An integrative bioinspired venation network with ultra-contrasting wettability for large-scale strongly self-driven and efficient water collection

A UV-Resistant Heterogeneous Wettability-Patterned Surface

Preparing UV-resistant heterogeneous wettability patterns is critical for the practical application of surfaces with heterogeneous wettability. However, combining UV-resistant superhydrophobic and superhydrophilic materials on heterogeneous surfaces is challenging. Inspired by the structure of cell membranes, a UV-resistant heterogeneous wettability-patterned surface (UPS) is designed via laser ablation of the coating of multilayer structures. UV-resistant superhydrophobic silica patterns can be created in situ on surfaces covered with superhydrophilic TiO2 nanoparticles. The UV resistance time of the UPS with a TiO2 -based surface is more than two orders of magnitude higher than that obtained with other surface molecular modification methods that require a mask. The cell-membrane-like structure of the UPS regulates the migration of internal siloxane chain segments in the hydrophilic and hydrophobic regions of the surface. The UPS enables efficient patterning of functional materials under UV irradiation, controlling the wetting behavior of liquids in open-air systems. Furthermore, its heterogeneous wettability remains stable even after 50 h of intense UV irradiation (365 nm, 500 mW cm-2 ). These UV-resistant heterogeneous wettability patterned surfaces will likely be applied in microfluidics, cell culture, energy conversion, and water collection in the future.

Determining the Role of Oxygen in Obtaining Long-Term Stable Superhydrophilic Surfaces on Metals Treated with a Femtosecond Laser

Laser processing is a simple way to obtain hydrophobic or even superhydrophobic properties of metal surfaces. However, preparation of superhydrophilic surfaces by this method, the properties of which do not change under the influence of various factors, remains a difficult task. In this work, we show that with increasing laser power, the degree of oxidation of the treated metal surface also increases. As a result, highly oxidized samples showed highly stable superhydrophilic properties. A Janus membrane fabricated from a stainless steel mesh with asymmetric hydrophilic-hydrophobic wettability demonstrated stable water diode properties. In addition, it was found that during the examination of sample surfaces by Raman spectroscopy, organic compounds adsorbed on the hydrophobic surface were decomposed by the laser of the spectrometer, which imposes limitations on the laser power when using this method in characterizing hydrophobic surfaces of metals fabricated by laser processing.

Nature-Inspired Surface Engineering for Efficient Atmospheric Water Harvesting

Atmospheric water harvesting is a sustainable solution to global water shortage, which requires high efficiency, high durability, low cost, and environmentally friendly water collectors. In this paper, we report a novel water collector design based on a nature-inspired hybrid superhydrophilic/superhydrophobic aluminum surface. The surface is fabricated by combining laser and chemical treatments. We achieve a 163° contrast in contact angles between the superhydrophilic pattern and the superhydrophobic background. Such a unique superhydrophilic/superhydrophobic combination presents a self-pumped mechanism, providing the hybrid collector with highly efficient water harvesting performance. Based on simulations and experimental measurements, the water harvesting rate of the repeating units of the pattern was optimized, and the corresponding hybrid collector achieves a water harvesting rate of 0.85 kg m-2 h-1. Additionally, our hybrid collector also exhibits good stability, flexibility, as well as thermal conductivity and hence shows great potential for practical application.

Comparing Water Transport Properties of Janus Membranes Fabricated from Copper Mesh and Foam Using a Femtosecond Laser

