Integrative Bioinspired Surface with Wettable Patterns and Gradient for Enhancement of Fog Collection

Biomimetic Fog Collector with Hybrid and Gradient Wettabilities

Water scarcity is a global problem and collecting water from the air is a viable solution to this crisis. Inspired by Namib Desert beetle, leaf venation and spider silk, we designed an integrated biomimetic system with hybrid wettability and wettability gradient. The hybrid hydrophilic-hydrophobic wettability design that bionomics desert beetle's back can construct a three-dimensional topography with a water layer on the surface, expanding the contact area with the fog flow and thus improving the droplet trapping efficiency. The venation-like structure with wettability gradient not only provides a planned path for water transportation, but also accelerates water removal under the synergistic effect of gravity and wettability driving force, thus further improving the surface regeneration rate. The collector combines droplet capture, coalescence, transportation, separation, and storage capabilities, which provides new ideas for the design of future high-efficiency fog collectors.

Synergistically Utilizing a Liquid Bridge and Interconnected Porous Superhydrophilic Structures to Achieve a One-Step Fog Collection Mode

Conventional fog collection efficiency is subject to the inherent inefficiencies of its three constituent steps: fog capture, coalescence, and transportation. This study presents a liquid bridge synergistic fog collection system (LSFCS) by synergistically utilizing a liquid bridge and interconnected porous superhydrophilic structures (IPHS). The results indicate that the introduction of liquid bridge not only greatly accelerates water droplet transportation, but also facilitates the IPHS in maintaining rough structures that realize stable and efficient fog capture. During fog collection, the lower section of the IPHS is covered by a water layer, however due to the effect of the liquid bridge, the upper section protrudes out, while covered by a connective thin water film that does not obscure the microstructures of the upper section. Under these conditions, a one-step fog collection mode is realized. Once captured by the IPHS, fog droplets immediately coalesce with the water film, and are simultaneously transported into a container under the effect of the liquid bridge. The LSFCS achieves a collection efficiency of 6.5 kg m-2 h-1, 2.3 times that of a system without a liquid bridge. This study offers insight on improving fog collection efficiency, and holds promise for condensation water collection or droplet manipulation.

One-Step Microfluidic Fabrication of Bioinspired Microfibers with a Spindle-Knot Structure for Fog Harvest

Many biomimetic microfibers have been designed from spider silk to collect water efficiently from humid air as a result of its periodic spindle-knot structure, which enhances the direct movement and convergence of captured fog droplets. Here, a hydrodynamic flow-focusing microfluidic device with a theta-shaped tube is designed for the one-step fabrication of bioinspired microfibers with a spindle-knot structure for fog harvest. The morphology of the structured microfibers, including height, width, and spacing of spindle knots, can be adjusted readily by regulating the flow rate of specific phases. The production rate of these structured microfibers can reach 1100 cm/min. Moreover, the capture, transportation, and collection performance of fog droplets on various microfibers are investigated in a fog collection platform. It is demonstrated that our one-step microfluidic device presents a ready method for the fabrication of structured microfibers with spindle knots, which provide a significant facilitation on efficient fog capture and water collection.

Research progress of bionic fog collection surfaces based on special structures from natural organisms

With the increasing shortage of water resources, people are seeking more innovative ways to collect fog to meet the growing need for production and the demand for livelihood. It has been proven that fog collection is efficient for collecting water in dry but foggy areas. As a hot research topic in recent years, bionic surfaces with fog collection functions have attracted widespread attention in practical applications and basic research. By studying natural organisms and bionic surfaces, more avenues are provided for the development of fog collection devices. Firstly, starting from biological prototypes, this article explored the structural characteristics and fog collection mechanisms of natural organisms such as spider silk, desert beetles, cactus, Nepenthes and other animals and plants (Sarracenia, shorebird and wheat awn), revealing the fog collection mechanism of the natural organisms based on microstructures. Secondly, based on the theory of interfacial tension, we would delve into the fog collection function's theoretical basis and wetting model, expounding the fog collection mechanism from a theoretical perspective. Thirdly, a detailed introduction was given to prepare bionic surfaces and recently explore fog collection devices. For bionic surfaces of a single biological prototype, the fog collection efficiency is about 2000-4000 mg cm-2 h-1. For bionic surfaces of multiple biological prototypes, the fog collection efficiency reaches 7000 mg cm-2 h-1. Finally, a critical analysis was conducted on the current challenges and future developments, aiming to promote the next generation of fog collection devices from a scientific perspective from research to practical applications.

