Fog Collection on Polyethylene Terephthalate (PET) Fibers: Influence of Cross Section and Surface Structure

Effect of Droplet-Removal Processes on Fog-Harvesting Performance on Wettability-Controlled Wire Array with Staggered Arrangement

Development of freshwater resources is vital to overcoming severe worldwide water scarcity. Fog harvesting has attracted attention as a candidate technology that can be used to obtain fresh water from a stream of foggy air without energy input. Drainage of captured droplets from fog harvesters is necessary to maintain the permeability of harp-shaped harvesters. In the present study, we investigated the effect of the droplet-removal process on the amount of water harvested using a harvester constructed by wettability-controlled wires with an alternating and staggered arrangement. Droplet transfer from hydrophobic to hydrophilic wires, located upstream and downstream of the fog flow, respectively, was observed with a fog velocity greater than 1.5 m/s. The proportion of harvesting resulting from droplet transfer exceeded 30% of the total, and it reflected more than 20% increase of the harvesting performance compared with that of a harvester with wires of the same wettability: this value varied with the adhesive property of the wires and fog velocity. Scaled-up and multilayered harvesters were developed to enhance harvesting performance. We demonstrated certain enhancements under multilayered conditions and obtained 15.99 g/30 min as the maximum harvested amount, which corresponds to 13.3% of the liquid contained in the fog stream and is enhanced by 10% compared with that without droplet transfer.

Geometry for low-inertia aerosol capture: Lessons from fog-basking beetles

Water in the form of windborne fog droplets supports life in many coastal arid regions, where natural selection has driven nontrivial physical adaptation toward its separation and collection. For two species of Namib desert beetle whose body geometry makes for a poor filter, subtle modifications in shape and texture have been previously associated with improved performance by facilitating water drainage from its collecting surface. However, little is known about the relevance of these modifications to the flow physics that underlies droplets' impaction in the first place. We find, through coupled experiments and simulations, that such alterations can produce large relative gains in water collection by encouraging droplets to "slip" toward targets at the millimetric scale, and by disrupting boundary and lubrication layer effects at the microscopic scale. Our results offer a lesson in biological fog collection and design principles for controlling particle separation beyond the specific case of fog-basking beetles.

Characterization of leaf trichomes and their influence on surface wettability of Salsola ferganica, an annual halophyte in the desert

Many organisms use functional surfaces to collect water from the atmosphere. Salsola ferganica Drob. is one of the most abundant plants in desert regions and thrives in extreme environments with multiple but limited water resources, including dew and fog; however, its mechanisms of water harvesting remain unclear. We investigated trichome structural characteristics and their influence on the surface wettability of S. ferganica leaves using a variety of approaches (scanning electron microscopy, optical microscopy, immunolabelling staining, X-ray diffractometry, and infrared spectroscopy). Microstructural observations revealed that the trichomes of S. ferganica presented a curved upper part, the 'spindle node'-like structure in the middle, and the micro-grooves structure in between; such unique structures may aid in capturing moisture from the air. The physicochemical characteristics of the trichome surface, including hydrophobic functional groups, hydrophilic pectins, and low crystallinity, may enhance the adhesion of water drops to trichomes. Furthermore, we discovered that the piliferous S. ferganica leaves were more effective in retaining water than the glabrous S. aralocaspica leaves, and the dense trichome layer exhibited a significantly unwettable surface (high contact angle with droplets), whereas the individual trichomes retained water effectively (more so under drought conditions). The combination of these two properties is consistent with the 'rose petal effect', which describes rough surfaces that are hydrophobic but exhibit high adhesion with water. These factors suggest that the evolutionary optimisation of water acquisition by coupling relevant microstructures with the physicochemical properties of trichomes enables S. ferganica to survive harsh conditions in the seedling stage.

Mangifera indica Leaf (MIL) as a Novel Material in Atmospheric Water Collection

Here, Mangifera indica leaves (MILs) have been used to collect atmospheric water for the first time. This novel material has been viewed by mankind as environmental waste and is mostly discarded or incinerated, causing environmental pollution. By turning waste into wealth, MILs have proven resourceful and can help ameliorate the water crisis, especially in tropical countries. The unprecedented water collection result is enough to describe MILs as an ideal material for atmospheric water collection when compared to other natural plants. Both the physical and chemical surface morphologies were extensively characterized. This comparative study shows that MIL surface droplet termination and hydrophilic nature differ from those of other materials, with the apex playing a key role in the roll-off of the droplet. The surface wettability and its interaction with the droplet are of keen interest in this study.

Fog collection behavior of bionic surface and large fog collector: A review

Water shortages are currently becoming more and more serious due to complicated factors such as the development of the economy, environmental pollution, and climate deterioration. And it is the best solution to the problems faced by people in today's world to investigate the bionic structure of nature and explore effective methods for fog collection. Herein, we've illustrated the bionic structures of the Namib desert beetle, cactus spines, and spider silk, and we imitate and further modify the respective bionic structures, as well as construct multifunctional bionic structures to improve fog collection. In addition, we also expound the fog collection behavior of a large fog collector, and an excellent fog capture effect was achieved through studying the mesh structure, the surface modification of the mesh, and the construction of the fog collector. The advantages and limitations of fog collection by a harp fog collector were also explored. We hope that through this review, relevant researchers can have a deeper understanding of this field and thus promote the development of fog collection.

