Wetting Of Droplets Research Articles

Light-Triggered Droplet Gating Strategy Based on Janus Membrane Fabricated by Femtosecond Laser.

The characteristics of the directed transport of liquids based on Janus membranes play a crucial role in practical applications in energy, materials, physics, chemistry, medicine, biology, and other fields. Although extensive progress has been made, it is still difficult to realize the accurate controllability of liquid directional transmembrane transport. The current gating strategies for the directed transport of liquids based on Janus membranes still have some limitations: (a) using magnetic fluid may cause contamination due to the addition of new substances and (b) utilizing hydrophobicity/hydrophilicity conversion of titanium dioxide requires a long switching time (over 30 min). Herein, a strategy is proposed to precisely control liquid directional transport by altering the wettability of droplets on Janus films prepared by a femtosecond laser through photothermal effects. Infrared laser irradiation on Janus film coated with CNTs can effectively convert light energy into thermal energy, rapidly increase the surface temperature of Janus film, and change the wettability of the liquid on the film. Liquid transmembrane directional transport can be achieved within a few seconds without contaminating the transported liquid. The proposed gating strategy can enable the application of Janus membranes in various scenarios such as microchemical reactions, biological cell culture, and interface self-propulsion.

Wetting dynamics and heat transfer of droplet evaporation around boiling point: Facile manipulation by surface roughness

The evaporation of water–ethanol droplets on solid surfaces has vast potential for applications in thermal management, microfluidics, and biomedical sciences, while the approach of manipulating wetting dynamics and heat transfer of droplet evaporation around the boiling point remains open for investigations. The present study proposes a facile approach: fabricating solid surfaces with diverse roughness by sandpaper milling with different grit densities, which can alter the wetting and heat transfer characteristics of droplet evaporation. We discover that when the temperature of the heated surface exceeds the droplet boiling point, the existence of bubbles affects the solid–liquid wetting state; surface roughness alters the ability of bubble departure, thus determining the solid–liquid contact and the contact line retraction time. Consequently, in this case, the solid–liquid contact area and the heat absorbed from heated surfaces for droplets changes with surface roughness, altering the droplet evaporation time and the average heat flux at the solid–liquid interface. Bridged by droplet wetting dynamics, the heat transfer of droplet evaporation above the boiling point can be manipulated by adjusting surface roughness, offering a facile approach to tuning the process of droplet evaporation in the related industrial applications.

Coalescence Mechanism Induced by Different Wetting States of Ti and Al Droplets on Rough Surfaces.

There is currently increasing interest in droplet transportation and coalescence on rough surfaces. However, the relationship among wettability, coalescence mode, and substrate characteristics (roughness and nanopillar height) remains unclear. In this work, two coalescence modes, climbing coalescence and contacting coalescence, are first observed in the dynamic behaviors of Ti and Al droplets on rough substrates. Due to the nonsynchronized wetting state transition of the droplets, the coalescence mode with increasing substrate characteristics differs, transitioning from contacting coalescence to climbing coalescence and then returning to the contacting mode. In general, the mode of coalescence correlates closely with the respective wetting states. Typically, Ti and Al droplets coalesce in the contacting mode when they have the same wetting state, but if they have different wetting states, they coalesce in the climbing mode. Our results emphasize the complicated relationship between the surface structure and the wettability of droplets, which could provide insights into self-assembly, three-dimensional printing, and microfluidic devices.

A hydrophobic organic coating on micro/nano-texture fabricated by galvanic corrosion

This paper is focused on the electrochemical corrosion protection issue of marine equipment during actual service. The two metals (45# steel and 6061 aluminum alloy) are brought into contact to form the galvanic corrosion. The micro/nano-structure with honeycomb-like protrusions (1.82 μm) and network-like texture (370–460 nm) is fabricated on the steel surface, which becomes superhydrophilic (water contact angle (WCA) ≈ 0°). Perfluorooctanoic acid, a low surface energy modifier, is used to modify the corroded micro/nano-texture surface. Thus, a hydrophobic organic coating (WCA = 158° ± 4°) is fabricated, which effectively inhibits droplet wetting and penetrating into the metal layer. Furthermore, it is found that the coating surface still maintained its hydrophobicity (WCA = 101.5°) after 2 h of the coating immersed in the corrosive solution. Thus, a new surface remanufacture method, utilizing the galvanic corrosive surface to prepare the hydrophobic coating, is proposed. This study hopes to provide new ideas for designing the protective coating applied to engineering maintenance.

