Bioinspired gas bubble spontaneous and directional transportation effects in an aqueous medium

Facile fabrication of robust superhydrophobic tapered needles for collection and transportation of underwater bubbles

The collection and transportation of underwater bubbles has attracted significant attention due to their wide range of applications in the mining, petroleum, and chemical industries. In this study, robust superhydrophobic tapered needles were successfully fabricated by spraying a superhydrophobic coating prepared by an organic-inorganic hybrid method. The prepared tapered needles present excellent surface stability and good superhydrophobicity with a contact angle (CA) of about 156°. The fabricated tapered needles demonstrate excellent performance in collection and transportation of underwater bubbles and the working mechanism was also thoroughly studied. The prepared robust superhydrophobic tapered needles provide a simple, efficient and economical way for collection and transportation of underwater bubbles.

Gas Evolution in Water Electrolysis

Gas bubbles generated by the hydrogen evolution reaction and oxygen evolution reaction during water electrolysis influence the energy conversion efficiency of hydrogen production. Here, we survey what is known about the interaction of gas bubbles and electrode surfaces and the influence of gas evolution on practicable devices used for water electrolysis. We outline the physical processes occurring during the life cycle of a bubble, summarize techniques used to characterize gas evolution phenomena in situ and in practical device environments, and discuss ways that electrodes can be tailored to facilitate gas removal at high current densities. Lastly, we review efforts to model the behavior of individual gas bubbles and multiphase flows produced at gas-evolving electrodes. We conclude our review with a short summary of outstanding questions that could be answered by future efforts to characterize gas evolution in electrochemical device environments or by improved simulations of multiphase flows.

Locomotion behavior of air bubbles on solid surfaces

Air bubbles are a common occurrence in both natural and industrial settings and are a significant topic in the fields of physics, chemistry, engineering, and medicine. The physical phenomena of the contact between bubbles and submerged solid surfaces, as well as the locomotion behavior of bubbles, are worth exploring. Bubbles are generated in an unbounded liquid environment and rise due to unbalanced external forces. Bubbles of different diameters follow different ascending paths, after which they approach, touch, collide, bounce, and finally adsorb to the solid surface, forming a stable three-phase contact line (TPCL). The bubbles are in an unstable state due to the unbalanced external forces on the solid surface and the effects generated by the two-phase contact surface, resulting in different locomotion behaviors on the solid surface. Studying the formation, transport, aggregation, and rupture behaviors of bubbles on solid surfaces can enable the controllable operation of bubbles. This, in turn, can effectively reduce the loss of mechanical apparatus in agro-industrial production activities and improve corresponding production efficiency. Recent research has shown that the degree of bubble wetting on a solid surface is a crucial factor in the locomotion behavior of bubbles on that surface. This has led to significant progress in the study of bubble wetting, which has in turn greatly advanced our understanding of bubble behavior. Based on this, exploring the manipulation process of the directional motion of bubbles is a promising research direction. The locomotion behavior of bubbles on solid surfaces can be controlled by changing external conditions, leading to the integration of bubble behavior in various scientific and technological fields. Studying the dynamics of bubbles in liquids with infinite boundaries is worthwhile. Additionally, the manipulation process and mode of these bubbles is a popular research direction.

On-Demand Transport Bubbles Adhering to Noncontiguous Patterned Superhydrophobic Surfaces Using a Superhydrophobic Tweezer

Bubble transportation and related flotation are ubiquitous phenomena in nature and industry. Various surfaces with distinct morphologies and specific wettability properties have been engineered by organisms in nature and by humans to facilitate the targeted movement of bubbles. However, existing methods predominantly rely on continuous surfaces, limiting the ability of bubbles to deviate from their path before reaching their intended destination. Therefore, directional transportation of bubbles using noncontiguous surfaces still remains a significant challenge. Inspired by water spiders' ability to capture bubbles underwater using their hydrophobic surface for survival, we propose a novel transport strategy that utilizes patterned superhydrophobic surfaces (PSHSs) and a superhydrophobic tweezer. This strategy is implemented by switching between the hood mode and puncture mode of the moving three-phase contact lines to load and unload the bubble. To quantitatively evaluate the loss ratio of the bubble during transportation, a simple and exquisite bubble-weighing apparatus is devised. Our findings indicate that circular PSHSs demonstrate superior bubble adhesion and achieve the highest bubble transport ratio of 95.1%. In order to validate the promising application of this novel method, we employ the computer numerical control (CNC) technology to facilitate the autonomous loading and precise transportation of underwater bubbles, as well as the blending and ionization of combustible gas bubbles with air bubbles at different volume ratios.