One of the important aspects of manipulating and controlling liquid transport is the design of membrane surfaces. Janus membranes with opposite wettability characteristics can be manufactured for efficient directional water transfer. In this work, two types of materials were used to fabricate membranes with an asymmetric wettability behavior: copper foam and copper mesh. One side of the membranes was treated by scanning with a femtosecond laser beam, as a result of which it was converted to a superhydrophilic state, while the untreated side remained hydrophobic. Both membranes demonstrated excellent properties of a water diode through which water droplets could easily pass from the hydrophobic side to the hydrophilic side, but not vice versa. This behavior was achieved by finding the optimal laser scanning speed. This type of Janus membrane has found applications in collecting water droplets from fog; therefore, the samples obtained were also tested in terms of harvesting micro-droplets. The Janus mesh-based structure has demonstrated a higher water collection efficiency (3.9 g/cm² h) compared to the foam-based membrane (2.5 g/cm² h). Since the fog-water conversion efficiency decreased over time (to 0.5 g/cm² h in 2 weeks) due to the absorbance of organic pollutants, a coating of titanium oxide was applied to the laser-treated side of the Janus membranes. As a result, the effective function of the systems became distinctly long-lasting and was well maintained for at least 60 days. Moreover, the fabricated systems were protected from further degradation by simply placing them under sunlight for several hours. Our results prove to be useful in developing asymmetric hydrophobic-superhydrophilic membranes, which have potential applications in high-precision drop control and in harvesting water from arid environments.

Fabrication of Superhydrophobic Gully-Structured Surfaces by Femtosecond Laser and Imprinting for High-Efficiency Self-Cleaning Rain Collection

Freshwater is considered an essential need for humanity. Moreover, it is important to collect and make full use of rainwater. This work utilizes a femtosecond laser to fabricate micro-nanostructures on aluminum alloy substrates as molds. Then, the structures are imprinted on cheap and wildly used polypropylene (PP) materials. The just-imprinted PP surfaces with instinctive surface energy and replicated micro-nanostructures have an excellent superhydrophobic property with contact angles greater than 160° and anisotropic sliding angles smaller than 5° in parallel directions and smaller than 10° in the vertical directions. A small-scale rain collection device formed by a combination of the superhydrophobic PP surfaces is used to investigate the effects of the rain collection efficiency and total surface area relating to manufacturing cost. The rain collection device formed by the imprinted PP surfaces has high rain collection efficiency in terms of the volume of the collected water per square centimeter. For the light rain, the rain collection efficiency can reach an approximated maximum of 90%, more than 100% efficiency improvement of the device formed by flat PP surfaces in some cases. Therefore, the rain collection device is helpful in collecting water from rains in arid areas.

Role of the microridges on cactus spines

Cactus spines have inspired a wide range of micro- and nano-structures that cause droplets to move spontaneously and directionally. The conical shape and the surface wettability gradient are two typical characteristics in such systems. The cross section of the existing conical fibers is usually assumed to be an ideal circle. In fact, microridges are observed on the spine surface of the cactus, and the function is not yet fully understood. The present work thus focuses on how microridges affect droplet self-transport. Structures mimicking microridges are first investigated by constructing pyramidal cross sections with concave or convex lateral faces. The dissipative particle dynamics method is then employed to numerically investigate and theoretically analyze the dynamic behaviors of droplets on these conical fibers with different cross sections. The results show that the microridges reduce the base radius and the contact area of the droplet, thereby increasing the driving force and reducing the friction force. Moreover, by mimicking the microridges structure, we propose a conical fiber with a triple concave cross section, which increases the droplet velocity and the distance traveled over the traditional circular fiber. This work reveals the role of the microridges in the droplet self-transport, which opens up new prospects for the manufacture of fiber systems for microfluidics and liquid manipulation.

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.

Directional anchoring patterned liquid-infused superamphiphobic surfaces for high-throughput droplet manipulation

High-throughput experiments involving isolated droplets based on patterned superwettable surfaces are important for various applications related to biology, chemistry, and medicine, and they have attracted a large amount of interest. This paper provides a directional anchoring liquid-infused superamphiphobic surface (DAS), via combining concepts based on the droplet-anchoring behavior of beetle backs with patterned wettability, the directional adhesion of butterfly wings, and the slippery liquid-infused surfaces (SLISs) of pitcher plants. Regularly arranged ">"-shaped SLIS patterns were created on a superamphiphobic (SAM) background through ultrafast-laser-based technology. Improved directional anchoring abilities with a sliding angle difference of 77° were achieved; this is the largest sliding angle difference in a one-dimensional direction achieved using an artificial surface, to the best of the authors' knowledge. Thanks to the directional anchoring abilities, the DAS coupled droplet 'anchoring' and 'releasing' abilities. Furthermore, a high-throughput droplet manipulation device was designed, on which a micro-droplet array with a large number of droplets can be 'captured', 'transferred', or 'released' in a single step. With the addition of lubricant, the DAS can work continuously for even more than 30 cycles without cross-contamination between different droplets. The DAS also shows good stability under an ambient atmosphere and can maintain its functionality when manipulating corrosive droplets. The DAS and corresponding high-throughput droplet manipulation method are excellent candidates for practical applications.