A novel quaternary ammonium triethanolamine modified polyester polyether for rapid wetting and penetration pretreatment for digital inkjet dyeing of polyester fabric

Green environmental protection and digitisation are vital for the future development of the textile printing and dyeing industry. Dispersed dye digital inkjet dyeing, as a new technology, enables on-demand low-liquor dyeing and controllable segment dyeing of polyester fabric. This facilitates the transformation, upgrading and sustainable development of the printing and dyeing industry. Due to its inherent hydrophobicity, polyester fabric poses challenges in terms of quick wetting and penetration of ink droplets. To enhance its moisture absorption and rapid wettability, a novel pre-treatment agent, i.e. cationic polyester polyether (CPP), was synthesised via the triethanolamine quaternisation modification of polyester polyether. The wetting and penetration behaviours of ink droplets on the CPP-pre-treated fabric were studied, and the colour performance of CPP-pre-treated dyed fabric was investigated. Results showed that the optimal conditions for CPP synthesis were a DMT to PEG molar ratio of 3:1, a molecular weight of PEG 2000 and a PEG to NAB molar ratio of 1:0.2. At a CPP concentration of 0.3 %o.w.f., the CPP-pre-treated fabric exhibited excellent moisture absorption and rapid wetting with water spreading time of 4 s, wicking height of 9.5 cm/5 min and ink droplet spreading time of 960 ms. Compared with the original inkjet-dyed fabric, the front K/S value of CPP-pre-treated dyed fabric decreased to 17.1 and the back K/S value increased to 16.0. The colour penetration was 93.3 %, which was 13 % higher than that of the original fabric. Thus, CPP pre-treatment of polyester fabric can facilitate complete dyeing of the fabric even within the interior of yarns.

Exceptional anisotropic superhydrophobicity of sword-lily striated leaf surface and soft lithographic biomimicking using polystyrene replica

Herein, we report unusually high anisotropic superhydrophobicity, unidirectional self-cleaning, and biomimicking of adaxial sword-lily (Gladiolus hortulanus) leaf comprising three distinct levels of surface textures. Observably, the static anisotropic wetting and rolling of water droplets are more favourable in the parallel (or, striation) direction than in the perpendicular direction. Inspired from such water repellency of the sword lily leaf surface, here bio-mimicked polystyrene (PS) leaf construct is developed through a soft lithographic technique. Considering different water droplet sizes (4–10 μl) on natural lily leaf and bio-mimicked PS construct surfaces, the respective parallel (θ ||) and perpendicular (θ ⊥) water contact angles (WCAs) stand at, θ || ∼143°–147°, θ ⊥ ∼156°–169°; and θ || ∼130°–139°, θ ⊥ ∼142°–145°. Moreover, the specimens under study exhibit roll-off angles ranging, α || ∼8°–23° (α ⊥ ∼16°–41°) and α || ∼21°–49° (α ⊥ ∼40°–55°) along parallel (and perpendicular) directions; respectively. A noticeable difference in α ⊥ and α || values can be ascribed to the profound three-phase contact line (TCL) pinning along the perpendicular direction taking advantage of striation as means of barrier. The roll-off angles can also alter due to a variation in the droplet volume. The unusual anisotropic superhydrophobicity and unidirectional droplet roll-off can be attributed to the entrapped air within the micro-nano texture beneath the water droplet along with the pinning effect in the perpendicular direction caused by the striated heights.

