Three-Dimensionally Structured Flexible Fog Harvesting Surfaces Inspired by Namib Desert Beetles

Droplet Morphology-Based Wettability Tuning and Design of Fog Harvesting Mesh to Minimize Mesh-Clogging

Fog harvesting relies on intercepting atmospheric or industrial fog by placing a porous obstacle, for example, a mesh and collecting the deposited water. In the face of global water scarcity, such fog harvesting has emerged as a viable alternative source of potable water. Typical fog harvesting meshes suffer from poor collection efficiency due to aerodynamic bypassing of the oncoming fog stream and poor collection of the deposited water from the mesh. One pestering challenge in this context is the frequent clogging up of mesh pores by the deposited fog water, which not only yields low drainage efficiency but also generates high aerodynamic resistance to the oncoming fog stream, thereby negatively impacting the fog collection efficiency. Minimizing the clogging is possible by rendering the mesh fibers superhydrophobic, but that entails other detrimental effects like premature dripping and flow-induced re-entrainment of water droplets into the fog stream from the mesh fiber. Herein, we improvise on traditional interweaved metal mesh designs by defining critical parameters, viz., mesh pitch, shade coefficient, and fiber wettability, and deducing their optimal values from numerically and experimentally observed morphology of collected fog water droplets under various operating scenarios. We extend our investigations over a varying range of mesh-wettability, including superhydrophilic and hydrophobic fibers, and go on to find optimal shade coefficients which would theoretically render clog-proof fog harvesting meshes. The aerodynamic, deposition, and overall collection efficiencies are characterized. Hydrophobic meshes with square pores, having fiber diameters smaller than the capillary length scale of water, and an optimal shade coefficient are found to be the most effective design of such clog-proof meshes.

Aluminum-Based Heterogeneous Surface for Efficient Solar Desalination and Fog Harvesting Processed by a Picosecond Laser

Solar desalination and fog harvesting are two common ways to obtain fresh water, and both are promising methods to solve the water shortage problem. However, through either the fabrication of interfacial evaporators for solar desalination or the preparation of superwetting surfaces for fog harvesting, current methods suffer from long preparation times, high costs, and low efficiency. Herein, we report an efficient and simple method to process heterogeneous surfaces (HSs) on aluminum (Al) by picosecond laser processing combined with chemical treatment used for fog harvesting and seawater desalination. The as-prepared HS simultaneously consists of regular periodic stripe structures with superhydrophilicity and superhydrophobicity. The spacing of the superhydrophilic and superhydrophobic regions can be adjusted through the processing path. This surface has a 44% improvement in fog harvesting efficiency compared to a pristine Al sheet, which is 0.53 kg·m-2·h-1. Furthermore, it shows a high evaporation rate of 2.35 kg·m-2·h-1 under one sun irradiation with an energy efficiency of 52.39%. Such functional surfaces can be applied to obtain fresh water resources in both coastal regions and arid areas, where water mist is relatively abundant, providing reference and guidance for fresh water collection, and being a promising way to solve the water shortage problem.

3D-printing-assisted fabrication of hierarchically structured biomimetic surfaces with dual-wettability for water harvesting

Freshwater acquisition methods under various environments are required because water scarcity has intensified worldwide. Furthermore, as water is an essential resource for humans, a freshwater acquisition method that can be utilized even under harsh conditions, such as waterless and polluted water environments, is highly required. In this study, a three-dimensional (3D) printing-assisted hierarchically structured surface with dual-wettability (i.e., surface with both hydrophobic and hydrophilic region) for fog harvesting was developed by mimicking the biological features (i.e., cactus spines and elytra of Namib Desert beetles) that have effective characteristics for fog harvesting. The cactus-shaped surface exhibited self-transportation ability of water droplet, derived from the Laplace pressure gradient. Additionally, microgrooved patterns of the cactus spines were implemented using the staircase effect of 3D printing. Moreover, a partial metal deposition method using wax-based masking was introduced to realize the dual wettability of the elytra of the Namib Desert beetle. Consequently, the proposed surface exhibited the best performance (average weight of 7.85 g for 10 min) for fog harvesting, which was enhanced by the synergetic effect between the Laplace pressure gradient and surface energy gradient. These results support a novel freshwater production system that can be utilized even in harsh conditions, such as waterless and polluted water environments.