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.

Laser Direct Structuring of Bioinspired Spine with Backward Microbarbs and Hierarchical Microchannels for Ultrafast Water Transport and Efficient Fog Harvesting

Achieving effective dropwise capture and ultrafast water transport is essential for fog harvesting. In nature, cactus uses the conical spine with microbarbs to effectively capture fog, while Sarracenia utilizes the trichome with hierarchical microchannels to quickly transport water. Herein, we combined their advantages to present a novel configuration, a spine with barbs and hierarchical channels (SBHC), for simultaneous ultrafast water transport and high-efficient fog harvesting. This bioinspired SBHC exhibited the fastest water transport ability and the highest fog harvesting efficiency in comparison with the spine with hierarchical channels (SHCs), the spine with barbs and grooves (SBG), and the spine with barbs (SB). Based on the fundamental SBHC unit, we further designed and fabricated a two-dimensional (2D) spider-web-like fog collector and a three-dimensional (3D) cactus-like fog collector using direct laser structuring and origami techniques. The 2D spider-web and 3D cactus-like fog collectors showed high-efficient fog collection capacity. We envision that this fundamental understanding and rational design strategy can be applied in fog harvesting, heat transfer, liquid manipulation, and microfluidics.

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.

Ultrafast Laser Enabling Hierarchical Structures for Versatile Superhydrophobicity with Enhanced Cassie-Baxter Stability and Durability

The controllable and facile fabrication of surface micro/nanostructures with the required dimensions and morphologies is the key to achieving surface superhydrophobicity. With the advantages of being a noncontact, maskless, programmable, and one-step process, ultrafast laser irradiation is a very flexible and adaptive technique for fabricating various microscale, nanoscale, and micro/nanomultiscale surface structures on diverse solids, thus realizing superhydrophobicity on their surfaces. In this feature article, a comprehensive review of our recent research advances on versatile superhydrophobic surfaces enabled by ultrafast lasers is presented from the perspectives of materials, methodologies, and functionalization. The realization of superhydrophobicity and even superamphiphobicity on varied solid surfaces through ultrafast laser treatment and the underlying mechanisms for the wettability transition of ultrafast-laser-processed surfaces from superhydrophilicity to superhydrophobicity will be discussed. For the sake of practical applications, the ultrafast-laser-based strategies for the large-scale and cost-effective fabrication of superhydrophobic surface micro/nanostructures will be introduced. A special focus will be devoted to the enhancement of structural durability and the Cassie-Baxter stability of ultrafast-laser-enabled superhydrophobic surfaces. Beyond that, the achievement of integrated surface functions including remarkable wetting functions such as the directional collection of water droplets and superhydrophobic surfaces simultaneously with unique optical properties will also be presented.

Femtosecond Laser Fabricated Elastomeric Superhydrophobic Surface with Stretching-Enhanced Water Repellency

Highly stretchable and robust superhydrophobic surfaces have attracted tremendous interest due to their broad application prospects. In this work, silicone elastomers were chosen to fabricate superhydrophobic surfaces with femtosecond laser texturing method, and high stretchability and tunable adhesion of the superhydrophobic surfaces were demonstrated successfully. To our best knowledge, it is the first time flexible superhydrophobic surfaces with a bearable strain up to 400% are fabricated by simple laser ablation. The test also shows that the strain brings no decline of water repellency but an enhancement to the superhydrophobic surfaces. In addition, a stretching-induced transition from "petal" state to "lotus" state of the laser-textured surface was also demonstrated by non-loss transportation of liquid droplets. Our results manifest that femtosecond laser ablating silicone elastomer could be a promising way for fabricating superhydrophobic surface with distinct merits of high stretchability, tunable adhesion, robustness, and non-fluorination, which is potentially useful for microfluidics, biomedicine, and liquid repellent skin.