Bioaerosol sampling and bioanalysis: Applicability of the next generation impactor for quantifying Legionella pneumophila in droplet aerosols by flow cytometry

Bioaerosol generation, sampling, and cultivation-independent quantification of pathogenic bacteria play a crucial role in studying dose-response effects of Legionella pneumophila. Here, the Next Generation Impactor (NGI), initially created for pharmaceutical inhaling studies, was assessed for its potential to sample airborne bioaerosols and to separate size-dependent wet droplets by incrementally increasing the airflow speed. This stainless-steel sampler was shown in this study to be suitable for sampling prior to cultivation-independent analysis of pathogen-containing bioaerosols using washable cups. The applicability was studied by quantifying the total and intact cell count of L. pneumophila by flow cytometry after being dispersed into a droplet aerosol. Our results demonstrate a high total sampling efficiency of 95.5% ± 11.8% despite a lower biological sampling efficiency of 59.7% ± 16.5% for dry aerosols. However, by elevating the relative humidity (RH) to 100% in a liquid aerosolization unit, the biological sampling efficiency increased to over 90% for L. pneumophila. More than 50% of the cells were found in stage 1 using the liquid aerosolization unit. In comparison, 80% of the cells were sampled in stages 4–6 at 30% RH. Specifically, while at 100% RH, the droplet size mattered, at 30% RH, the size distribution of dry particles, in this case L. pneumophila, was relevant due to evaporation processes, which explains the size differences. These findings indicate the potential of the NGI for further exploration and application in studying other aerosol-borne pathogens, especially concerning the size distribution of wet droplets, viability, or effect-based bioanalysis.

Mixed superoleophilic/superoleophobic hard granular media for coalescence of oil-in-water-emulsion

To enhance the coalescence separation efficiency of oil/water emulsions, the coalescence mechanism of oil droplets between two media of opposite wettability was investigated. A coalescent-bed comprising a mix of superhydrophobic and superoleophilic quartz sand, along with superhydrophilic and underwater superoleophobic quartz sand, was constructed in a coalescer. Single-factor and response surface experiments were used to study the effects of oleophilic/oleophobic sand mixing ratio, apparent velocity, initial oil concentration, bed thickness, sand particle size on coalescence-separation performance, and to determine the optimal experimental conditions. Employing the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory, the agglomeration mechanism of oil droplets within the mixed particle bed was analyzed based on interfacial energy dynamics. Results indicate that the mixing ratio of the two sands significantly influences coalescence separation performance. An optimal oleophilic/oleophobic sand mixing ratio of 1:1 yielded a mass factor of 0.69 kPa−1, showcasing superior coalescence separation efficiency and an optimal balance of separation efficiency and pressure drop. Under the optimal conditions of apparent velocity of 2.7 m/h, initial oil concentration of 471.9 mg/L, bed thickness of 12.6 cm and mixed sand particle size of 0.5 mm, the oil/water separation efficiency reached 98.08 % ± 0.87 %, with effluent oil droplet size being 2.5 to 3 times larger than at the inlet. Analysis of the oil droplet coalescence mechanism revealed a synergistic effect between the two quartz sands, enhancing oil removal performance. The strong affinity of oil to the superhydrophobic and superoleophilic sand surface increases oil droplet wetting and coalescence efficiency, while the water attraction on the superhydrophilic and underwater superoleophobic sand surface forms a stable water film, improving the flux of the continuous water phase and reducing pressure drop across the bed. This study demonstrates that combining quartz sands of opposite wettability not only overcomes the limitations of single-wettability surfaces in coalescing oil removal but also achieves high efficiency and low energy consumption in oil removal.

Droplet impinging on sparse micropillar-arrayed non-wetting surfaces

Wettability of droplets and droplet impinging on sparse micropillar-arrayed polydimethylsiloxane (PDMS) surfaces were experimentally investigated. For droplets wetting on these surfaces, the contact line density model combining stability factor and droplet sagging depth was developed to predict whether the droplets were in the Wenzel or Cassie–Baxter wetting state. It was found that droplets on the sparser micropillar-arrayed PDMS surfaces were in the Wenzel wetting state, indicating that a complete rebound cannot happen for droplets impinging on these surfaces. For the case of droplets impinging on sparse micropillar-arrayed PDMS surfaces, it was found that there existed a range of impact velocity for bouncing droplets on the micropatterned surfaces with a solid fraction of 0.022. To predict the upper limit of impact velocity for bouncing droplets, a theoretical model considering the immersion depth of liquid into the micropillar structure was established to make the prediction, and the lower limit of impact velocity for bouncing droplets can be obtained by balancing kinetic energy with energy barrier due to contact angle hysteresis. In addition, the droplet maximum spreading parameter was fitted and found to follow the scale law of We1/4.

