Underwater Superhydrophobicity Research Articles

Janus stainless steel mesh-based membrane with asymmetric wettability for highly efficient gravity-driven oil-water emulsion separation

With the asymmetric wettability of the membrane due to its unique transmission features and functions, the application of oil-water separation achieved rapid development. Nevertheless, their practical application is severely restricted due to poor mechanical durability and low flux in the absence of external pressure. Herein, the Janus ZIF-8/stainless steel mesh/polydopamine (DZIF-8/SSM/PDA) membrane was successfully prepared using electrodeposition, in-situ growth, and shell ligand exchange reaction methods. The Janus membrane surface was created through the combination of 5, 6-dimethylbenzimidazole (DMBIM) modified superhydrophobic nanoparticles ZIF-8 (DZIF), and PDA, exhibiting underwater superoleophobicity (UWOCA 156°), superhydrophobicity (WCA 154°), and excellent anti-oil (light and heavy oil) pollutants ability. By manipulating the molecular repulsion forces on the Janus membrane surface, a stable interfacial hydration layer is created in conjunction with the micro-nano structure. This combination enables the achievement of interfacial demulsification and gravity-driven separation of emulsified oil, according to the Molecular Dynamics (MD) simulation process. The water-in-oil emulsion achieved an oil flux of 601 L m−2 h−1 with a separation efficiency of 99.1 %, while the oil-in-water emulsion demonstrated a water flux of 62 L m−2 h−1 with a separation efficiency of 99.7 %. The Janus DZIF-8/SSM/PDA demonstrated acid and alkali resistance, mechanical stability and reusability, and resistance to E. coli bacteria. Overall, this research anticipates advancing the production of durable asymmetric Janus membranes with superior separation efficiency under gravity, providing a novel solution for oil-water separation.

Recent Advances in Superhydrophobic Materials Development for Maritime Applications.

Underwater superhydrophobic surfaces stand as a promising frontier in materials science, holding immense potential for applications in underwater infrastructure, vehicles, pipelines, robots, and sensors. Despite this potential, widespread commercial adoption of these surfaces faces limitations, primarily rooted in challenges related to material durability and the stability of the air plastron during prolonged submersion. Factors such as pressure, flow, and temperature further complicate the operational viability of underwater superhydrophobic technology. This comprehensive review navigates the evolving landscape of underwater superhydrophobic technology, providing a deep dive into the introduction, advancements, and innovations in design, fabrication, and testing techniques. Recent breakthroughs in nanotechnology, magnetic-responsive coatings, additive manufacturing, and machine learning are highlighted, showcasing the diverse avenues of progress. Notable research endeavors concentrate on enhancing the longevity of plastrons, the fundamental element governing superhydrophobic behavior. The review explores the multifaceted applications of superhydrophobic coatings in the underwater environment, encompassing areas such as drag reduction, anti-biofouling, and corrosion resistance. A critical examination of commercial offerings in the superhydrophobic coating landscape offers a current perspective on available solutions. In conclusion, the review provides valuable insights and forward-looking recommendations to propel the field of underwater superhydrophobicity toward new dimensions of innovation and practical utility.

Tannic acid induced in situ growth of an underwater superoleophobic ZIF-8 composite membrane for emulsion separation

Tannic acid induced in situ growth of ZIF-8 crystals on a nylon membrane, creating a multifunctional composite membrane that exhibits remarkable underwater superhydrophobicity and resistance to oil adhesion.

In-situ electrostatic self-assembly superhydrophilic UiO-66-NH2@h-PPS membranes for highly efficient oil-water emulsion separation

