater superoleophobicity, and underoil superhydrophobicity. The SMS can be utilized in treating both light and heavy oil/water mixtures through the prewetting process. It demonstrated fast permeation fluxes (reaching 108,600 L m-2 h-1 for a light oil/water mixture and 147,700 L m-2 h-1 for a heavy oil/water mixture) and exhibited good separation efficiency (exceeding 99.56%). The compressed SMS was employed in separating surfactant-stabilized water-in-oil emulsions (SWOEs), as well as surfactant-stabilized oil-in-water emulsions (SOWEs), giving high permeation fluxes (reaching 7210 and 5054 L m-2 h-1, respectively). The oil purity for SWOEs' filtrates surpassed 99.98 wt% and the separation efficiencies of SOWEs exceeded 98.84%. Owing to their remarkable capability for separating oil/water mixtures and emulsions, eco-friendly fabrication method, and feasibility for large-scale production, our SMS has a promising potential for practical applications.

Development of a spiropyran-assisted cellulose aerogel with switchable wettability as oil sorbent for oil spill cleanup

This research presents the successful development and optimization of a spiropyran-assisted cellulose aerogel (CNF-SP) aerogel with UV-induced switchable wettability, and the evaluation of its performance as an effective oil sorbent for oil spill cleanup. The aerogel initially exhibited strong hydrophobicity (124°) and showed UV-induced switchable wettability due to the photo-response structure of spiropyran. Upon UV irradiation, the hydrophobicity of the aerogel could be switched to hydrophilicity (31°), while visible light irradiation could restore its hydrophobicity. The three-dimensional (3D) porous structure of the CNF-SP aerogel combined with the hydrophobic properties of spiropyranol led to its great oil adsorption performance (27–30 g/g of oil adsorption ratio). The central composite design (CCD) was applied to optimize the aerogel and investigate the effects of raw material ratio (i.e., carboxymethyl cellulose, carboxyethyl spiropyran, polyvinyl alcohol, and nano zinc oxide) on the oil sorption performance of the aerogel. The optimized CNF-SP aerogel demonstrated a high oil sorption efficiency, particularly in acid and cold environments. Moreover, the switchable function indicated that the aerogel exhibited reusability and renewability, with the added benefit of UV-induced oil recovery.