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Dynamic control of liquid wetting behavior on smart surfaces has attracted considerable concern owing to their important applications in directional motion, confined wetting and selective separation. Despite much progress in this regard, there still remains challenges in dynamic liquid droplet manipulation with fast response, no loss and anti‐contamination. Herein, a strategy to achieve dynamic droplet manipulation and transportation on the electric field adaptive superhydrophobic elastomer surface is demonstrated. The superhydrophobic elastomer surface is fabricated by combining the micro/nanostructured clusters of hydrophobic TiO2 nanoparticles with the elastomer film, on which the micro/nanostructure can be dynamically and reversibly tuned by electric field due to the electric field adaptive deformation of elastomer film. Accordingly, fast and reversible transition of wetting state between Cassie state and Wenzel state and tunable adhesion on the surface via electric field induced morphology transformation can be obtained. Moreover, the motion states of the surface droplets can be controlled dynamically and precisely, such as jumping and pinning, catching and releasing, and controllable liquid transfer without loss and contamination. Thus this work would open the avenue for dynamic liquid manipulation and transportation, and gear up the broad application prospects in liquid transfer, selective separation, anti‐fog, anti‐ice, microfluidics devices, etc. Dynamic liquid droplet manipulation and transportation are demonstrated on the superhydrophobic micro/nanostructured elastomer surface via electric field, based on electric adaptive switchable wettability and tunable adhesion. This work provides a new route for precise liquid manipulation and transfer, and is promising in designing and constructing smart interface materials.

In order to satisfy the needs of different applications and more complex intelligent devices, smart control of surface wettability will be necessary and desirable, which gradually become a hot spot and focus in the field of interface wetting. Herein, we review interfacial wetting states related to switchable wettability on superwettable materials, including several classical wetting models and liquid adhesive behaviors based on the surface of natural creatures with special wettability. This review mainly focuses on the recent developments of the smart surfaces with switchable wettability and the corresponding regulatory mechanisms under external stimuli, which is mainly governed by the transformation of surface chemical composition and geometrical structures. Among that, various external stimuli such as physical stimulation (temperature, light, electric, magnetic, mechanical stress), chemical stimulation (pH, ion, solvent) and dual or multi-triggered stimulation have been sought out to realize the regulation of surface wettability. Moreover, we also summarize the applications of smart surfaces in different fields, such as oil/water separation, programmable transportation, anti-biofouling, detection and delivery, smart soft robotic etc. Furthermore, current limitations and future perspective in the development of smart wetting surfaces are also given. This review aims to offer deep insights into the recent developments and responsive mechanisms in smart biomimetic surfaces with switchable wettability under external various stimuli, so as to provide a guidance for the design of smart surfaces and expand the scope of both fundamental research and practical applications.

In recent years, wettability materials with pH-responsive have attracted increasing attention in oil/water separation applications. However, these materials were limited by the pH range and infiltration time. Herein, a simple operational procedure is proposed to prepare banana nanocellulose cryogels with pH-responsive switchable wettability to realize these outstanding performances. Alkyl-modified cryogels (BCNC-MS) are obtained by adding methyltrimethoxysilane (MTMS) to the banana nanocellulose (BCNF) suspension. BCNC-MS are soaked in the carboxyl-modified solution to produce pH-responsive cryogels (BCNC-MS-SA). The carboxyl-modified solution is made from succinic anhydride (SA), (3-aminopropyl)triethoxysilane (KH550), and N,N-dimethylformamide (DMF) in a molar mass ratio of 1:1:18. The key to achieving the pH-response is the protonation and deprotonation of the carboxyl groups. SEM demonstrates that the modification keeps the three-dimensional porous structure of the cryogel, and the results of EDS, FTIR, and XPS show the success of alkyl and carboxyl modifications. BCNC-MS-SA can realize hydrophilic/underwater oleophobic (θ water  = 0°) and hydrophobic/underwater oleophilic (maximal θ water  = 135°) wettability transitions after treatment with different pH solutions. Compared with other pH-responsive oil/water separation materials, BCNC-MS-SA performs well in pH = 1 and pH = 13 environments, and the shortest infiltration time is only 3 s. With a porosity of 93.80%, BCNC-MS-SA possesses excellent adsorption capacity (10–40 g/g), oil/water separation efficiency (> 92%), and adsorption cycle performance (15 cycles) even for viscous oils. Moreover, BCNC-MS-SA has satisfactory stability. Cryogels are made of banana nanocellulose, and they are inexpensive and can be easily degraded. BCNC-MS-SA has great potential in practical applications such as oil removal and purification of oily wastewater.

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.

The global challenge of oil pollution calls for the efficient selective recovery of oil or organics from oil–water mixtures. A pH-responsive carbon foam (CF) made from liquefied larch sawdust (LLS) with switchable wettability was fabricated in this work. After grafted with poly 4-vinyl pyridine (P4vp), the CF obtained a switchable wettability surface, which allowed the CF to exhibit superhydrophilicity and superhydrophobicity at different pH levels, respectively. The results revealed that the pH-responsive CF possessed a three-dimensional (3D) spongy-like skeleton and porous structure with a diameter between 50 and 200 µm. Thus, the pH-responsive CF could absorb 15–35 g/g of oil/organics in a neutral aqueous solution at pH = 7 and desorb all the absorbate within 40 s after immersion in an aqueous solution at pH = 1. Moreover, only about 2.8% loss was observed for organic (chloroform) absorption and recovery after reusing up to 15 cycles, which indicated promising prospects in oil and organic recovery.