Super-Fast Fog Collector Based on Self-Driven Jet of Mini Fog Droplets

Freshwater scarcity crisis threatens human life and economic security. Collecting water from the fog seems to be an effective method to defuse this crisis. Nonetheless, the existing fog collection methods have the limitations of the low fog collection rate and efficiency because of their gravity-based droplet shedding. Here, the aforementioned limitations are resolved by proposing a new fog collection method based on the self-driven jet phenomenon of the mini fog droplets. A prototype fog collector (PFC) composed of a square container that is filled with water is first designed. Both sides of the PFC are superhydrophobic but covered with superhydrophilic pore array. The mini fog droplets touching the side wall are easily captured and spontaneously and rapidly penetrate into the pores to form jellyfish-like jets, which greatly increases the droplet shedding frequency, guaranteeing a higher fog collection rate and efficiency compared with the existing fog collection methods. Based on this, a more practical super-fast fog collector is finally successfully designed and fabricated which is assembled by several PFCs. This work is hoping to resolve the water crisis in some arid but foggy regions.

Origami-like 3D Fog Water Harvestor with Hybrid Wettability for Efficient Fog Harvesting

Collecting water in fog has also become a breakthrough to solve the hidden danger of water shortage in some arid areas. The three-dimensional (3D) structure fog collection material can increase the surface area in direct contact with the fog flow and reduce the quick flow of fog, which can effectively improve the fog collection efficiency. Imitating the three-dimensional structure of corrugated paper, the 3D fog collecting material with hybrid wettability was prepared by chemical and physical means. We discuss the influence of different wettability combinations on the fog collection efficiency of 3D structures and study the influence of spraying times and illumination times on the surface wettability during the construction of wettability. We also study the influence of the concavity and tip as well as the bending angle on the fog collection in the 3D structure and obtain the most reasonable concavity and convex ratio and bending angle. The superhydrophilic and superhydrophobic 3D structure fog harvesting material prepared by us performs well in the fog harvesting process, and the fog harvesting efficiency reaches 1.442 g cm-2 h-1. The fog collection efficiency is 418% of the original zinc sheet. At the same time, compared with the superhydrophilic and superhydrophobic hybrid two-dimensional (2D) plane, the increase is 168%, and compared with the superhydrophobic 3D structure, the increase is 150%.

Bioinspired Green Fabricating Design of Multidimensional Surfaces for Atmospheric Water Harvesting

Across the globe, the quest for clean water is escalating for both households as well as agricultural exigencies. With the industrial revolution and swift population growth, the contamination of natural water bodies has impacted the lives of more than two billion people around the world. A spectrum of water-saving solutions has been examined. Nonetheless, most of them are either energy-inefficient or limited to only a particular region. Thus, the pursuit of clean and potable drinking water is an assignment that invites collective discourse from scientists, policymakers, and innovators. In this connection, the presence of moisture in the atmosphere is considered one of the major sources of potential freshwater. Thus, fishing in atmospheric water is a mammoth opportunity. Atmospheric water harvesting (AWH) by some plants and animals in nature (particularly in deserts or arid regions) at low humidity serves as an inspiration for crafting state-of-the-art water harvesting structures and surfaces to buffer the menace of acute water scarcity. Though a lot of research articles and reviews have been reported on bioinspired structures with applications in water and energy harvesting, the area is still open for significant improvisation. This work will address the multidimensional-based AWH ability of natural surfaces or fabricated structures without the involvement of toxic chemicals. Moreover, the review will discuss the availability of clean technologies for emulating fascinating natural surfaces on an industrial scale. In the end, the current challenges and the future scope of bioinspired water harvesters will be discussed for pushing greener technologies to confront climate change.

