Water Collection Efficiency Research Articles

Efficient Aerial Water Harvesting with Self-Sensing Dynamic Janus Crystals.

Water scarcity is one of the most pressing issues of contemporary societal development that requires innovative technologies where the material not only harvests water but also plays an active role in the process. Here, we demonstrate a highly efficient optical self-sensing approach to humidity capture from the air, where both humidity-harvesting and water-transduction functionalities are imparted on slender organic crystals by partial silanization via layer-by-layer hybridization. We report that due to the integration of the harvesting of aerial moisture and the collection of the condensed water, the ensuing Janus-type crystals capture humidity with the highest-to-date water collection efficiency of 15.96 ± 0.63 g cm-2 h-1. The water-collecting elements are also capable of delivering the water by reversible and periodic elastic deformation, and their high optical transparency allows real-time monitoring of the periodic fog collection process by deformational modulation of passively or actively transduced light that outcouples at the crystal-droplet interface. The results could inspire sophisticated approaches to humidity harvesting where optically transparent crystals combine fog capture with self-sensing capabilities for continuous and optimized operation to maximize the cost-gain balance of aerial fog capture.

Efficient Fog-Harvesting Origami Fan.

Fog harvesting represents a promising strategy to address the global freshwater shortage. To enhance the water collection efficiency, diverse geometric structures that can effectively drive water droplet movement are essential. Inspired by the Livistona chinensis leaf, which naturally facilitates directional droplet motion through its unique gradually varying V-groove structure, we have developed a novel origami fan structure for fog harvesting through theoretical analysis. A key feature is that we can modulate the speed of droplet transport by adjusting the opening angle of the V-shaped grooves positioned at the outer circumference. Interestingly, the water collection efficiency exhibits a linear correlation with the opening angle. The highest efficiency of the origami fan can reach 5.75 times that of the control group calculated by the projected area and 3.76 times that of the control group calculated by the real area, showcasing its significant potential for enhancing water collection from fog. The simulations demonstrate that the hollow structure enhances the condensation rate of droplets, the geometric gradient of the gradual-variation V-groove drives the condensed droplets to move rapidly on the surface, and the Janus membrane permits the aggregated droplets to transit to the fan's rear side. The synergistic action of these three components ensures a clean surface for the subsequent water-collecting cycle, contributing to the high fog-harvesting efficiency. Given its simple fabrication and superior water transfer efficiency, the origami fan holds substantial promise for widespread application in the field of droplet manipulation.

Siphon-based scalable and salt-resistant multistage thermal desalination system

Recent advances in thermal localization-based passive solar desalination provide a great opportunity for the economical generation of freshwater, particularly in regions with insufficient energy and water infrastructure. Yet, the capillary-assisted passive desalination systems with a high-water productivity flux (measured in Lm−2 h−1) suffer from the issue of performance degradation due to salt accumulation and the inability to be scaled up. In this work, we propose siphon-based supply of saline water over the evaporator that enables scale up of the desalination system to a size significantly higher than the capillary rise height of the hydrophilic evaporator while preventing salt accumulation on the evaporator. The composite siphon comprises insulating fabric wick and a metallic grooved surface for localizing heat to evaporate a thin layer of saline liquid over the evaporator. We perform heat and mass transfer analysis to show that the thermal-to-vapor efficiency depends on the inlet mass flow rate and air gap between the evaporator and the condenser. We propose a methodology to passively control the mass flow rate to maximize the thermal to vapor efficiency at different input heat flux and initial concentration of the brine. A grooved condenser avoids mixing of brine and the freshwater, even at air gaps as low as 2 mm. A siphon-assisted 10-stage desalination system with a footprint area 15 cm × 15 cm and an air gap of 2 mm is shown to have a high water productivity flux of ~5.73 Lm−2 h−1 from 3.5 wt% saline water at an applied heat flux 1000 W/m2, which increases to a record high distillate flux of ~6.23 Lm−2 h−1 and thermal to water collection efficiency of ~423 % for a 15-stage system. The ability of the desalination system to maintain a high-water productivity flux even when the evaporator area is increased by 4 times demonstrates its scalability to achieve higher desalinated water productivity rate.