Highly efficient treatment of oil-water emulsions in severe environments (organic solvents, strong acids) is recognized as a challenging subject. Polyphenylene sulfide (PPS) is an ideal membrane material with superior thermal stability as well as chemical resistance that is expected to settle this challenge. However, the hydrophobicity of PPS (h-PPS) membrane inherently prevents its efficient treatment of oil-in-water (O/W) emulsions. In this study, superhydrophilic modification of PPS substrates was achieved by in situ electrostatic self-assembly of hydrophilic Zr-MOFs to obtain UiO-66-NH2@h-PPS membranes. The introduction of UiO-66-NH2 crystals constructed the micro/nanoscale hierarchical structure for the substrate, which transforms the PPS substrate from a hydrophobic to a superhydrophilic state (water droplets require only 0.6 s to be completely immersed in the membrane). Meanwhile, the UiO-66-NH2@h-PPS membranes gain under-water superhydrophobicity (the under-water oil contact angle exceeds 150° for toluene, hexane, dichloromethane, and carbon tetrachloride, of which toluene reaches 163.1°), thereby improving wettability selectivity and decreasing oil adhesion, permitting the fabricated UiO-66-NH2@h-PPS membranes to effectively separate different types of O/W emulsions. The UiO-66-NH2@h-PPS membranes acquire excellent permeate fluxes (for the surfactant-free toluene-in-water emulsion, it reaches up to 7638.1 L m−2 h−1) and separation accuracy (99.1% oil rejection). Moreover, the UiO-66-NH2@h-PPS membrane exhibited outstanding harsh environment resistance. Interestingly, the combination of UiO-66-NH2 and h-PPS substrate was applicable to degrade the organic compound in water, and the degradation efficiency of the methylene blue achieved 85.7% under visible light within 120 min. In summary, loading Zr-MOFs onto h-PPS substrates provides a novel strategy for the efficient treatment of wastewater under complex conditions with great potential for practical applications.

Robust self-cleaning membrane with superhydrophilicity and underwater superoleophobicity for oil-in-water separation

The discharge of oily wastewater has increased dramatically and will bring serious environmental problems. In this work, a self-cleaning and anti-fouling g-C3N4/TiO2/PVDF composite membrane was fabricated via the layer-by-layer approach. The surface of as-prepared composite membrane displayed a superhydrophilic and underwater superoleophobic behavior under irradiation with visible light. Also, upon irradiation with visible light, the fabricated g-C3N4/TiO2/PVDF composite membrane displayed enhanced permeation flux and improved oil removal efficiency as a result of the generation of hydroxyl free radicals during the photocatalytic filtration process. Significantly, irradiation with visible light remarkably improved reusability of the composite membrane by initiating photocatalytic decomposition of deposited oil foulants, which enabled removal of over 99.75% of oils, thus reaching a nearly 100% flux recovery ratio. Furthermore, the g-C3N4/TiO2/PVDF composite membrane exhibited great anti-fouling behavior in photocatalysis-assisted filtration. The mechanistic study revealed that underwater superhydrophobicity and the generation of free hydroxyl radicals jointly contributed to membrane anti-fouling. The greatest advantages of this g-C3N4/TiO2/PVDF composite membrane are that not only does it degrades the oil pollutants, but it also makes the membrane less vulnerable to fouling.

Mineralized cupric phosphate/alginate gel alternately multilayer-wrapped nanofibrous membrane with robust anti-crude oil pollution for oily wastewater purification

The high oil-adhesion of crude oil results in flux decline. Therefore, the purification of crude oil/water emulsions is an ongoing and arduous challenge. To improve oil stain resistance and reusability, alginate gel and mineralized Cu3(PO4)2 were alternately layer-by-layer self-assembled on a thermally stabilized PAN/PEI nanofibrous membrane. Inspired by a natural formation water purification system, the hydrophilic and antifouling ‘soil-stone’-like hierarchical structures were constructed. The prepared membrane has excellent mechanical properties (Modulus:198.36 MPa) and large specific surface area. In addition, the synergistic effect of the alginate gel layer and mineralized copper phosphate endows the membrane with excellent underwater superhydrophobicity and outstanding anti-oil-fouling properties, so the (Alg/CuP)6-PAN/PEI membrane can effectively separate crude oil-in-water emulsions and other various SDS-stabilized oil-in-water emulsions, with a high separation efficiency of approximately 99.5%. In the continuous test of separating 1000 mL emulsion, the flux and separation efficiency have no obvious attenuation, and the flux recovery rate (FRR) reach up to 98.4%. Furthermore, the modified membrane has good mechanical and chemical stability in environments with ultrasonic waves, salt, solvents, bases and weak acids, showing great promise in the decontamination of actual oily wastewater.