Survival in desert: Extreme water adaptations and bioinspired structural designs

Deserts are the driest places in the world, desert creatures have evolved special adaptations to survive in this extreme water shortage environment. The collection and transport of condensed water have been of particular interest regarding the potential transfer of the underlying mechanisms to technical applications. In this review, the mechanisms of water capture and transport were first summarized. Secondly, an introduction of four typical desert creatures including cactus, desert beetles, lizards, and snakes which have special adaptations to manage water was elaborated. Thirdly, the recent progress of biomimetic water-collecting structures including cactus, desert beetles, and lizards inspired designs and the influence of overflow on water collection was demonstrated. Finally, the conclusions were drawn, and future issues were pointed out. The present study will further promote research on bioinspired water management strategies.

Water harvesting on biomimetic material inspired by bettles

Many organisms in nature such as beetles and cacti can survive in arid places by their own surface structures that are still able to collect mist. These surfaces have micro-nano structures that maintain a very low adhesion, allowing them to continuously collect and transport water. Here, we used a light curing three dimensional molding process to create a template for a water harvesting system inspired by the back of a beetle, a hydrogel-like beetle back surface for water transport. By changing the curvature structure of the water evacuation channels and altering the hydrophilic and hydrophobic properties of the surface, the designed large-scale artificial water harvesting study was made possible. The results show that if the surface has a proper curvature structure and hydrophobic density, the water collection on the super-impregnated surface is much higher than that on an ordinary hydrophobic surface. Based on this, a new efficient and environmentally friendly water collection scheme is proposed. The data show that the triangular tip structure imitating beetle-backed hydrogel surface collects the highest amount of water with a water weight of 16 g in 2 h. This study offers interesting prospects for designing a new generation of structural materials with a bionic structure distribution for high-efficiency water harvesting. The results of the study are useful for pushing the improvement of environmental-friendly water collection, transport and separation devices.

Abbreviations

The dorsal shape of the beetle's back is critical for water collection. In this work, while redesigning the shape of the back of the beetle, the method of 3D printing the beetle back template was used to prepare the beetle back made of hydrogel, which greatly improved the water collection performance and has certain engineering application prospects.

Unidirectional self-actuation transport of a liquid metal nanodroplet in a two-plate confinement microchannel

Controllable directional transport of a liquid metal nanodroplet in a microchannel has been a challenge in the field of nanosensors, nanofluidics, and nanofabrication. In this paper, we report a novel design that the self-actuation of a gallium nanodroplet in a two-plate confinement microchannel could be achieved via a continuous wetting gradient. More importantly, suitable channel parameters could be used to manipulate the dynamic behavior of the gallium nanodroplet. The self-actuation transport in the two-plate confinement microchannel is the result of the competition between the driving force from the difference of the Laplace pressure and energy dissipation from the viscous resistance. Furthermore, we have identified the conditions to assess whether the droplet will pass through the contractive cross-section or not. This work can provide guidance for manipulating liquid metal nanodroplets in microchannels.

Hierarchically Structured, All-Aqueous-Coated Hydrophobic Surfaces with pH-Selective Droplet Transfer Capability

Often inspired by nature, techniques for precise droplet manipulation have found applications in microfluidics, microreactors, and water harvesting. However, a widely applicable strategy for surface modification combining simultaneous hydrophobicity and pH-sensitivity has not yet been achieved by employing environmentally friendly assembly conditions. The introduction of pH-responsive groups to an otherwise fluorinated polyphosphazene (PPZ) unlocks pH-selective droplet capture and transfer. Here, an all-aqueous layer-by-layer (LbL) deposition of polyelectrolytes is used to create unique hydrophobic coatings, endowing surfaces with the ability to sense environmental pH. The high hydrophobicity of these coatings (ultimately reaching a contact angle >120° on flat surfaces) is enabled by the formation of hydrophobic nanoscale domains and controllable by the degree of fluorination of PPZs, polyamine-binding partners, deposition pH, and coating thickness. Inspired by the hierarchical structure of rose petals, these versatile coatings reach a contact angle >150° when deposited on structured surfaces while introducing a tunable adhesivity that enables precise droplet manipulation. The films exhibited a strongly pronounced parahydrophobic rose petal behavior characterized through the contact angle hysteresis. Depositing as few as five bilayers (∼25 nm) on microstructured rather than smooth substrates resulted in superhydrophobicity with water contact angles >150° and the attenuation of the contact angle hysteresis, enabling highly controlled transfer of aqueous droplets. The pH-selective droplet transfer was achieved between surfaces with either the same microstructure and LbL film building blocks, which were assembled at different pH, or between surfaces with different microstructures coated with identical films. The demonstrated capability of these hydrophobic LbL films to endow surfaces with controlled hydrophobicity through adsorption from aqueous solutions and control the adhesion and transfer of water droplets between surfaces can be used in droplet-based microfluidics applications and water collection/harvesting.