Underwater Bubble Manipulation on Surfaces with Patterned Regions with Infused Lubricants

The flexible manipulation of underwater gas bubbles on solid substrates has attracted considerable research interest from scientists in the fields of water electrolysis, bubble microreactions, drug delivery, and heat transfer. Inspired by the oxygen-binding mechanisms of aquatic organisms, scientists have designed a series of interfacial materials for use in collecting gases, detecting and grading bubbles, and conducting microbubble reactions. Aerophilic surfaces are commonly used in underwater bubble manipulation platforms due to their excellent gas-trapping properties. However, during bubble transport, some of the bubbles are retained in the rough structure of the aerophilic surface and cause gas loss, which in the long run reduces the gas transport function. In addition, the aerophilic surface is prone to failure in high-humidity and high-pressure underwater environments. The lubricant-infused surface features an oil layer that remains stable on a rough substrate and is immiscible with water. Additionally, the bubbles are transported over the oil layer without causing losses other than those dissolved in water. These attributes make it more favorable than the aerophilic surface. Inspired by the unique properties of Nepenthes and cactus spines, we developed a patterned slippery surface [patterned lubricant-infused surface (PLIS)] through laser etching and ammonia etching that facilitates the coexistence of superaerophobic and aerophilic surfaces. The PLIS executes bubble capture utilizing a difference in wettability measuring 78°, transports bubbles through Laplace force and buoyancy, and regulates bubble release by restricting the contact area on the PLIS. The PLIS can be prepared rapidly and affordably in just about an hour, and its potential for large-scale production is high. Following tests for shear, acid and alkali resistance, and corrosion resistance, the PLIS exhibited impressive weathering resistance and appears to have potential for application in some extreme environments.

Ultrahigh Efficient Collection of Underwater Bubbles by High Adsorption and Transport, Coalescence, and Collection Integrating a Conical Arrayed Surface

The capture and utilization of underwater fuel bubbles such as methane can alleviate the greenhouse effect, solve the global energy crisis, and possibly improve the endurance of underwater equipment. However, previous research routinely failed to achieve the integrated process of continuous adsorption, transportation, and collection of bubbles limited by the trade-off between the bubble adhesion and transport efficiency dependent on interfacial pinning, tremendously hindering the direct capture and utilization of underwater fuel bubbles. To break through this bottleneck, a magnetic-guided conical arrayed surface (CAS) associated with a laser etching technique is fabricated conveniently to realize superhydrophobicity. The bubbles on laser-etched CAS have higher adhesiveness and low-pinning transport compared with those on the nonlaser-etched surface. Intriguingly, the gas film adsorbed within the CAS seems to be a gas channel, which accelerates the bubble coalescence and fast spreading to eventually realize the integration of transport, coalescence, and collection. The dynamic behaviors of bubble adsorption, transportation, and coalescence on CAS are probed to reveal the mechanism of the gas film-generating process within conical arrays. Furthermore, a novel underwater bubble-collecting device with multiangled CAS is proposed to achieve multidirectional capture, highly efficient transportation, and collection of rising bubbles. The results advance our understanding of dynamic behaviors of bubbles at solid-liquid interfaces and facilitate design and manufacturing of an apparatus for bubble collection.

Directional Transport of Underwater Bubbles on Solid Substrates: Principles and Applications

The manipulation of underwater bubbles on substrates has received extensive research interest from both the scientific community and industry, including the chemical industry, machinery, biology, medicine, and other fields. Recent advances in "smart" substrates have enabled the bubbles to be transported on demand. Herein, the progress in the directional transport of underwater bubbles on various types of substrates is summarized, including planes, wires, and cones. The transport mechanism can be classified as buoyancy-driven, Laplace-pressure-difference-driven, and external-force-driven according to the driven force of the bubble. Moreover, the wide applications of directional bubble transport are reported, ranging from gas collection, microbubble reaction, bubble detection and classification, bubble switch, and bubble microrobots. Lastly, the advantages and challenges of various directional bubble transportation methods are discussed, and the current challenges and future prospects in this field are also discussed. This Review outlines the fundamental mechanisms of underwater bubble transportation on solid substrates and helps to understand the methods of optimizing bubble transportation performances.