Acoustic Tweezer-Modulated Biomimetic Patterned Particle-Polymer Composite for Water Vapor Harvesting

With the recent threat of climate change and global warming, ensuring access to safe drinking water is a great challenge in many areas worldwide. Designing functional materials for capturing water from natural resources like fog and mist has become one of the key research areas to maximize the production of clean water. From this aspect, nature is a great source for designing bioinspired functional materials as some of the plant leaves and animal exoskeletons can harness water and then store it to save themselves from arid, xeric conditions. Inspired by the Stenocara beetle, we have designed a composite surface structure with periodic islands made of aluminum microparticles surrounded by poly(dimethylenesiloxane) (PDMS). An acoustic tweezer-based method was used to fabricate the bioinspired composite structures, where surface acoustic waves at specific frequencies and amplitudes are applied to align the microparticles as islands in the polymer matrix. An oxygen plasma etching step was applied to expose the microparticles on the PDMS surface. The average water harvesting efficiencies for structures made with 120 and 80 kHz acoustic frequencies and 1 hour etching time were found to be 9.41 and 8.84 g cm^-2 h^-1, respectively. The acoustically patterned biomimetic composite surface showed higher water harvesting efficiency compared with completely hydrophobic PDMS and hydrophilic aluminum surfaces, demonstrating the advantages of the bioinspired composite material design and acoustic-assisted manufacturing technique. The biomimetic fog water harvesting material is a promising avenue to fulfill the demand for a cost-effective, sustainable, and energy-efficient solution to safe drinking water.

Spherical Micro/Nano Hierarchical Structures for Energy and Water Harvesting Devices

Three-dimensional (3D) hierarchical structures have been explored for various applications owing to the synergistic effects of micro- and nanostructures. However, the development of spherical micro/nano hierarchical structures (S-HSs), which can be used as energy/water harvesting systems and sensing devices, remains challenging owing to the trade-off between structural complexity and fabrication difficulty. This paper presents a new strategy for facile, scalable S-HS fabrication using a thermal expansion of microspheres and nanopatterned structures. When a specific temperature is applied to a composite film of microspheres and elastomers with nanopatterned surfaces, microspheres are expanded and 3D spherical microstructures are generated. Various nanopatterns and densities of spherical microstructures can thereby be quantitatively controlled. The fabricated S-HSs have been used in renewable electrical energy harvesting and sustainable water management applications. Compared to a triboelectric nanogenerator (TENG) with bare film, the S-HS-based TENG exhibited 4.48 times higher triboelectric performance with high mechanical durability. Furthermore, an S-HS is used as a water harvesting device to capture water in a fog environment. The water collection rate is dramatically enhanced by the increased surface area and locally concentrated vapor diffusion flux due to the spherical microstructures.

Multi-Scale Structure and Directional Hydrophobicity of Titanium Alloy Surface Using Electrical Discharge

Titanium alloys with special macro-micro composite structures of directional hydrophobicity are difficult to prepare due to poor thermal conductivity and good corrosion resistance, inhibiting the wide engineering applications for aerospace, marine engineering, and biomedicine. To prepare macro-micro composite structures on the surface of titanium alloys and achieve directional hydrophobicity, the sub-millimeter structures with an edge width of 150 μm, a groove width of 250 μm, and a depth of 250 μm were fabricated on the titanium alloy by wire electrical discharge machining (WEDM) technology, and high voltage-induced weak electric arc machining (HV-μEAM) was used to fabricate micro-scale feature size micro-structures on the processed macro-structure edges. The influence of process parameters on the morphology of microstructures was studied experimentally. The smooth surface of the titanium alloy is isotropically hydrophilic, and its contact angle is 68°. After processing the macrostructure on the titanium alloy surface, it shows directional hydrophobicity after being modified by low surface energy materials. The macro-micro composite structure formed by HV-μEAM realizes a directional hydrophobic surface with contact angles (CA) of 140° (parallel direction) and 130° (perpendicular direction), respectively. This surface has been modified with low surface energy to achieve contact angles of 154° and 143°. The results of the abrasion resistance test show that under the load of 100 g, it retains directional hydrophobicity at a friction distance of 700 mm with 600# sandpaper. The existence of the sub-millimeter macrostructure is the reason for the directionality of surface hydrophobicity. The microstructure can realize the transformation of the titanium alloy surface from hydrophilic to hydrophobic. Under the combined effects of the macro and micro composite structure, the surface of the titanium alloy shows obvious directional hydrophobicity.