A confined-etching strategy for intrinsic anisotropic surface wetting patterning

Anisotropic functional patterned surfaces have shown significant applications in microfluidics, biomedicine and optoelectronics. However, surface patterning relies heavily on high-end apparatuses and expensive moulds/masks and photoresists. Decomposition behaviors of polymers have been widely studied in material science, but as-created chemical and physical structural changes have been rarely considered as an opportunity for wettability manipulation. Here, a facile mask-free confined-etching strategy is reported for intrinsic wettable surface patterning. With printing technology, the surface wetting state is regulated, enabling the chemical etching of setting locations and efficient fabrication of complex patterns. Notably, the created anisotropic patterns can be used for realizing water-responsive information storage and encryption as well as fabricating flexible electrodes. Featuring advantages of simple operation and economic friendliness, this patterning approach brings a bright prospect in developing functional materials with versatile applications.

Anisotropic functional patterned surfaces have shown significant applications in microfluidics, biomedicine, and optoelectronics. Here, authors demonstrate a fast and mask-free etching method for accurate surface patterning by confined decomposition, enabling the efficient fabrication of complex patterns.

Bioinspired functional SLIPSs and wettability gradient surfaces and their synergistic cooperation and opportunities for enhanced condensate and fluid transport

Bioinspired smart functional surfaces have received increasing attention in recent years owed to their tunable wettability and enhanced droplet transport suggesting them as excellent candidates for industrial and nanotechnology-related applications. More specifically, bioinspired slippery lubricant infused porous surfaces (SLIPSs) have been proposed for their low adhesion enabling continuous dropwise condensation (DWC) even of low-surface tension fluids. In addition, functional surfaces with chemical and/or structural wettability gradients have also been exploited empowering spontaneous droplet transport in a controlled manner. Current research has focused on the better understanding of the mechanisms and intimate interactions taking place between liquid droplets and functional surfaces or on the forces imposed by differences in surface wettability and/or by Laplace pressure owed to chemical or structural gradients. Nonetheless, less attention has been paid to the synergistic cooperation of efficiently driving droplet transport via chemical and/or structural patterns/gradients on a low surface energy/adhesion background imposed by SLIPSs, with the consequent promising potential for microfluidics and condensation heat transfer applications amongst others. This review provides a detailed and timely overview and summary on recent advances and developments on bioinspired SLIPSs and on wettability gradient surfaces with focus on their synergistic cooperation for condensation and fluid transport related applications. Firstly, the fundamental theory and mechanisms governing complex droplet transport on homogeneous, on wettability gradient surfaces and on inclined SLIPSs are introduced. Secondly, recent advances on the fabrication and characterization of SLIPSs and functional surfaces are presented. Then, the condensation performance on such functional surfaces comprising chemical or structural wettability gradients is reviewed and their applications on condensation heat transfer are summarized. Last a summary outlook highlighting the opportunities and challenges on the synergistic cooperation of SLIPSs and wettability gradient surfaces for heat transfer as well as future perspective in modern applications are presented.

Regular Self-Actuation of Liquid Metal Nanodroplets in Radial Texture Gradient Surfaces

Liquid metal movement in microfluidic devices generally requires an external stimulus to achieve its motion, which results in many difficulties to precisely manipulate its motion at a nanoscale. Therefore, there is an attempt to control the motion of a liquid metal droplet without the input of an external force. In this paper, we report an approach to achieve the self-actuation of a gallium nanodroplet in radial texture gradients on substrates. The results have proved the validity of this method. It is suggested that there are four stages in the self-motion of the droplet and that the precursor film forming on the second stage plays a pivotal role in the motion. Furthermore, how the impact velocity affects the self-actuation of the nanodroplet on the gradient surface is also studied. We find that the moderate impacting velocity hinders the self-actuation of the gallium nanodroplet. This study is very helpful to regulate the self-actuation on patterned substrates and facilitate their applications in the fields of microfluidics devices, soft robots, and liquid sensors.