New Nanofiber Composition for Multiscale Bubble Capture and Separation

Bubble dynamics inside a liquid medium and its interactions with hydrophobic and hydrophilic surfaces are crucial for many industrial processes. Electrospinning of polymers has emerged as a promising fabrication technique capable of producing a wide variety of hydrophobic and hydrophilic polymer nanofibers and membranes at a low cost. Thus, knowledge about the bubble interactions on electrospun hydrophobic and hydrophilic nanofibers can be utilized for capturing; separating; and transporting macro-, micro-, and nanobubbles. In this study, poly(methyl methacrylate) (PMMA) and PMMA-poly(ethylene glycol) (PEG) electrospun nanofibers were fabricated to investigate gas bubble interactions with submerged nanofiber mats. To improve their durability, the nanofibers were reinforced with a plastic mesh. The ultimate tensile strengths of PMMA and PMMA-30%PEG nanofibers were measured as 0.35 and 0.30 MPa, respectively. With the use of reinforcement mesh, the mechanical properties of final membranes could be improved by a factor of 70. The gas permeability of the electrospun and reinforced nanofibers was also studied using the high-speed visualization technique and a homemade setup to investigate the effect of electrospun nanofibers on the bubble coalescence and size in addition to the frequency of released bubbles from the nanofiber mat. The diffusion rate of air bubbles in hydrophobic PMMA electrospun nanofibers was measured as 10 L/s for each square meter of the nanofiber. However, the PMMA-30%PEG mat was able to restrict the diffusion of gas bubbles through its pores owing to the van der Waals force between the water molecules and nanofiber surface as well as the high stability of the thin water layer. It has been shown that the hydrophobic electrospun nanofibers can capture and coalesce the rising gas bubbles and release them with predictable size and frequency. Consequently, the diameter of bubbles introduced to the hydrophobic PMMA membrane ranged between 2 and 25 mm, whereas the diameter of bubbles released from the hydrophobic electrospun nanofibers was measured as 8 ± 1 mm. The proposed mechanism and fabricated electrospun nanofibers can enhance the efficiency of various systems such as heat exchangers, liquid-gas separation filters, and direct air capture (DAC) systems.

Stimuli-Responsive Liquid-Crystal-Infused Porous Surfaces for Manipulation of Underwater Gas Bubble Transport and Adhesion

Biomimetic artificial surfaces that enable the manipulation of gas bubble mobility have been explored in a wide range of applications in nanomaterial synthesis, surface defouling, biomedical diagnostics, and therapeutics. Although many superhydrophobic surfaces and isotropic-lubricant-infused porous surfaces have been developed to manipulate gas bubbles, the simultaneous control over the adhesion and transport of gas bubbles underwater remains a challenge. Thermotropic liquid crystals (LCs), a class of structured fluids, provide an opportunity to tune the behavior of gas bubbles through LC mesophase transitions using a variety of external stimuli. Using this central idea, the design and synthesis of LC-infused porous surfaces (LCIPS) is reported and the effects of the LC mesophase on the transport and adhesion of gas bubbles on LCIPS immersed in water elucidated. LCIPS are demonstrated to be a promising class of surfaces with an unprecedented level of responsiveness and functionality, which enables the design of cyanobacteria-inspired object movement, smart catalysts, and bubble gating devices to sense and sort volatile organic compounds and control oxygen levels in biomimetic cell cultures.

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 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.