Highly Efficient Multiscale Fog Collector Inspired by Sarracenia Trichome Hierarchical Structure

Fog harvesting through bionic strategies to solve water shortage has drawn considerable attention. Recently, an ultrafast fog harvesting and transport mode was identified in Sarracenia trichome, which is mainly attributed to its superslippery capillary force induced by its unique hierarchical microchannel. However, the underlying effect of hierarchical microchannel-induced ultrafast transport on fog harvesting and the multiscale structural coupling effect on highly efficient fog harvesting are still great challenges. Herein, a bionic Sarracenia trichome (BST) with an on-demand regular hierarchical microchannel is designed using a one-step thermoplastic stretching approach on a glass fiber bundle. The BST is engineered to harbor major channels confined by an inner gear pattern along with junior microchannels that are automatically assembled by the glass fiber monofilaments. The BST shows enhanced capillary condensation and fog harvesting performance, in part due to its coupling effect with a Janus membrane (JM). Hence, a highly efficient multiscale fog collector is developed, in which a gradient high-pressure field is purposely formed to improve by threefold fog harvesting performance compared with a single-scale structure. This easy manufacturing and low-cost fog collector may represent a useful tool for harvesting fog water for production and living and pave the way for further investigations.

Biomimicking spider webs for effective fog water harvesting with electrospun polymer fibers

Fog is an underestimated source of water, especially in regions where conventional methods of water harvesting are impossible, ineffective, or challenging for low-cost water resources. Interestingly, many novel methods and developments for effective water harvesting are inspired by nature. Therefore, in this review, we focused on one of the most researched and developing forms of electrospun polymer fibers, which successfully imitate many fascinating natural materials for instance spider webs. We showed how fiber morphology and wetting properties can increase the fog collection rate, and also observed the influence of fog water collection parameters on testing their efficiency. This review summarizes the current state of the art on water collection by fibrous meshes and offers suggestions for the testing of new designs under laboratory conditions by classifying the parameters already reported in experimental set-ups. This is extremely important, as fog collection under laboratory conditions is the first step toward creating a new water harvesting technology. This review summarizes all the approaches taken so far to develop the most effective water collection systems based on electrospun polymer fibers.

Aerodynamics-assisted, efficient and scalable kirigami fog collectors

To address the global water shortage crisis, one of the promising solutions is to collect freshwater from the environmental resources such as fog. However, the efficiency of conventional fog collectors remains low due to the viscous drag of fog-laden wind deflected around the collecting surface. Here, we show that the three-dimensional and centimetric kirigami structures can control the wind flow, forming quasi-stable counter-rotating vortices. The vortices regulate the trajectories of incoming fog clusters and eject extensive droplets to the substrate. As the characteristic structural length is increased to the size of vortices, we greatly reduce the dependence of fog collection on the structural delicacy. Together with gravity-directed gathering by the folds, the kirigami fog collector yields a collection efficiency of 16.1% at a low wind speed of 0.8 m/s and is robust against surface characteristics. The collection efficiency is maintained even on a 1 m² collector in an outdoor setting.

Water shortage not only occurs in arid regions, but also in humid area with little precipitation, despite abundant fog. Authors develop robust and scalable 3D centimetric kirigami structures to control wind flow and regulate the trajectories of incoming fog, yielding high fog collection efficiency.

The importance of nanofiber hydrophobicity for effective fog water collection

To increase fog collection efficiency in a fiber system, controlled wetting properties are desirable. In this work, hydrophobic (PA11) and hydrophilic (PA6) polyamides were tested to verify the surface wetting effect on fog water collection rate. Highly porous fiber meshes were obtained from both polymer solutions. Randomly oriented fibers with average diameter of approximately 150 nm were observed with a scanning electron microscope (SEM). Despite the similar geometry and zeta potential of PA6 and PA11 meshes, it was shown that the hydrophobic PA11 nanofibers are more effective at water collection than hydrophilic PA6. These results indicate that wetting properties of electrospun nanofiber mesh have a significant effect on the process of draining from the mesh, as discussed in this paper. The results obtained are crucial for designing more efficient fog water collectors that include nanofibers in their construction.

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.