Multifuctional Janus Materials for Rapid One-Way Water Transportation and Fog Collection

A novel micronano wire channels Janus membrane (WCJM) was fabricated by a convenience spraying method. We prepared a series of samples with different (super)hydrophobic energy barriers and studied the effects of WCJMs on one-way transportation and fog collection. The droplets can be one-way transported from the (super)hydrophobic side to the superhydrophilic side, forming a transport channel when they contact the superhydrophilic micronano wire under hydrostatic pressure. In the experiment, when droplets touch the exposed micronano wires, they will be rapidly absorbed by the superhydrophilic side. However, when the superhydrophobic energy barrier is thick and the superhydrophobic layer completely covers the micronano wires on the substrate surface, the droplets cannot achieve one way transport behavior. Besides, we observed three different fog collection modes. They have a significant difference in fog collection efficiency. In WCJM-3, the superhydrophobic side collects fog in a dropwise condensation mode, and then transported to the superhydrophilic side through the micronano wire channels for storage, with the highest fog collection efficiency (1.1 g/cm²·h). The results show that the WCJM surface not only makes full use of the difference in wettability and micronano wire structure to promote the droplets one-way transportation, but also improves the fog collection performance by accelerating surface regeneration, which has potential application value in fog collection, droplet treatment and related engineering.

A combined structural and wettability gradient surface for directional droplet transport and efficient fog collection

HYPOTHESIS

The droplet manipulation behavior is affected by chemical structural driving force (including the superposition of electric, magnetic, optical and thermal fields), which directly determine transportation velocity. A lot of research has focused on a single driving force that induces the directional transportation behavior, which affects its performance.

EXPERIMENTS

A simple method for preparing wettability gradient conical copper needles (WGCCN) combining structural gradient and chemical gradient was formulated. The effect of droplet volume and tilt angles on droplet transport velocity was systematically studied. The process of droplet transport was revealed through theoretical model and mechanical analysis. Finally, the application of WGCCN and its array model in fog collection were explored.

FINDINGS

A continuous chemical gradient in the conical structure gradient induces the droplet directional transportation, and the transportation velocity depends on the droplet volume. In addition, under the cooperation effect of multiple driving force, the droplet can still be transported in a directional orientation even if it is tilted at a certain angle. The simple droplet manipulation behavior portends that the droplets directional transport behavior can be applied in microfluidic manipulation by cooperation of effective multiple driving force with satisfactory results.

Fog Collection Based on Secondary Electrohydrodynamic-Induced Hybrid Structures with Anisotropic Hydrophilicity

The outcomes of the study of plant surfaces, such as rice leaves or bamboo leaves, have led to extensive efforts being devoted to fabricating anisotropic arrays of micro/nanoscale features for exploring anisotropic droplet spreading. Nonetheless, precise engineering of the density and continuity of three-phase contact lines for anisotropic wetting remains a significant challenge without resorting to chemical modifications and costly procedures. In this work, we investigated secondary electrohydrodynamic instability in polymer films for producing secondary nanosized patterns between the micrometer-sized grooves by controlling the timescale parameter, 1/τ_m (>10^-4 s^-1). We experimentally demonstrated facile morphological control of anisotropic wettability without the use of any chemical modifications. Thus, anisotropic hydrophilic surfaces fabricated by the secondary phase instability of polymer films are advantageous for both droplet condensation and removal, thereby outperforming the water collection efficiency of conventional (isotropic) hydrophilic surfaces in water harvesting applications (∼200 mg·cm^-2·h^-1) with excellent durability.