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Superhydrophobicity is recovered from fully wetted state in submerged system

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The dynamic spreading of bubbles is theoretically analyzed

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The geometric criteria provide direction in designing superhydrophobic surfaces

Bioinspired Universal Approaches for Cavity Regulation during Cylinder Impact Processes for Drag Reduction in Aqueous Media: Macrogeometry Vanquishing Wettability

Stabilizing lubricating gas films at the solid-liquid interface is a promising strategy for underwater drag reduction. It has been restricted by the enormous extra energy input and the poor stability of superhydrophobic coatings. Cavity encapsulation is a valid method to improve and maintain the formation of the air layer on the solid surface, which is created by the rapidly impacting process on a water surface. The wettability of solid objects (the combination of the surface roughness and chemical component) and liquid properties played a key factor in determining the water impact process for cavity entrainment. However, inspired by the striking behavior of basilisk lizards and their toe's shape, we found that the geometric shape of solid objects plays an equally important role in cavity entrainment and stabilization, which is often ignored. Herein, we present a universal strategy to retain the air cavity on the cylinder surfaces. The cavity can be retained not only on the surface of superhydrophobic cylinders but also on the surface of hydrophobic, hydrophilic, and even superhydrophilic cylinders, without bursting at a depth of 70.0-90.0 cm underwater. The retaining cavity enfolds the profile and upper sides of the cylinder and changes its shape to a streamlined body to achieve underwater drag reduction. In addition, optimizing the cylinders' shapes by increasing the fillet radii significantly improved the drag reduction efficiency from 64.2 to 70.5%.

The challenges, achievements and applications of submersible superhydrophobic materials

Superhydrophobic materials have been widely reported throughout the scientific literature. Their properties originate from a highly rough morphology and inherently water repellent surface chemistry. Despite promising an array of functionalities, these materials have seen limited commercial development. This could be attributed to many factors, like material compatibility, low physical resilience, scaling-up complications, etc. In applications where persistent water contact is required, another limitation arises as a major concern, which is the stability of the air layer trapped at the surface when submerged or impacted by water. This review is aimed at examining the diverse array of research focused on monitoring/improving air layer stability, and highlighting the most successful approaches. The reported complexity of monitoring and enhancing air layer stability, in conjunction with the variety of approaches adopted, results in an assortment of suggested routes to achieving success. The review is addressing the challenge of finding a balance between maximising water repulsion and incorporating structures that protect air pockets from removal, along with challenges related to the variant approaches to testing air-layer stability across the research field, and the gap between the achieved progress and the required performance in real-life applications.

Laser-Induced Wettability Gradient Surface of the Aluminum Matrix Used for Directional Transportation and Collection of Underwater Bubbles

The control of underwater bubble behavior on a solid surface has great research significance. However, the control of the spontaneous directional transport and collection of numerous underwater bubbles remains a challenge. A new technique of a metal mesh with superhydrophobic/hydrophobic properties is demonstrated here, which creates a wettability gradient coupled with a microporous array by means of pulsed fiber laser ablation and chemical modification of the aluminum sheet. The resultant wettability surface effectively achieved the spontaneous movement of bubbles along the directional wettability gradient (superaerophobicity to aerophilicity) and through the metal mesh (aerophilicity to superaerophilicity) in the direction of decreasing free energy. Theoretical analysis accounted first for the spontaneous sliding of bubbles on the wettability gradient surface as a result of the action of an unbalanced surface tension force and second for the spontaneous transition of bubbles from the aerophilic to superaerophilic side as a result of the combined action of Laplace pressure and buoyancy. A device with the capability of directional transportation and collection of underwater bubbles was designed based on the samples with a wettability gradient and a superhydrophobic/hydrophobic microporous array as the core components. The potential application is laser ablation of wettability gradient surfaces and metal mesh with superhydrophobic/hydrophobic properties for directional transportation and collection of underwater bubbles.

Bioinspired Slippery Cone for Controllable Manipulation of Gas Bubbles in Low-Surface-Tension Environment

Manipulating bubbles in surfactant solutions or oil mediums is of vital importance in daily life and industries concerned with cosmetics, food, fermentation, mineral flotation, etc. However, realizing controllable regulation of a bubble's behavior is quite challenging in a low-surface-tension aqueous environment, which is mainly attributed to the strong affinity of liquid molecules to a solid surface to prevent the efficient interaction of gas bubbles with the solid surface. To address these issues, herein, we have taken inspiration from cactus spines and pitcher plants to develop a slippery copper cone (SCC), which can facilely manipulate gas bubble in surfactant solutions (as low as ∼29.9 mN/m, 20 °C), e. g., directional and continuous transportation of gas bubbles. This intriguing capability mainly originates from the cooperation of the conical morphology engendering a Laplace pressure and the slippery surface with low friction force but high affinity to bubbles. In addition, the SCC also shows an elegant capability of transporting gas bubbles in various organic solvents, such as formamide (57.4 mN/m, 20 °C), glycol (46.5 mN/m, 20 °C), dibutyl phthalate (37.0 mN/m, 20 °C), and dimethylformamide (35.8 mN/m, 20 °C). Furthermore, the prepared SCC also demonstrated distinguished feasibility in antibuoyancy bubble delivery, efficient collection of acidic CO₂ microbubbles, and the underwater reaction of hydrogen and oxygen, endowing it with promising applications in various complex low-surface-tension environments.