Dynamical Adsorption Behavior Prediction of Dried Tobacco Leaf Heterogeneous Interfaces through Simulation and Image Recognition Techniques.

The process of spraying water and flavorings on dry tobacco is an important factor in the industrial environment and product quality. Tobacco as a complex porous fiber material, the interfacial transfer process of water is complex. In this study, machine learning and image recognition techniques were utilized to quickly obtain the structural parameters of the tobacco surface and construct a cellular structure model of the tobacco surface. In situ observation of the droplet impact spreading process was carried out using a high-speed camera to explore the droplet dissipation dynamics on different surfaces. And the competing processes of droplet wetting and evaporation under the influence of surface microstructure were determined by combining experimental studies and finite element simulation calculations. Based on the characteristics of tobacco pore size distribution, the infiltration under gas-liquid two-phase action was transformed into single-phase flow transfer under capillary force, and the continuous droplet infiltration process was simulated. A parallel artificial membrane permeability measurement method of bionic tobacco waxy layer was constructed for the screening of spray dosing copenetrant. This study brings new insights into the wetting of porous fibrous materials and is important for exploring the wetting process and additive development process influenced by the microstructure.

Regulation mechanism of droplets wetting on banana leaf surface and its dynamic contact angle wetting model

The effective wetting and deposition of pesticide droplets on the leaf surface significantly affect the control of pests and diseases, and its mainly affected by the leaf surface properties and pesticide formulation properties. This article aims to investigate the wettability regulation mechanism and dynamic wetting properties of banana leaves, in order to intelligently design appropriate pesticide formulations. The micro-morphology of the banana leaves exhibits a micro-nano dual-scale structure with different microstructures and roughness on the adaxial side and abaxial side. Correspondingly, the wettability on the adaxial side of the banana leaf with higher roughness (Rq = 71.1 nm) is always better than that on the abaxial side (Rq = 42.5 nm). The droplet contact performance of droplets exhibits a strong concentration dependence of surfactants, which regulate the inter-species differences in the wettability of different pesticide formulations on both sides. Besides, the dynamic wetting process of pesticide droplets exhibits strong time dependence, by using a time series method to establish an AR(2) model, precise simulation and prediction of the contact angle changes on the adaxial side (R2 = 0.9560) and abaxial side (R2 = 0.8903) during the dynamic wetting process were achieved. The dynamic wetting model provides new insights into the spreading and deposition processes of droplets on leaves. This work provides a favorable reference for the study of the dynamic balance between wetting and adhesion properties of pesticide spray on leaf surfaces.

Micro-structure design of black ceramic composite surface towards photothermal superhydrophobic anti-icing

Conventional superhydrophobic surfaces have limitations in the melting and icing process in response to ambient temperature. In this study, we fabricated a composite coating using plasma-etched black ceramic as a low layer on the surface of AZ31 magnesium alloy, and loaded-polydimethylsiloxane nanoparticles (PDMS NPs) as a top layer to achieve a photothermal and superhydrophobic anti-icing. The absorbance and forbidden bandwidth of the ceramic layer exhibit the excellent photothermal properties of solid solution (Mg1-XCuXO). The ceramic phase exhibits a bandgap of 3.62 eV and achieves a temperature of up to 69.4 °C, demonstrating desirable light absorption and photothermal properties. The microstructure and polar bonds of Si-O-Si provide excellent superhydrophobic properties, allowing the surface contact angle to reach 156°. The wetting of water droplets on the composite coating’s surface is simulated using molecular dynamics, which is consistent with experimental findings and emphasizes the coating’s exceptional superhydrophobic. Additionally, the transition process between Cassie-Baxter and Wenzel was analyzed by researching the dynamic icing and ice-melting processes. This composite surface converts light energy into heat, melts snow/ice, and uses hydrophobicity to promptly desorb it from the surface, enabling a photothermal active and superhydrophobic passive anti-icing strategy.