Study on the enhancing water collection efficiency of cactus- and beetle-like biomimetic structure using UV-induced controllable diffusion method and 3D printing technology

Collecting water from fog flow has emerged as a promising strategy for the relief of water shortage problems. Herein, using a UV-induced (ultraviolet light induced) controllable diffusion method combined with technology of three-dimensional (3D) printing, we fabricate biomimetic materials incorporating beetle-like hydrophobic-hydrophilic character and cactus-like cone arrays with various structure parameters, and then systematically study their fog-harvesting performance. The UV-induced controllable diffusion method can break away from the photomask to regulate the hybrid wettability. Moreover, employing 3D printing technology can flexibly control the structure parameters to improve the water collection efficiency. It is found that the water collection rate (WCR) can be optimized by controlling the hybrid wettability of the sample surface and cone distance and using substrates with printed holes, which lead to a 109% increase of WCR.

Bioinspired wettable-nonwettable micropatterns for emerging applications

Superhydrophilic and superhydrophobic surfaces are prevalent in nature and have received tremendous attention due to their importance in both fundamental research and practical applications. With the high interdisciplinary research and great development of microfabrication techniques, artificial wettable-nonwettable micropatterns inspired by the water-collection behavior of desert beetles have been successfully fabricated. A combination of the two extreme states of superhydrophilicity and superhydrophobicity on the same surface precisely, wettable-nonwettable micropatterns possess unique functionalities, such as controllable superwetting, anisotropic wetting, oriented adhesion, and other properties. In this review, we briefly describe the methods for fabricating wettable-nonwettable patterns, including self-assembly, electrodeposition, inkjet printing, and photolithography. We also highlight some of the emerging applications such as water collection, controllable bioadhesion, cell arrays, microreactors, printing techniques, and biosensors combined with various detection methods. Finally, the current challenges and prospects of this renascent and rapidly developing field are proposed and discussed.

Progress in Bioinspired Dry and Wet Gradient Materials from Design Principles to Engineering Applications

Nature does nothing in vain. Through millions of years of revolution, living organisms have evolved hierarchical and anisotropic structures to maximize their survival in complex and dynamic environments. Many of these structures are intrinsically heterogeneous and often with functional gradient distributions. Understanding the convergent and divergent gradient designs in the natural material systems may lead to a new paradigm shift in the development of next-generation high-performance bio-/nano-materials and devices that are critically needed in energy, environmental remediation, and biomedical fields. Herein, we review the basic design principles and highlight some of the prominent examples of gradient biological materials/structures discovered over the past few decades. Interestingly, despite the anisotropic features in one direction (i.e., in terms of gradient compositions and properties), these natural structures retain certain levels of symmetry, including point symmetry, axial symmetry, mirror symmetry, and 3D symmetry. We further demonstrate the state-of-the-art fabrication techniques and procedures in making the biomimetic counterparts. Some prototypes showcase optimized properties surpassing those seen in the biological model systems. Finally, we summarize the latest applications of these synthetic functional gradient materials and structures in robotics, biomedical, energy, and environmental fields, along with their future perspectives. This review may stimulate scientists, engineers, and inventors to explore this emerging and disruptive research methodology and endeavors.

Lattice Boltzmann Modeling of Condensation Heat Transfer on Downward-Facing Surfaces with Different Wettabilities

The model of vapor condensation heat transfer on downward-facing surfaces with different wettabilities is built by a two-dimensional (2D) lattice Boltzmann method. Dynamic evolution of condensate microdroplets on different wettability surfaces is simulated and the influence on heat transfer performance is analyzed. Moreover, the mechanism of a heterogeneous wettability surface enhancing condensation heat transfer is explored by investigating the condensate behaviors in the process of condensation. The numerical results indicate that as the contact angle of the homogeneous wettability surface increases, the initial nucleation time of the condensate is prolonged, while the departure time of the condensate is reduced significantly. The temperature adjacent to the gas-liquid interface, especially in the three-phase contact line region, is much higher than elsewhere due to the release of latent heat during condensation. Coalescence and detachment behaviors of condensate droplets cause the average heat flux to fluctuate locally with time. For the hybrid wettability surface, if the proportion of hydrophobic regions is small, the condensation heat transfer performance will be deteriorated. However, increasing the hydrophobic-hydrophilic ratio has a positive effect on enhancing heat transfer. It is found that a critical hydrophobic-hydrophilic ratio exists to optimize the heat transfer performance. For the gradient wettability surface, directional migration induced by capillary force facilitates the removal of condensate droplets, thereby enhancing the condensation heat transfer. Furthermore, a larger wetting gradient benefits to further improve the heat transfer performance. The results are valuable for optimally designing the heat transfer enhancement of vapor condensation on functionalized surfaces with heterogeneous wettability.