Pattern transfer of large-scale thin membranes with controllable self-delamination interface for integrated functional systems

Direct transfer of pre-patterned device-grade nano-to-microscale materials highly benefits many existing and potential, high performance, heterogeneously integrated functional systems over conventional lithography-based microfabrication. We present, in combined theory and experiment, a self-delamination-driven pattern transfer of a single crystalline silicon thin membrane via well-controlled interfacial design in liquid media. This pattern transfer allows the usage of an intermediate or mediator substrate where both front and back sides of a thin membrane are capable of being integrated with standard lithographical processing, thereby achieving deterministic assembly of the thin membrane into a multi-functional system. Implementations of these capabilities are demonstrated in broad variety of applications ranging from electronics to microelectromechanical systems, wetting and filtration, and metamaterials.

Bioinspired Integrative Surface with Hierarchical Texture and Wettable Gradient-Driven Water Collection

At present, collecting water directly from the atmosphere has become an effective means to solve the growing shortage of fresh water. Inspired by the structures of trichomes (hairs) of Sarracenia to capture fog and transport water, a series of different high-low rib-like hierarchical texture surfaces were prepared based on the laser method. These surfaces have gradient superwetting and adhesion because of the differences in subsequent preparation methods. In addition, this work discusses the effect of the above performance differences on the efficiency of fog collection and the surface condensation characteristics during fog collection. The results show that the surface of the laser-prepared sample with the mixing unit combination has more efficient fog collection efficiency and droplet removal rate. After 30 min, the amount of drip measured in the atmospheric environment is 8.4 times that of the polished surface. This indicates that the multihierarchical textured surface and superhydrophobicity are essential for improving the droplet removal rate and coagulation efficiency.

Bio-Inspired Design of Bi/Tridirectionally Anisotropic Sliding Superhydrophobic Titanium Alloy Surfaces

Many biological surfaces with the multi-scale microstructure show obvious anisotropic wetting characteristics, which have many potential applications in microfluidic systems, biomedicine, and biological excitation systems. However, it is still a challenge to accurately prepare a metal microstructured surface with multidirectional anisotropy using a simple but effective method. In this paper, inspired by the microstructures of rice leaves and butterfly wings, wire electrical discharge machining was used to build dual-level (submillimeter/micrometer) periodic groove structures on the surface of titanium alloy, and then a nanometer structure was obtained after alkali-hydrothermal reaction, forming a three-level (submillimeter/micrometer/nanometer) structure. The surface shows the obvious difference of bidirectional superhydrophobic and tridirectional anisotropic sliding after modification, and the special wettability is easily adjusted by changing the spacing and angle of the inclined groove. In addition, the results indicate that the ability of water droplets to spread along parallel and perpendicular directions on the submillimeter groove structure and the different resistances generated by the inclined groove surface are the main reasons for the multi-anisotropic wettability. The research gives insights into the potential applications of metal materials with multidirectional anisotropic wetting properties.

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.

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.

Development of Micropatterns on Curved Surfaces Using Two-Step Ultrasonic Forming

Nanoimprint lithography (NIL) is a micro/nanoscale patterning technology on thermoplastic polymer films, and has been widely used to fabricate functional micro/nanoscale patterns. NIL was also used to develop micro/nanoscale patterns on curved surfaces by employing flexible polymer stamps or micropatterned metal molds with macroscopic curvatures. In this study, two-step ultrasonic forming was used to develop micropatterns on a curved surface out of a flat metal stamp, by connecting ultrasonic imprinting and stretching processes. Ultrasonic imprinting was used to replicate functional micropatterns on a flat polymer film, using a flat ultrasonic horn and micropatterned metal stamps with prism and dot micropatterns. An ultrasonic stretching process was then used to form a curvature on the patterned film using a curved ultrasonic horn and a soft mold insert, to avoid damage to the pre-developed micropatterns. The ultrasonic horn was designed to have three different tip radii, and the resulting forming depth and curvature formation were investigated experimentally. As a result, three different curved surfaces containing two different micropatterns were obtained. The developed curved films containing micropatterns were then evaluated optically, and showed different optical diffusion and illumination characteristics according to the film curvature and micropattern type. These results indicate that the proposed technology can extend the functionality of conventional micropatterned products by imposing appropriate curvatures.