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

Improved Fog Collection on a Hybrid Surface with Acylated Cellulose Coating

Fog collection serves as an efficient method to alleviate water scarcity in foggy, water-stressed regions. Recent research has focused on constructing a hybrid surface to enhance fog collection efficiency, with one approach being the prevention of liquid film formation at hydrophilic sites. Inspired by the desert beetle, a coating (10-MCC) made by partially acylating microcrystalline cellulose (MCC) exhibits hydrophilic sites alongside a hydrophobic skeleton enabling rapid droplet capture despite its overall hydrophobicity. The captured droplets quickly coalesce into a large droplet driven by the wetting gradient created by the hydrophobic backbone and hydrophilic sites. To achieve greater fog collection efficiency, a hydrophobic-superhydrophobic hybrid surface is formed by combining a coating of 10-MCC with a superhydrophobic surface. The construction of superhydrophobic surfaces typically involves creating a rough surface with a distinctive structure produced by the anodization technique and modifying it with stearic acid. The superhydrophobic surface exhibits excellent corrosion resistance and mechanical stability. Moreover, the hybrid surface shows high efficiency in fog collection, with a tested maximum efficiency of approximately 1.5092 g/cm2/h, 1.77 times that of the original Al sheets. The results demonstrate a remarkable enhancement in fog collection capacity. Furthermore, this work serves as an inspiration for the low-cost and innovative design of engineered surfaces for efficient fog collection.

Capillary Filling in Open Rectangular Microchannels with a Spatially Varying Contact Angle

Capillary flow in microchannels is important for many technologies, such as microfluidic devices, heat exchangers, and fabrication of printed electronics. Due to a readily accessible interior, open rectangular microchannels are particularly attractive for these applications. Here, we develop modifications of the Lucas-Washburn model to explore how a spatially varying contact angle influences capillary flow in open rectangular microchannels. Four cases are considered: (i) different uniform contact angles on channel sidewalls and channel bottom, (ii) contact angles varying along the channel cross section, (iii) contact angle varying monotonically along the channel length, and (iv) contact angle varying periodically along the channel length. For case (i), it is found that the maximum filling velocity is more sensitive to changes in the wall contact angle. For case (ii), the contact angles can be averaged to transform the problem into that of case (i). For case (iii), the time evolution of the meniscus position no longer follows the simple square-root law at short times. Finally, for case (iv), the problem is well described by using a uniform contact angle that is a suitable average. These results provide insights into how to design contact-angle variations to control capillary filling and into the influence of naturally occurring contact-angle variations on capillary flow.

Laser-Textured Hybrid Brass Pattern Array Surface for High-Efficiency Fog Collection

Fog collection holds promise for addressing water shortage. However, the conventional fabrication of fog collection devices, normally chemical methods, suffers many challenges, such as complicated preparation and environmental issues. Herein, we proposed a green fabrication strategy to construct superhydrophobic/hydrophilic surfaces on the brass substrate via the combination of laser fabrication and heat treatment. The wettability of brass is directly dictated by the laser process parameters. The different superhydrophobic/hydrophilic hybrid pattern surface with a rectangular/triangular array was designed for an optimal fog collection performance. The maximum water collection efficiency of the prepared surface is measured up to 427.36 mg h-1 cm-2, which is 97% higher than that of the control sample. Furthermore, the surface can be folded into different forms to realize a flexible collector. We envision that our work provides a green fabrication strategy to construct a superwetting surface for highly efficient fog collection.

Ultrafast Self-Healing Superhydrophobic Surface for Underwater Drag Reduction

The self-healing superhydrophobic surfaces have attracted great interest owing to restoring superhydrophobicity without preparation crafts. However, the self-healing superhydrophobic surface still faces the dilemma of long repairing time. Especially in aqueous environments, superhydrophobic surfaces are highly susceptible to contamination and damage. In the current study, a superhydrophobic surface with ultrafast repairability was developed, which apply for drag reduction in aqueous medium. The prepared superhydrophobic surface can recover superhydrophobicity in only 30 s after severe physical and chemical damage. In addition, this research pioneered the combination of superhydrophobicity and porous structures for underwater drag reduction. The study of drag reduction confirms that the superhydrophobic surface can reduce the frictional drag by about 43% in the water. However, the drag reduction rate of the superhydrophobic surface with the porous structure can be improved to 76% due to increased stability of the air layer. More importantly, the porous structure with the average pore size of 50 μm has the most excellent stability through further experiments on the underwater air layer. This is attributed to the proper size of the pore to effectively balance the capillary force and resist wetting in the marginal region. This study will bring inspiration for the large-scale application of superhydrophobic surfaces and long-term drag reduction.

Cactus-Inspired Janus Membrane with a Conical Array of Wettability Gradient for Efficient Fog Collection

Fog collection plays an important role in alleviating the global water shortage. Despite great progress in creating bionic surfaces to collect fog, water droplets still could adhere to the microscale hydrophilic region and reach the thermodynamic stable state before falling, which delays the transport of water and hinders the continuous fog collection. Inspired by lotus leaves and cactuses, we designed a Janus membrane that functions to both collect fog from the air and transport it to a certain region. The Janus membrane with opposite wettability contains conical microcolumns with a wettability gradient and hydrophilic copper mesh surface. The apexes of conical microcolumns are superhydrophobic and the rest are hydrophobic. The fog droplets were deposited, coalesced, and directionally transported to the bottom of the conical microcolumns. Then, the droplets unidirectionally passed through the membrane and flowed into the water film on the surface of the copper mesh. The asymmetric structural and wettability merits endow the Janus membrane with an improved fog collection of ∼7.05 g/cm²/h. The study is valuable for designing and developing fluid control equipment in fog collection, liquid manipulation, and microfluidics.

Artificial Leaf for Switchable Droplet Manipulation

Droplet manipulation plays an important role in scientific research, daily life, and practical production such as biological and chemical analysis. Inspired by the structure and function of three typical leaf veins, the bionic texture was replicated by the template method, and the artificial leaf was selectively treated by nanoparticles to obtain a quasi-three-dimensional hybrid superhydrophobic-hydrophilic surface. When the droplet touches the surface of the leaf, it will be attracted to the bottom of the main vein from different directions even in horizontal conditions due to the Laplace pressure gradient and energy gradient. The simulation analysis demonstrates that the reason for directional transportation is the energy gradient of the droplets on the different levels of veins, including the thin veins, lateral veins, and main vein. Meanwhile, the experimental result of water collection also showed an outstanding directional transportation effect and excellent water collection efficiency. In addition, when the sample is tilted upside down, the droplet will flow back to the main vein along the lateral vein and then flow down the main vein, showing a good droplet pumping effect. Therefore, the directional and polydirectional transportation of droplets on the same sample is successfully realized, and the conversion between executing single and multiple tasks simultaneously can be realized only by upright and inverted samples. This work provided a new strategy for directional and polydirectional water manipulation, water collection, directional drainage, and microfluidic devices.