One-step process for dual-scale ratchets with enhanced mobility of Leidenfrost droplets

HYPOTHESIS

Droplet depositing onto hot surfaces above the so-called Leidenfrost temperature will float on a cushion of its own vapor. The vapor flow below the drop could be rectified by asymmetric surface textures, resulting the self-propelled droplet motion. Asymmetric structures like ratchets are used to rectify Leidenfrost droplet movement. Hence, it is possible to enhance the droplet mobility using surfaces with combined asymmetric macro/micro-structures.

EXPERIMENTS

Continuous scale-like microcraters stacked end-to-end were fabricated on steel surfaces by wire electrical discharge machining (WEDM). The crater orientation always vectored towards the machining direction (MD), which oriented the droplet motion. Further, by integrating micro and macro-ratchets, dual-scale ratchets were constructed by one-step process using WEDM. The travelling velocities of Leidenfrost droplets on dual-scale and traditional single-scale ratchets were compared and the enhanced mechanism on dual-scale ratchets was analyzed.

FINDINGS

One-step process was developed to fabricate transport platforms for Leidenfrost droplets, that continuous scale-like microcraters formed simultaneously on the macroratchets. The highest droplet travelling velocity was achieved compared to previous research. Further study shows that the enhanced drop mobility is attributed to the dual-scale roughness which endows a larger propelling force. This finding presents a high-efficiency method to fabricate transport platforms for Leidenfrost droplets.

A fog-collecting surface mimicking the Namib beetle: its water collection efficiency and influencing factors

Shortage of water resources and deterioration of water quality are becoming more and more serious today. Inspired by Namib Desert beetles, scientists designed biomimetic fog collection materials to obtain fresh water. The overview of this field is limited and mainly concerned with the preparation and application. In this paper, we focused on the water collection efficiency of surfaces inspired by beetles and discussed their influence on the water collection efficiency from three aspects: surface wettability, surface structure and surface pattern distribution.

Micro-/Nanostructured Interface for Liquid Manipulation and Its Applications

Understanding the relationship between liquid manipulation and micro-/nanostructured interfaces has gained much attention due to the wide potential applications in many fields, such as chemical and biomedical assays, environmental protection, industry, and even daily life. Much work has been done to construct various materials with interfacial liquid manipulation abilities, leading to a range of interesting applications. Herein, different fabrication methods from the top-down approach to the bottom-up approach and subsequent surface modifications of micro-/nanostructured interfaces are first introduced. Then, interactions between the surface and liquid, including liquid wetting, liquid transportation, and a number of corresponding models, together with the definition of hydrophilic/hydrophobic, oleophilic/olephobic, the definition and mechanism of superwetting, including superhydrophobicity, superhydrophilicity, and superoleophobicity, are presented. The micro-/nanostructured interface, with major applications in self-cleaning, antifogging, anti-icing, anticorrosion, drag-reduction, oil-water separation, water collection, droplet (micro)array, and surface-directed liquid transport, is summarized, and the mechanisms underlying each application are discussed. Finally, the remaining challenges and future perspectives in this area are included.

Hybrid Hydrophilic-Hydrophobic CuO@TiO2-Coated Copper Mesh for Efficient Water Harvesting

Fresh water scarcity has been a worldwide problem to be solved urgently. Inspired by the outstanding hydrophobic-hydrophilic patterns on the back of Namib desert beetles, hierarchical CuO@TiO₂-coated surface with alternating hydrophilic-hydrophobic chemistry patterns were fabricated utilizing the photocatalysis of titanium dioxide through ultraviolet irradiation. The results indicated that the as-prepared hybrid dual-coated copper mesh enhanced the fog-collection efficiency compared with the uniformly superhydrophobic or superhydrophilic surface. This enhancement can be regulated by controlling the deposition cycle times of TiO₂ multilayers on CuO and UV irradiation time. The best water harvesting behavior was determined at the deposition cycle times of 10 times and UV irradiation time of 4 h. This work findings offer new insights into the fabrication of hybrid hydrophilic-hydrophobic surfaces for highly efficient water harvesting.