The wettability of gas bubbles: from macro behavior to nano structures to applications

In recent years, various interfaces related to bubble wettability have been fabricated, which have already been widely applied in various disciplines and fields. Therefore, to better research and understand the wettability of gas bubbles, recent progress with interfaces and wettability of bubbles in aqueous media, including superaerophilicity and superaerophobicity, is summarized. Many biological interfaces which exhibit marvelous characteristics are discussed for reference. Because of the similar behavior between gas bubbles in aqueous media and droplets in air, the two wetting conditions are compared together to better illustrate theories of gas bubble wettability. Based on these theories, effective and available manipulation of gas bubbles' wettability provides a novel idea and method to solve practical problems in various aspects, i.e., superaerophobic electrodes for gas evolution reactions, superaerophilic electrodes for gas compensation reactions, superaerophilic interfaces for directional collection and transportation of gas bubbles, and so on.

Is Superhydrophobicity Equal to Underwater Superaerophilicity: Regulating the Gas Behavior on Superaerophilic Surface via Hydrophilic Defects

Superhydrophobic surfaces have long been considered as superaerophilic surfaces while being placed in the aqueous environment. However, versatile gas/solid interacting phenomena were reported by utilizing different superhydrophobic substrates, indicating that these two wetting states cannot be simply equated. Herein, we demonstrate how the hydrophilic defects on the superhydrophobic track manipulate the underwater gas delivery, without deteriorating the water repellency of the surface in air. The versatile gas-transporting processes can be achieved on the defected superhydrophobic surfaces; on the contrary, in air, a water droplet is able to roll on those surfaces indistinguishably. Results show that the different media pressures applied on the two wetting states determine the diversified fluid-delivering phenomena; that is, the pressure-induced hydrophilic defects act as a gas barrier to regulate the bubble motion behavior under water. Through the rational incorporation of hydrophilic defects, a series of gas-transporting behaviors are achieved purposively, for example, gas film delivery, bubble transporting, and anisotropic bubble gating, which proves the feasibility of this underwater air-controlling strategy.

Spontaneous and Directional Bubble Transport on Porous Copper Wires with Complex Shapes in Aqueous Media

Manipulation of gas bubble behaviors is crucial for gas bubble-related applications. Generally, the manipulation of gas bubble behaviors generally takes advantage of their buoyancy force. It is very difficult to control the transportation of gas bubbles in a specific direction. Several approaches have been developed to collect and transport bubbles in aqueous media; however, most reliable and effective manipulation of gas bubbles in aqueous media occurs on the interfaces with simple shapes (i.e., cylinder and cone shapes). Reliable strategies for spontaneous and directional transport of gas bubbles on interfaces with complex shapes remain enormously challenging. Herein, a type of 3D gradient porous network was constructed on copper wire interfaces, with rectangle, wave, and helix shapes. The superhydrophobic copper wires were immersed in water, and continuous and stable gas films then formed on the interfaces. With the assistance of the Laplace pressure gradient between two bubbles, gas bubbles (including microscopic gas bubbles) in the aqueous media were subsequently transported, continuously and directionally, on the copper wires with complex shapes. The small gas bubbles always moved to the larger ones.