Controllable wettability is important for a wide range of applications, including intelligent switching, self-cleaning and oil/water separation. In this work, rapid switching and extreme wettability changes upon ultraviolet (UV) illumination were investigated. TiO 2 nanoparticles were modified in solutions of trimethoxy(alkyl)silane, and the suspensions were sprayed on glass substrates. For such samples, the water contact angle (WCA) was shown to transition from a superhydrophobic (WCA ≈ 165°) to a superhydrophilic (WCA ≈ 0°) state within 10 min upon UV illumination and subsequent recovery to superhydrophobicity occurred after heat treatment. It was found that the changes in the trimethoxy(alkyl)silane upon UV illumination can explain the rapid decrease of the WCA from more than 165° to almost 0°. To further investigate the wettability transition, trimethoxy(alkyl)silane and Al 2 O 3 nanoparticles (which are not photocatalytic) were mixed and spray-coated onto the glass substrates as the control samples. Then the unrecoverable change of trimethoxy(alkyl)silane under UV illumination can be confirmed. It was found that the presence of trimethoxy(alkyl)silane in the TiO 2 –trimethoxy(alkyl)silane coating served to speed up the super-wettability transition time from superhydrophobicity to superhydrophilicity, but also limited the number of wettability recycle times. With this understanding, the effect of the trimethoxy(alkyl)silane concentration on the number of recycle cycles was investigated.

Water pollution caused by oil spills or sewage discharges has become a serious ecological environmental issue. Despite the membrane separation technique having a promising application in wastewater purification, the membrane fabrication method and separation robustness have remained unsatisfactory until now. Herein, we developed a novel strategy, spacer-assisted sequential phase conversion, to create a patterned polyvinylidene fluoride@polypropylene (P-PVDF@PP) substrate membrane with a multiscale roughened surface. Based on that surface structure, the underwater oil resistance behavior of the P-PVDF@PP membrane was improved. Moreover, owing to the abundant active sites on the P-PVDF@PP surface, the polydopamine/P-PVDF@PP (PDA/P-PVDF@PP) Janus membrane could be readily fabricated via wet chemical modification, which exhibited excellent switchable oil–water separation performance. Regarding surfactant-stabilized oil-water emulsion, the as-prepared PDA/P-PVDF@PP Janus membrane also had robust separation efficiency (as high as 99% in the n-hexane/water, chloroform/water, and toluene/water emulsion separation cases) and desirable reusability. Finally, the underlying mechanism of emulsion separation in the PDA/P-PVDF@PP Janus membrane was specified. The as-designed PDA/P-PVDF@PP Janus membrane with high-efficiency oil–water separation shows potential application in oily wastewater treatment, and the developed fabrication method has implications for the fabrication of advanced separation membranes.

Zinc oxide (ZnO) nanoparticles were grown on graphene oxide (GO) surface via reflux method. Electrophoretic deposition (EPD) process was employed to deposit graphene oxide/zinc oxide (GO/ZnO) nanocomposite on Al plate with 1 mg/ml of GO/ZnO suspension, 30 V and 3 min. X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), energy-dispersive X-ray spectrometry (EDX) and elemental mapping (MAP) investigation were used to characterize the samples. The hydrophobic and hydrophilic behavior of samples were obtained by heat treatment and UV irradiation, respectively. Heat treatment and UV-irradiation affect wettability of samples. Moreover, switchable wettability of samples was investigated. The WCA changes from 5° to 137° by heat treatment and vice versa by UV-irradiation. Graphic Abstract

Advanced application of the superwettable surface with intelligent stimulus-responsive property invents a new way for designing controllable and effective oil-water separation material. Here, a smart superwettable fabric with tunable wettability has been fabricated via a low-cost and eco-friendly approach. With synergistic effect of the well-define micro/nano-hierarchical structure and pH-sensitive undecylenic acid, the as-prepared superwettable coating exhibits an excellent switchable surface wettability, which can reversibly transit between superhydrophobicity and superhydrophilicity in accordance with pH values of the contacting aqueous water. By virtue of the special wetting property, the obtained superwettable coating can be exploited to controllably and efficiently separate various oil-water mixtures, showing attractive separation efficiency (99 %) and favorable durability. Moreover, the complex oil/water/oil three-phase system also can be successfully separated with the superwettable coating as separation membrane, revealing an excellent in situ pH-responsiveness. Besides, it is worth noting that the superwettable coating is competent to selectively remove organic from oil, which has been seldom reported in previous works and providing a viable strategy for separating organic from oil. In light of the above advantages, it is believed that the functionalized smart coating with controllable tunable surface wettability will be a promising candidate for oily wastewater remediation under various harsh settings.

In this work the preparation of fibers array with electric field switchable wettability is described. Piezoelectric properties of polyvinylidene fluoride (PVDF) and chemical reactivity of polymethylmethacrylate (PMMA) are used. Electrospinning blend fibers were fabricated from both of polymers and deposited on glass (non-conductive) and aluminum (conductive) substrates. The samples were modified with diazonium salts (ADT-C8F17), causing the hydrophobic surface characteristics. Applying an external electric field (EF) on these surfaces results in fibers rearrangement and dramatic changes in surface hydrophilicity. The wettability of the surface and the shape of the droplet were studied reporting the effect of varying the orientation of the electric field and using different substrates with different conductivity, which allowed us to control the shape of the deionized water drop interfering with the applied voltage. Moreover, reporting the results of shifting the polarity of the water by dissolving NaCl salt in different concentrations was reported.