Sepiolite@TiO2/Graphene oxide composite membrane for long-term Separation of oily wastewater

Microwave synthesis was taken to synthesis A-SEP@TiO2 which has a steady chemical property. The A-SEP@TiO2 composite materials and GO (with different proportion)were mixed and then pumped on commercial PVDF membranes to filter. Finally, the composite membrane A-SEP@TiO2/GO was obtained Under electrostatic self-assembly. Due to the superhydrophilicity and underwater super hydrophobicity of the surface of A-SEP@TiO2/GO membrane, A-SEP@TiO2/GO composite has shown great potential in the field of oil in water. The separation rates of four different emulsions (petroleum ether, toluene, Mesitylene, n-hexane) are all above 90%. With the steady chemical properties, A-SEP@TiO2/GO composite membrane is able to separate oil from water in harsh environment in long-run. After being immersed in 1 mol/L acid and alkali salt solution for one month respectively, all the sample membranes showed a better stability than commercial PVDF membranes. Also, the oil and water removal flux of A-SEP@TiO2/GO composite membrane only decreased by less than 10%, and that was why the composite membranecan can still maintain good cleaning performance even after ten times’ repetition of experiments,. Moreover, the membrane could efficiently remove dyes and heavy metal ions. Therefore, it has a great application prospect in complex water environment treatment.

Natural grass to all-biomass biodegradable tape and superior oil-water separation fabric

Valorization of agricultural and forestry wastes is a promising and meaningful strategy to turn wastes into treasure. Here, we developed a simple and easy-to-scale method to fabricate all-biomass biodegradable tape and superior oil-water separation fabric from the natural grass. The natural Hybrid Pennisetum grass was transformed into the biodegradable and low-cost tapes by partially removing hemicellulose to destroy “biomass recalcitrance” and following a direct dissolution/regeneration process in ionic liquids, because the regenerated all-biomass films from grass exhibit high tensile strength of 58.6 ± 2.9 MPa, which achieves the standard of engineering plastics. In addition, the all-biomass ultrafine fibers with micron-scale diameters were prepared by the air-blowing spinning process. Due to the outstanding hydrophilicity and under-water super-hydrophobicity, the non-woven fabric consisting of 8–12 μm fibers had excellent oil-water separation performance with a super-high flux of exceed 25,000 L•m −2•h −1 (only with gravity) and an extremely high separation efficiency of 100%, which are much better than the previous reports. Furthermore, the all-biomass oil-water separation fabric can be easily prepared and is completely feasible in the high salty solution and acid/alkaline solution. Therefore, a simple and practical process is demonstrated to construct high-performance matetials from low-cost biomass wastes, providing a huge potential in circular economy, environmental protection and bioresource utilization.

Pore structure and surface property design of silicon carbide membrane for water-in-oil emulsification

In recent years, ceramic membranes have been applied as emulsification media in preparing emulsified heavy oils for residue fluid catalytic cracking units. In this case, silicon carbide (SiC) material is used for harsh environments due to its excellent chemical and thermal stabilities. Herein, a superhydrophobic SiC emulsified functional membrane was prepared by using an in-situ sintering method and subsequent grafting modification. The open porosity of these prepared membranes can be controlled from 26% to 48%, and the water contact angle of the membranes under oil was approximately 170°. The decrease of open porosity improved the bending strength of the membranes and also affected the emulsification performance. The size of water droplets decreased from 1.16 μm to 0.98 μm as the open porosity decreased from 48% to 26%. Moreover, the SiC membranes exhibited high flux and stability during the preparation of water-in-oil (W/O) emulsions, and the fouled membranes can be conveniently and effectively regenerated by ultrasonic cleaning with ethanol. Hence, the SiC membranes produced W/O emulsions with small droplets in harsh environments, such as the emulsification of heavy oil and high-viscosity oil at high temperature. • Pore structure and surface property of SiC membranes were successfully tailored. • High bending strength and excellent underwater superhydrophobicity were achieved. • Open porosity was revealed to dominate the membrane emulsification performance. • SiC membranes showed high flux and stability in preparing water-in-oil emulsions. • SiC membranes can be effectively regenerated by ultrasonically cleaning in ethanol.