Bioinspired Nanostructured Surfaces for On-Demand Bubble Transportation

The maneuver of small bubbles in a programmed way will advance numerous processes, including gas evolution reaction and aeration. Unlike in-air droplets, rapidly rising bubbles in liquid medium can hardly be steered through interaction with solid substrates, causing difficulties in maneuvering bubbles. We pattern and lubricate nanoporous substrates with regions of contrasting wettability that is similar to the back of Namib desert beetles and subsequently immerse the lubricated surface underwater to spontaneously form spatially patterned Nepenthes-inspired slippery surfaces after the dewetting of lubricants. As a result, bubbles are confined on lubricant-infused surfaces, with their high mobility well preserved. The interfacial states of attached bubbles are analyzed, and their dynamic sliding velocities are quantified. Using the lubricated patterned surfaces, we further demonstrate the predefined motion of bubbles driven by buoyancy at a small tiling angle, as well as a self-propulsion of bubbles driven by surface tension force at a tilting angle of 0°, respectively. The spatially lubricated surfaces simplify gas handling in liquid medium and have potential applications in fields where bubble handling is crucial.

Superwettability of Gas Bubbles and Its Application: From Bioinspiration to Advanced Materials

Gas bubbles in aqueous media are common and inevitable in, for example, agriculture and industrial processes. The behaviors of gas bubbles on solid interfaces, including generation, growth, coalescence, release, transport, and collection, are crucial to gas-bubble-related applications, which are always determined by gas-bubble wettability on solid interfaces. Here, the recent progress regarding the study of interfaces with gas-bubble superwettability in aqueous media, i.e., superaerophilicity and superaerophobicity, is summarized. Some examples illustrate how to design microstructures and chemical compositions to achieve reliable and effective manipulation of gas-bubble wettability on artificial interfaces. These designed interfaces exhibit excellent performance in gas-evolution reactions, gas-adsorption reactions, and directional gas-bubble transportation. Moreover, progress in the theoretical investigation of gas-bubble superwettability is reported. Lastly, some challenges are presented, such as the reliable manipulation of gas-bubble wettability and the establishment of mature theory for exactly and systematically explaining gas-bubble wetting phenomena.

Bioinspired Design of Underwater Superaerophobic and Superaerophilic Surfaces by Femtosecond Laser Ablation for Anti- or Capturing Bubbles

A micro-/nanoscale hierarchical rough structure inspired by the underwater superaerophobicity of fish scales was fabricated by ablation of a silicon surface by a femtosecond laser. The resultant silicon surface showed superhydrophilicity in air and became superaerophobic after immersion in water. Additionally, inspired by the underwater superaerophilicity of lotus leaves, we showed that the polydimethylsiloxane surface after femtosecond laser ablation exhibits superhydrophobicity in air and becomes superaerophilic in water. The underwater superaerophobic surface showed excellent antibubble ability, whereas the underwater superaerophilic surface could absorb and capture air bubbles in a water medium. The experimental results revealed that the in-air superhydrophilic surface generally shows superaerophobicity in water and that the in-air superhydrophobic surface generally shows underwater superaerophilicity. An underwater superaerophobic porous aluminum sheet with through microholes was prepared, and this sheet was able to intercept underwater bubbles and further remove bubbles from water. In contrast, the underwater superaerophilic porous polytetrafluoroethylene sheet could allow the bubbles to pass through the sheet. We believe that these results are highly significant for providing guidance to researchers and engineers for obtaining excellent control of bubbles' behavior on a solid surface in a water medium.

Biomimetic self-cleaning surfaces: synthesis, mechanism and applications

With millions of years of natural evolution, organisms have achieved sophisticated structures, patterns or textures with complex, spontaneous multifunctionality. Among all the fascinating characteristics observed in biosystems, self-cleaning ability is regarded as one of the most interesting topics in biomimicry because of its potential applications in various fields such as aerospace, energy conversion and biomedical and environmental protection. Recently, in-depth studies have been carried out on various compelling biostructures including lotus leaves, shark skins, butterfly wings and gecko feet. To understand and mimic their self-cleaning mechanisms in artificial structures, in this article, recent progress in self-cleaning techniques is discussed and summarized. Based on the underlying self-cleaning mechanisms, the methods are classified into two categories: self-cleaning with water and without water. The review gives a succinct account of the detailed mechanisms and biomimetic processes applied to create artificial self-cleaning materials and surfaces, and provides some examples of cutting-edge applications such as anti-reflection, water repellence, self-healing, anti-fogging and micro-manipulators. The prospectives and directions of future development are also briefly proposed.