Comparative Molecular Dynamics Simulation of Wetting on Liquid-like Surfaces.

We utilize molecular dynamics simulations to comparably investigate the wetting and motion behavior of droplets on liquid-like surfaces (LLS) with varying grafting conditions. Polydimethylsiloxane (PDMS) and perfluoropolyether (PFPE) have been considered to be flexible molecules versus rigid molecules of trichloro(octadecyl) silane (OTS) and trichloro(1H,1H,2H,2H-perfluorooctyl) silane (PFOS), respectively. Our findings reveal that droplets on surfaces tethered with either PDMS or PFPE brushes can generate indentations and wetting ridges, providing microscopic evidence of their liquid-like nature. The grafting density of mobile chains exerts a dominant influence on the wetting properties compared to the molecular weight. A parameter map is created to pinpoint the precise range of grafting densities essential for the optimal construction of LLS at predetermined molecular weights. Furthermore, the investigation of droplet motion dynamics on LLS demonstrates that droplets consistently exhibit a rolling state, regardless of the intensity of the applied lateral force. The movement pattern of the droplet shifts only under conditions where the grafting density is significantly reduced and the substrate exhibits hydrophilic tendencies. These findings and the developed model are anticipated to offer valuable guidelines for optimal designs of LLS.

The kinetics of droplet wetting gas hydrate surfaces regulated by anti-agglomerant monolayers: Insights from molecular dynamics study

Anti-agglomerants (AAs) have become a promising strategy for mitigating gas hydrate risks in hydrocarbon transport systems. However, a notable limitation of AAs is their diminished effectiveness in systems with high water content. The density of the AA monolayer is a critical factor influencing the coalescence and agglomeration of hydrate particles with water droplets, but the mechanisms driving these effects remain largely unknown. In this study, molecular dynamics (MD) simulations were employed to explore the anti-wetting performance and interaction properties of Cocamide Monoethanolamine (CMEA) monolayers at varying densities on hydrate surfaces. The results indicate that in systems with low-density monolayers, CMEA molecules exhibit high degrees of freedom and increased fluctuation frequencies, allowing the water molecules of droplet to penetrate the monolayer and rapidly wet the hydrate surface. In contrast, high-density monolayer configurations show properties that hinder droplet wetting on hydrate surfaces and inhibit hydrate growth. Further investigation into the adsorption behavior of CMEA molecules on hydrate surfaces highlights the substantial impact of the solution phase on adsorption free energy, with gas-phase environments enhancing the binding strength between CMEA molecules and hydrate surfaces. These molecular insights offer valuable observations regarding the structural adaptability of AA monolayers during droplet permeation and wetting processes, providing critical information for understanding the broader interactions between anti-agglomerant molecules and hydrates.

Study of the rolling wetting behavior of droplets on surfaces with T-shaped microstructures

The regulation of droplet wetting behavior encompasses multiple professional disciplines, and the surface microstructure has become the preferred approach for controlling droplet wetting behavior. Bionic studies have revealed that springtails can effortlessly move in water because of the T-shaped microstructures on their skin, which prompted the design of various T-shaped microstructures to effectively regulate droplet wetting behavior. Although the static wetting theory for T-shaped microstructure surfaces is well understood, the dynamic wetting theory regarding droplet rolling remains unexplored. This study aims to investigate the rolling behavior of droplets on inclined surfaces with T-shaped microstructures through theoretical analysis and numerical simulation. By analyzing the comprehensive energy transformation and gas-liquid interface morphology during the tilting processes, we revealed the mechanism by which the geometric parameters of the T-shaped microstructures influence the droplet rolling behavior and constructed an energy prediction analysis model for the final droplet morphology. The results showed that reducing the cantilever height, increasing the structural spacing, and enlarging the droplet size facilitated continuous rolling of the droplets. Additionally, we discovered that the driving energy for droplet rolling on inclined surfaces with T-shaped microstructures originates from the surface free energy. The maximum surface free energy of a droplet often occurs when the droplet begins to roll.