Fiber-Based Composite Meshes with Controlled Mechanical and Wetting Properties for Water Harvesting

Water is the basis of life in the world. Unfortunately, resources are shrinking at an alarming rate. The lack of access to water is still the biggest problem in the modern world. The key to solving it is to find new unconventional ways to obtain water from alternative sources. Fog collectors are becoming an increasingly important way of water harvesting as there are places in the world where fog is the only source of water. Our aim is to apply electrospun fiber technology, due to its high surface area, to increase fog collection efficiency. Therefore, composites consisting of hydrophobic and hydrophilic fibers were successfully fabricated using a two-nozzle electrospinning setup. This design enables the realization of optimal meshes for harvesting water from fog. In our studies we focused on combining hydrophobic polystyrene (PS) and hydrophilic polyamide 6 (PA6), surface properties in the produced meshes, without any chemical modifications, on the basis of new hierarchical composites for collecting water. This combination of hydrophobic and hydrophilic materials causes water to condense on the hydrophobic microfibers and to run down on the hydrophilic nanofibers. By adjusting the fraction of PA6 nanofibers, we were able to tune the mechanical properties of PS meshes and importantly increase the efficiency in collecting water. We combined a few characterization methods together with novel image processing protocols for the analysis of fiber fractions in the constructed meshes. The obtained results show a new single-step method to produce meshes with enhanced mechanical properties and water collecting abilities that can be applied in existing fog water collectors. This is a new promising design for fog collectors with nano- and macrofibers which are able to efficiently harvest water, showing great application in comparison to commercially available standard meshes.

Hydrophilic nanofibers in fog collectors for increased water harvesting efficiency

The water crisis is a big social problem and one of the solutions are the Fog Water Collectors (FWCs) that are placed in areas, where the use of conventional methods to collect water is impossible or inadequate. The most common fog collecting medium in FWC is Raschel mesh, which in our study is modified with electrospun polyamide 6 (PA6) nanofibers. The hydrophilic PA6 nanofibers were directly deposited on Raschel meshes to create the hierarchical structure that increases the effective surface area which enhances the ability to catch water droplets from fog. The meshes and the wetting behavior were investigated using a scanning electron microscope (SEM) and environmental SEM (ESEM). We performed the fog water collection experiments on various configurations of Raschel meshes with hydrophilic PA6 nanofibers. The addition of hydrophilic nanofibers allowed us to obtain 3 times higher water collection rate of collecting water from fog. Within this study, we show the innovative and straightforward way to modify the existing technology that improves water collection by changing the mechanisms of droplet formation on the mesh.

Modification of Raschel meshes used for fog water collectors with PA6 nanofibers allow to obtain 300% higher water collection rate in collecting water from fog.

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.

Biological and Engineered Topological Droplet Rectifiers

The power of the directional and spontaneous transport of liquid droplets is revealed through ubiquitous biological processes and numerous practical applications, where droplets are rectified to achieve preferential functions. Despite extensive progress, the fundamental understanding and the ability to exploit new strategies to rectify droplet transport remain elusive. Here, the latest progress in the fundamental understanding as well as the development of engineered droplet rectifiers that impart superior performance in a wide variety of working conditions, ranging from low temperature, ambient temperature, to high temperature, is discussed. For the first time, a phase diagram is formulated that naturally connects the droplet dynamics, including droplet formation modes, length scales, and phase states, with environmental conditions. Parallel approaches are then taken to discuss the basic physical mechanisms underlying biological droplet rectifiers, and a variety of strategies and manufacturing routes for the development of robust artificial droplet rectifiers. Finally, perspectives on how to create novel man-made rectifiers with functionalities beyond natural counterparts are presented.

Passive water collection with the integument: mechanisms and their biomimetic potential

Several mechanisms of water acquisition have evolved in animals living in arid habitats to cope with limited water supply. They enable access to water sources such as rain, dew, thermally facilitated condensation on the skin, fog, or moisture from a damp substrate. This Review describes how a significant number of animals - in excess of 39 species from 24 genera - have acquired the ability to passively collect water with their integument. This ability results from chemical and structural properties of the integument, which, in each species, facilitate one or more of six basic mechanisms: increased surface wettability, increased spreading area, transport of water over relatively large distances, accumulation and storage of collected water, condensation, and utilization of gravity. Details are described for each basic mechanism. The potential for bio-inspired improvement of technical applications has been demonstrated in many cases, in particular for several wetting phenomena, fog collection and passive, directional transport of liquids. Also considered here are potential applications in the fields of water supply, lubrication, heat exchangers, microfluidics and hygiene products. These present opportunities for innovations, not only in product functionality, but also for fabrication processes, where resources and environmental impact can be reduced.

Fog Harvesting with Harps

Fog harvesting is a useful technique for obtaining fresh water in arid climates. The wire meshes currently utilized for fog harvesting suffer from dual constraints: coarse meshes cannot efficiently capture microscopic fog droplets, whereas fine meshes suffer from clogging issues. Here, we design and fabricate fog harvesters comprising an array of vertical wires, which we call "fog harps". Under controlled laboratory conditions, the fog-harvesting rates for fog harps with three different wire diameters were compared to conventional meshes of equivalent dimensions. As expected for the mesh structures, the mid-sized wires exhibited the largest fog collection rate, with a drop-off in performance for the fine or coarse meshes. In contrast, the fog-harvesting rate continually increased with decreasing wire diameter for the fog harps due to efficient droplet shedding that prevented clogging. This resulted in a 3-fold enhancement in the fog-harvesting rate for the harp design compared to an equivalent mesh.