Real‐Time Monitoring and Quantification of Underwater Superhydrophobicity

AbstractLong‐lasting underwater superhydrophobicity involves stable‐in‐time air layer trapped and retained between a submersed surface and the overlying liquid. Due to its underwater nature, it is rather difficult to accurately observe this air layer in submerged systems, let alone to measure it. Here, white light reflectance spectroscopy is used for real‐time monitoring (acquisition speed 50–200 ms) of underwater superhydrophobicity as well as for the calculation of the entrapped air layer thickness on “everlasting” underwater polymeric superhydrophobic surfaces fabricated by plasma micro‐nanotexturing. The proposed method enables real‐time monitoring and accurate calculation of air layers with thicknesses ranging from a few nanometers to several tens of micrometers involving small underwater bubbles. In addition, different scale topographies of superhydrophobic plasma micro‐nanotextured surfaces are tested and found to retain underwater superhydrophobicity for at least 60 days. As a demonstration, the air layer thickness is also monitored in real time when titrating water with low surface tension liquids. Among several topographies tested, a superhydrophobic surface exhibiting hierarchical topography and a high roughness ratio is able to retain a stable air layer when submersed in liquids with surface tension down to 44 mN m−1.

Recoverable underwater superhydrophobicity from a fully wetted state via dynamic air spreading

SummaryMaintaining the superhydrophobicity underwater offers drag resistance reduction, antifouling, anti-corrosion, noise reduction, and gas collection for boat hulls and submarine vehicles. However, superhydrophobicity typically does not last long underwater since the Cassie state is metastable. Here, we report a reversible and localized recovery of superhydrophobicity from the fully wetted state via air bubble spreading. Composed of sparse fluorinated chained nanoparticles, the submerged surface shows super-low energy barrier for bubble attachment. Especially the recovered plastron exhibits excellent longevity. Based on a simplified, truncated nanocone model, the dynamic spreading of bubbles is analyzed considering two basic parameters, i.e., surface geometric structure and surface energy (which appeared as intrinsic water contact angle). Numerical simulation results via COMSOL confirms the effect of geometric structure on bubble spreading. This investigation will not only offer new insights for the design of robust recoverable superhydrophobic surfaces but also broaden the applications of superhydrophobic coatings.

How Multilayered Feathers Enhance Underwater Superhydrophobicity.

Inspired by ducks, we demonstrate that air pockets within stacked layers of porous superhydrophobic feathers can withstand up to five times more water pressure compared to a single feather. In addition to natural duck feathers, this "layer effect" was replicated with synthetic feathers created by laser cutting micrometric slots into aluminum foil and imparting a superhydrophobic nanostructure. It was revealed that adding layers promotes an increasingly redundant pathway for water impalement, which serves to pressurize the enclosed air pockets. This was validated by creating a probabilistic pore impalement model and also by filling the feathers with an incompressible oil, rather than air, to suppress the layer effect. In addition to revealing a utility of natural duck feathers, our findings suggest that multilayered engineered surfaces can maintain air pockets at high pressures, useful for reducing the drag and fouling of marine structures or enhancing desalination membranes.

Recoverable Underwater Superhydrophobicity From a Fully Wetted State via Dynamic Air Spreading

Maintaining the superhydrophobicity underwater offers drag resistance reduction, antifouling, anti-corrosion, noise reduction and gas collection for boat hulls and submarine vehicles. However, superhydrophobicity typically do not last long underwater since the Cassie state is metastable; any significant pressure fluctuation, shear flow or temperature change will lead the non-reversible transition to the fully wetted state. Here, we report a reversible and localized recovery of the superhydrophobic surface from the fully wetted state via air bubble spreading. Comprised of sparse fluorinated chained nanoparticles, the submerged surface shows super-low energy barrier for bubble attachment. In particular, by analyzing the dynamic spreading of bubbles on a simplified, truncated nanocone model, we enumerate the general geometric criteria for bubble pinning and lateral spreading without vertical growth. Numerical simulation results via COMSOL confirms the effect of geometric structure on bubble spreading. This investigation will not only offer new insights for the design of robust recoverable superhydrophobic surfaces, but also broaden the applications of superhydrophobic coatings for underwater vehicles.