Laser-induced selective metallization directly preparing repairable superhydrophobic copper layer on polymers and single-droplet ethanol detection

Ethanol is widely used in life and ethanol solutions with different concentrations have different uses, so it is important to detect ethanol concentration. Currently, methods for judging ethanol concentration according to the wetting of droplets on the surface of materials are gradually emerging. However, there are still problems, such as the need for measurement equipment, poor accuracy, and cannot be reused. Herein, a novel strategy was proposed to prepare a superhydrophobic (water contact angle = 155.4°) and super-ethanophilic (ethanol contact angle = 5.2°) copper layer on polymer substrate by laser-induced selective metallization (LISM) and atmosphere-induced hydrophobicity, which can be used for single-drop ethanol detection. X-ray photoelectron spectroscopy and Thermogravimetric analysis − Fourier transform infrared − gas chromatography − mass spectrometry showed that when the rough copper layer prepared by LISM was exposed to air, it spontaneously oxidized and adsorbed airborne volatile organic compounds gases to become a superhydrophobic copper layer. The superhydrophobic copper layer obtained by this strategy has good durability. In addition, the superhydrophobic copper layer has excellent repair ability. The ethanol detection array prepared based on this superhydrophobic copper layer can be used to perform single-droplet ethanol detection. The array does not require any equipment and only needs to read the number of droplets in the wetted grid in the 20–70 % ethanol solution concentration range, showing great potential in the field of ethanol detection.

Nanoscopic wetting behaviour of single oil droplets on a fibre

Multiphase fluids that contain dispersed oil droplets are fundamental to many industrial processes and formulated products. In the present work, the surface wetting and interaction between an emulsion droplet and a single fibre were quantitatively investigated as a function of surfactant chemistry, pH, and salt concentration using an optical tweezers apparatus. Silicone droplets were emulsified in water and stabilised by surfactants. An individual droplet was captured using an optical tweezers setup to allow a controlled contact with a fibre surface. During the approach process, the surface interaction was measured and spreading behaviour was quantified after contact was made between the droplet and fibre. Salt concentration was increased to change the surface interaction behaviour. The pH and ζ-potential of silicone emulsions decreased from 9.5 to 6.0 and from 39 mV to − 15 mV, respectively, when increasing salt concentration for both of the surfactants studied, SDS and CTAB. The attraction between the fibre and a silicone oil droplet was found to increase as the electrostatic repulsion was suppressed with an increased salt concentration. This was confirmed by an increased number of droplet-fibre adhesion events and in some cases droplet spreading on the fibre. Such observations for adhesion are based on electrostatic attractions exceeding the maximum force that can be exerted by the trapping laser. When adhesion was not observed, attraction could still be recorded through a thin-film hydrodynamic suction effect during the process of retraction, which becomes more pronounced on the polyester fibre. Our results provide a novel method to directly quantify the surface interaction and wetting of liquid droplets on microscopic fibres simultaneously, which could be valuable when investigating formulated products that involve emulsion droplets.

Investigating the Effects of Lubricant Infusion Methods on Polymer SLIPS.