Biomimetic fog collection and its influencing factors

This review starts with the main process of fog collection and then analyzes the influencing factors that affect the efficiency of fog collection.

Beetle-like droplet-jumping superamphiphobic coatings for enhancing fog collection of sheet arrays

Fog collection from atmosphere is an effective way to solve the water resource crisis in arid or semi-arid areas. Inspired by the bumpy surface of the desert beetle, this work provides a beetle-like superamphiphobic coating by adding silicon carbide particles to nano-SiO2 superamphiphobic coating in proportion, which shows excellent superamphiphobic performance, high nucleation rate, efficient drop removal efficiency and recommendable fog collection effect. In this work, drop removal is facilitated by the collisions of water droplets between the array sheets, and when the as-prepared samples are placed parallel to each other and with a space of ∼2 mm, the jumping drop collisions between two sample surfaces could promote the departure of droplets, and the water collection rate of the collision surface increased by ∼217% compared to that of the non-collision surface, which provides a new idea to promote water droplet removal. This work findings are instrumental in water collection and have wide application prospects in desalination, heat transfer, anti-fogging and other fields.

Large-scale efficient water harvesting using bioinspired micro-patterned copper oxide nanoneedle surfaces and guided droplet transport

As the Earth's atmosphere contains an abundant amount of water as vapors, a device which can capture a fraction of this water could be a cost-effective and practical way of solving the water crisis. There are many biological surfaces found in nature which display unique wettability due to the presence of hierarchical micro-nanostructures and play a major role in water deposition. Inspired by these biological microstructures, we present a large scale, facile and cost-effective method to fabricate water-harvesting functional surfaces consisting of high-density copper oxide nanoneedles. A controlled chemical oxidation approach on copper surfaces was employed to fabricate nanoneedles with controlled morphology, assisted by bisulfate ion adsorption on the surface. The fabricated surfaces with nanoneedles displayed high wettability and excellent fog harvesting capability. Furthermore, when the fabricated nanoneedles were subjected to hydrophobic coating, these were able to rapidly generate and shed coalesced droplets leading to further increase in fog harvesting efficiency. Overall, ∼99% and ∼150% increase in fog harvesting efficiency was achieved with non-coated and hydrophobic layer coated copper oxide nanoneedle surfaces respectively when compared to the control surfaces. As the transport of the harvested water is very important in any fog collection system, hydrophilic channels inspired by leaf veins were made on the surfaces via a milling technique which allowed an effective and sustainable way to transport the captured water and further enhanced the water collection efficiency by ∼9%. The system presented in this study can provide valuable insights towards the design and fabrication of fog harvesting systems, adaptable to arid or semi-arid environmental conditions.

Beetle-Inspired Hierarchical Antibacterial Interface for Reliable Fog Harvesting

The microdroplets in fog flow have been considered as an important resource for supplying fresh drinking water. Most of the reported works of fog collection focus on the water-collecting ability rather than the environmental reliability of selected materials. In this work, a beetle-inspired hierarchical fog-collecting interface based on the antibacterial needle-array (ABN) and hydrophilic/hydrophobic cooperative structure is displayed. The hydrophilic ABN is coated with zwitterionic carboxybetaine (CB) brushes that endow the fog collector with a long-term cleaning in harsh environment. Due to its strong affinity to water molecules, the tilted needles with a CB coating can facilitate the capture of fog and the rapid delivery of condensed water driven by gravity. After being transported to the connected hydrophobic sheet, the collected droplets can be rapidly detached and stored in the container, achieving a high fog-harvesting rate. Furthermore, CB-patterned channels are integrated on the hydrophobic sheet for the pathway-controlled water delivery. The CB coating is able to efficiently resist bacterial adhesion and contamination during fog harvesting, protecting the device from microbiological corrosion. The current design provides a promising method to incorporate antibacterial ability into fog collectors, which offer great opportunity to develop water harvesters for real-world applications.