Reversible Wettability between Underwater Superoleophobicity and Superhydrophobicity of Stainless Steel Mesh for Efficient Oil-Water Separation.

Design and fabrication of smart materials with reversible wettability for oil–water separation have attracted worldwide attention due to the increasingly serious water pollution problem. In this study, a rough oxide coating with micro/nanoscale structures is developed on the 304 stainless steel mesh (SSM) by laser ablation. The smart surface with ethanol immersion and natural drying treatments shows the wetting conversion between underwater superoleophobicity and superhydrophobicity. Based on the wettability transition behavior, both light and heavy oil–water mixtures can be separated with the high separation efficiency. Moreover, after being exposed to various corrosive solutions and high temperatures, the smart surface still shows prominent environmental stability. Switchable surface with excellent properties should be an optimal choice to solve the environmental conditions that need to be addressed urgently.

Engineered topographies and hydrodynamics in relation to biofouling control-a review.

Biofouling, the unwanted growth of microorganisms on submerged surfaces, has appeared as a significant impediment for underwater structures, water vessels, and medical devices. For fixing the biofouling issue, modification of the submerged surface is being experimented as a non-toxic approach worldwide. This technique necessitated altering the surface topography and roughness and developing a surface with a nano- to micro-structured pattern. The main objective of this study is to review the recent advancements in surface modification and hydrodynamic analysis concerning biofouling control. This study described the occurrence of the biofouling process, techniques suitable for biofouling control, and current state of research advancements comprehensively. Different biofilms under various hydrodynamic conditions have also been outlined in this study. Scenarios of biomimetic surfaces and underwater super-hydrophobicity, locomotion of microorganisms, nano- and micro-hydrodynamics on various surfaces around microorganisms, and material stiffness were explained thoroughly. The review also documented the approaches to inhibit the initial settlement of microorganisms and prolong the subsequent biofilm formation process for patterned surfaces. Though it is well documented that biofouling can be controlled to various degrees with different nano- and micro-structured patterned surfaces, the understanding of the underlying mechanism is still imprecise. Therefore, this review strived to present the possibilities of implementing the patterned surfaces as a physical deterrent against the settlement of fouling organisms and developing an active microfluidic environment to inhibit the initial bacterial settlement process. In general, microtopography equivalent to that of bacterial cells influences attachment via hydrodynamics, topography-induced cell placement, and air-entrapment, whereas nanotopography influences physicochemical forces through macromolecular conditioning.

Green fabrication of biodegradable cork membrane for switchable separation of oil/water mixtures

Cork is a renewable and biodegradable raw material, which was directly stripped from the periderm of oak bark. The natural amphiphilic chemical composition endowed its pristine surface with hydrophobicity and superoleophilicity in air. Based on the wettability of pristine cork, a stable superhydrophilic and superoleophilic cork membrane without any coating was fabricated via a facile, low-cost, and eco-friendly ethanol- and H2O2-treating process. The superamphiphilic treated membrane exhibited underwater superoleophobicity or underoil superhydrophobicity just by prewetting it with water or oil. This superamphiphilicity of the obtained membrane, combined with its natural permeability of tracheids, plasmodesmata and capillaries, enabled it to separate heavy oil from water or separate water from light oil. Furthermore, without using any continuous external stimulus, a good reversible switching between underwater superoleophobicity and underoil superhydrophobicity could be achieved simply by drying and washing alternately. With such superwettability, the membrane could selectively separate oil/water mixtures with high separation efficiency over multiple cycles solely driven by gravity. Moreover, the membrane could maintain superamphiphilicity in air and superamphiphobicity under liquids after scratching by a penknife or rubbing by a sandpaper. This simple but effective strategy will inspire the design and fabrication of other plant membranes for practical applications in oil/water separation.

Bubble Formation in a Finite Cone: More Pieces to the Puzzle.