Slippery lubricant infused porous surfaces (SLIPSs) are promising bioinspired surfaces with self-healing and droplet wetting properties, among many others, that are desirable due to their range of applications. Recently, there have been many developments in the SLIPS field regarding the creation of textured surfaces and lubricant selection. However, there is a lack of knowledge regarding the method of lubricant infusion. In this study, we aim to fill this void by investigating different infusion methods that impose external forces on the lubricant. We developed our SLIPS by hot embossing nanostructures onto polypropylene by using molds that were laser micromachined. These textured surfaces were then infused with silicone oil using three different infusion methods: ultrasonication, vacuum, and hydrostatic pressure. We analyzed the wettability and slipperiness of the SLIPS by evaluating the critical tilt angle and comparing the sliding velocities of water droplets on each sample at a tilt angle of 20°. Additionally, the durability of the SLIPS was tested by dropping 50 successive water drops onto the samples and evaluating the droplet-surface interactions throughout. The sonicated infusion method yielded SLIPS that performed the best with a contact angle hysteresis of 13°, a critical tilt angle of 18.3°, a sliding velocity of 1.66 mm/s, and the least accumulation of droplets over time with use. These values are greatly improved when compared to the control sample where lubricant was simply dripped on, which resulted in a contact angle hysteresis of 20°, a critical tilt angle of 26.3°, and a sliding velocity of 0.23 mm/s. The sonicated and drip infusion methods were also compared with different materials (stainless steel) and different textures (microstructures). It was found that the improvement in slipperiness using the sonicated infusion method is prominent for nanoscale textures on both stainless steel and polypropylene. In this study, we discuss the challenges with oil depletion in SLIPS (cloaking and wetting ridges) and with the selection of contact angle measurement methods. While further investigation as to why certain applied forces during infusion yield better SLIPS is warranted, these forces greatly affect the outcome. This work suggests that researchers should consider using sonication or other methods of lubricant infusion that apply external forces as infusion techniques to yield better SLIPS on the nanoscale.

Novel conductive Janus membranes with water diode effect assisted by an electrostatic field for efficient mass transfer and oil-water separation

Membrane separation technology is effective for treating oily wastewater but is limited by the lack of membranes that are both highly permeable and fouling-resistant. Janus membranes, with asymmetric wettability and electrostatic repulsion of charged oil droplets, offer a potential strategy for oil-water separation. This study developed novel electrically enhanced conductive Janus membranes using metal-modified organic membranes to achieve high-efficiency treatment. Conductive metal Janus membranes were fabricated using a straightforward two-step method involving simple electroless nickel plating and surface peeling. The resulting nickel-coated surface of the membrane demonstrated extreme hydrophilicity, underwater oleophobicity, and excellent conductivity (approximately 4.7 Ω). Fascinatingly, the developed membranes exhibited a “water diode” effect, facilitating unidirectional liquid transport without external pressure. Gravity-driven oil-water separation achieved a flux rate of 4874 L m−2 h−1 and over 99 % oil removal efficiency, surpassing most existing studies. During electro-filtration of oil-water emulsions, the conductive Janus membrane as a cathode generated electrostatic repulsion, preventing emulsified oil droplet adhesion. Applying an external voltage increased the flux by approximately 13 times and oil removal efficiency to 99.6 %. The membrane's separation capability remained consistent across six cycle tests, demonstrating robust stability. As all, this work first synthesized conductive “water diode” Janus membranes with high oil-water separation efficiency, offering fresh perspectives on the design and fabrication of Janus membranes as well as on the processes for treating oily wastewater.

The influence of geometric boundary features on droplet wetting and directional motion

Structured surfaces with micro and nanostructures have shown great potential for a wide range of applications, such as self-cleaning, deicing, and drag reduction. Previous research has extensively explored the interactions between droplets and microstructures, particularly in terms of Cassie-Baxter and Wenzel states. However, there is still a notable gap in systematically studying the interactions between droplets and various types of geometrical boundary features. In this study, we employed molecular dynamics simulations to investigate how different geometric boundary features affect the wetting behavior and motion of water droplets. Our detailed analysis led to the derivation of a formula to calculate the critical contact angle for a droplet interacting with any geometric boundary feature from a mechanical perspective. Based on these findings, we successfully controlled motion direction of water droplets along a separation membrane. We observed that due to the influence of geometric boundary features at the holes, droplets exhibited unidirectional motion on the separation membrane. By considering the influence of geometric boundary features, our research provides valuable insights for preventing wetting transition, enhancing the storage of lubricating oil in grooves, facilitating efficient oil–water separation, and regulating fluid flow in microchannels.