Hydraulic Strategy of Cactus Trichome for Absorption and Storage of Water under Arid Environment

Being an essential component in various metabolic activities, water is important for the survival of plants and animals. Cacti grown in arid areas have developed intrinsic water management systems, such as water collection through spines, water absorption through trichome, and water storage using mucilage. The water collection method of cactus is well-documented, but its water absorption and storage strategies remain to be elucidated. Thus, this study analyzed the morphology and wettability of cactus trichomes by using advanced bio-imaging techniques and by performing in vitro experiments on an artificial system mimicking these structures, respectively. In addition, the in situ water absorption process through the trichome cluster was quantitatively visualized. This paper proposes a new bio-inspired technique for dew collection based on information about the water management strategies of cactus. This study discusses the underlying water absorption and storage strategies of cactus and provides the experimental database required to develop a biomimetic water management device.

Multiphase Media Antiadhesive Coatings: Hierarchical Self-Assembled Porous Materials Generated Using Breath Figure Patterns

The cleaning of interface pollutants typically consumes a large amount of energy. Therefore, the development of multiphase media antiadhesive materials is urgently required to meet the demand of energy savings and environmental protection. In this study, the antiadhesive properties toward several liquid droplets and bubbles in multiple media are demonstrated on a porous Fe₂O₃ coating, which is prepared via a facile spin-coating-assisted breath figure approach and a phase separation strategy. The prominent antiadhesive characteristic of these porous surfaces lies in their high-surface-energy hierarchical micro/nanoscale structure, which easily entraps one medium (oil or water) in the pore and repels other unmixable liquids and air bubbles. In addition, we successfully demonstrate an antifouling application of the coating, which shows excellent antiadhesive and super-antiwetting characteristics under multiple liquids. Our work extends relevant antiadhesion research from a single medium to multiple media and promises to broaden the applications of antiadhesive materials in sophisticated activities performed under complicated liquid environments, such as marine antifouling or pipeline transportation.

Superhydrophobic Cones for Continuous Collection and Directional Transportation of CO2 Microbubbles in CO2 Supersaturated Solutions

Microbubbles are tiny bubbles with diameters below 50 μm. Because of their minute buoyant force, the microbubbles stagnate in aqueous media for a long time, and they sometimes cause serious damage. Most traditional methods chosen for elimination of gas bubbles utilize buoyancy forces including chemical methods and physical methods, and they only have a minor effect on microbubbles. Several approaches have been developed to collect and transport microbubbles in aqueous media. However, the realization of innovative strategies to directly collect and transport microbubbles in aqueous media remains a big challenge. In nature, both spider silk and cactus spines take advantage of their conical-shaped surface to yield the gradient of Laplace pressure and surface free energy for collecting fog droplets from the environment. Inspired by this, we introduce here the gradient of Laplace pressure and surface free energy to the interface of superhydrophobic copper cones (SCCs), which can continuously collect and directionally transport CO₂ microbubbles (from tip side to base side) in CO₂-supersaturated solution. A gas layer was formed when the microbubbles encounter the SCCs. This offers a channel for microbubble directional transportation. The efficiency of microbubble transport is significantly affected by the apex angle of SCCs and the carbon dioxide concentration. The former provides different gradients of Laplace pressure as the driving force. The latter represents the capacity, which offers the quantity of CO₂ microbubbles for collection and transportation. We believe that this approach provides a simple and valid way to remove microbubbles.

Piezo-phototronic effect enhanced UV photodetector based on CuI/ZnO double-shell grown on flexible copper microwire

In this work, we present a facile, low-cost, and effective approach to fabricate the UV photodetector with a CuI/ZnO double-shell nanostructure which was grown on common copper microwire. The enhanced performances of Cu/CuI/ZnO core/double-shell microwire photodetector resulted from the formation of heterojunction. Benefiting from the piezo-phototronic effect, the presentation of piezocharges can lower the barrier height and facilitate the charge transport across heterojunction. The photosensing abilities of the Cu/CuI/ZnO core/double-shell microwire detector are investigated under different UV light densities and strain conditions. We demonstrate the I-V characteristic of the as-prepared core/double-shell device; it is quite sensitive to applied strain, which indicates that the piezo-phototronic effect plays an essential role in facilitating charge carrier transport across the CuI/ZnO heterojunction, then the performance of the device is further boosted under external strain.