We investigate the stability of bubble formation, starting with a convex or a concave meniscus, from a liquid solution (of water and a dissolved gas) inside a finite cone at constant temperature and constant liquid pressure (above the saturation pressure of the pure solvent). It is assumed that the dissolved gas (nitrogen) forms a dilute solution at equilibrium, which can be described by Henry's law. The number and nature of equilibrium states are determined with Gibbsian composite-system thermodynamics, both from the intersection of the equilibrium Kelvin radius with the geometry radius and from the extrema in the plot of free energy of the system versus size of the new phase. Bubble stability is studied along the whole growth path, as the bubble grows inside, gets pinned, and grows further outside the finite cone. The changes in the concentration of the liquid bulk phase and the vapor phase during the growth of the bubble are carefully incorporated in the equations. The effects of various parameters, including cone apex angle, cone half mouth radius, contact angle, total number of moles, and initial degree of saturation, on the stability of the bubble are also investigated. Stability of bubble formation from a liquid solution inside a confined geometry such as a finite cone is of interest in areas such as restoring underwater superhydrophobicity and adhesion of particles to the roughness of synthetic biomaterials.

Schiff base reaction assisted one-step self-assembly method for efficient gravity-driven oil-water emulsion separation

Oil contamination is the most common pollution that is seriously harmful to ecological environments and human health. There is a huge demand to develop an effective and facile strategy for the separation of oily water. Here we report a universal self-assembly coating method via a one-step dopamine (DA) @γ-aminopropyl triethoxysilane (γ-APTS) Schiff base reaction for highly efficient separation of oil-water emulsions. The addition of γ-APTS not only accelerates the DA polymerization but also reduces the aggregation of polydopamine (PDA). Interestingly, the newly designed composite membrane possesses both high hydrophilicity and underwater superhydrophobicity. Under gravity-driven separation conditions, the membrane displays superior efficiency to separate oil-water emulsions with an ultrahigh water flux and splendid anti-oil-fouling ability. The coating layer also exhibits multiform robustness and stability owing to the formation of Schiff base covalent bonding. Importantly, the coated membrane has the ability to be used in a dry state without sacrificing its permeability ability. The desirable stability, simplicity of fabrication, and versatility of the DA@γ-APTS coating may open up new insights to molecularly design next-generation membranes with superior hydrophilicity and underwater superhydrophobic for oily water separation and purification.

Long-Term Underwater Hydrophobicity: Exploring Topographic and Chemical Requirements.

A family of hybrid organoinorganic silica-based particles with varied chemical natures and morphologies has been synthesized to test their ability to develop coatings with underwater hydrophobicity. The particles were characterized by elemental microanalysis, scanning electron microscopy, and dynamic light scattering to evaluate the organic content, observe the morphology, and estimate the aggregate size, respectively. These morphologies were transferred into surface topographies by spraycoating dispersions made from the particles onto glass supports, resulting in coatings with an ample range of profiles and roughness but all of them being superhydrophobic. Atomic force microscopy and optical profilometry were used to map the coating surfaces and analyze the topography. Then, underwater hydrophobicity endurance was tested by immersion under a 2 cm 20 °C water column perpendicular to circular glass supports coated with the particles. The so-called mirror effect derived from the occurrence of the primary plastron (continuous air layer occluded between the surface and the water) was observed on the surface of all of the coatings tested. Apart from the dependency of plastrons on the water temperature and substrate shape, the plastron quality and lifetime is notably different depending on the particle morphology and thus on the coating topography. These experiments have demonstrated that the most persistent mirror effects, and therefore underwater superhydrophobicity, were produced on coatings that exhibited the smoothest topographies at the micrometric scale. In addition, these particle-only coatings can be made mechanically stable and robust by blending with a polymer matrix.

A probe into the surface and interface phenomenon of WO3 endowing with superwettability and super gas sensing ability

The surface engineering to adjust superwettable and gas sensing properties is fascinating. To conduct these investigations, we successfully developed WO3 by a facile method with binary structures: micro sized spherical and nano sized particles or slices. The design and understanding of superwettability to water and sensing ability to acetone gases of the as-prepared WO3 are investigated by multiple technologies. The results of present study, superhydrophilicity (∼0°) and underwater ultra-oleophobicity (143.8°) or superhydrophobicity after surface modification (158.7°) and high sensitivity (%) to acetone of 100ppm: 82.6, are quite desirable and could be arouse the interest of other researchers and materials engineers who are engaged in surface-functionalization field.