Water collection through a directional leaf vein pattern by fast laser marker ablation of stainless-steel

Inspired by the natural water harvesting mechanisms of desert beetles, cactus thorns, and leaf veins, we designed a heterogeneous wettability surface with superhydrophilic pattern integrating leaf vein as the directional water transport main channel, attached capillary triangles as auxiliary channel plus a deep rough desorption channel on an overall superhydrophobic surface for an efficient water collection. A superhydrophilic surface was initially fabricated on the stainless steel disc by laser marker ablation allowing 1 μL droplet to spread completely to 0° within 0.12 seconds, followed by fluorine-containing coating transforming superhydrophilic surface to superhydrophobic one. Directional water transport patterns were then etched on the superhydrophobic surfaces by the secondary laser marker. The surface energy gradient and Laplace pressure induced by the pattern facilitated directional fast transport and efficient desorption of droplets, thus improving water collection efficiency. The enhancement mechanism of the water harvesting behavior for such surfaces was analyzed, with one focus on enhancing collection in hydrophobic regions with capillaries to reduce bouncing off loss and the other on improving balanced cycling of the collection process. At a fog flow rate of 1500ml/h and 20cm away from the fog outlet, the directional leaf vein-patterned 19.625cm2 sized surface demonstrated a fog water collection rate (WCR) of 5.6 Kg·m-2·h-1 and first drop collection at the 49th s, an impressively short time rarely reported. Compared to the superhydrophobic, superhydrophilic samples, and the reference, WCR increased by 180%, 62%, and 59%, respectively, and the first droplet collection time decreased by 73%, 46%, and 62%, respectively. This efficient water collection method has huge potential in arid regions.

Leaf Vein-Inspired Superhydrophilic Microchannels for Sustainable Fog Collection.

Fog collection is a promising solution for mitigating the urgent water shortage around the world. Despite the delicate design of various bionic fog harvesting surfaces with prowess to enable fast fog capture and programmed water transport, achieving sustainable and efficient fog collection by regulating the macroscale surface refreshment efficacy remains rarely concerned yet is effective. Here, we proposed a bioinspired structural design to achieve significant improvement on the surface refreshment efficacy to 46.47%, nearly 5 times larger than that of conventional design. Specifically, we constructed superhydrophilic vein-like microchannels on a superhydrophobic brass surface by using laser texture technology and hydrothermal treatment. Our microchannel design acts as a "highway" for synergically transporting and converging the collected fog droplets, as well as rapidly refreshing large surface area for the subsequent fog collection, reminiscent of the leaf veins responsible for the persistent mass transport between plant tissues. The practical implementation also convinced our design of a maximum water collection efficiency of up to 506.67 mg cm-2 h-1 and a long-term performance stability within a 10 h test. Our design is generic to most of the fog harvesting materials, showing great application potential for efficient atmospheric fog collection.

Engineering of Hierarchical Porous Composite Film (ZIF-8@PVDF/PMMA) with Superhydrophobicity for Highly Efficient Water Harvesting.

Atmospheric water harvesting has attracted much attention because of its potential to escalate the global freshwater shortage. However, the water collection efficiency is hindered by the trade-off between fast droplet nucleating and rapid droplet dripping due to the opposite requirements in the chemistry and the morphology of surfaces. Herein, the hierarchical porous composite film (ZIF-8@PVDF/PMMA, HPCF) with superhydrophobicity is designed for highly efficient and stable water harvesting. It indicates that the HPCF film has a large water contact angle (WCA) of 155.50° and ultralow sliding angle (SA) of 2°, exhibiting the self-cleaning function. Significantly, it is demonstrated that the water collection efficiency of HPCF can achieve 1.13 g·cm-2·h-1, which is much higher than the value of the blank sample, as well as most of the reported values. It is attributed to the hierarchical porous structure with the ZIF-8 crystals enhancing the surface roughness and endowing the film with the hydrophilic/superhydrophobic regions. This design promotes an optimal balance between droplet nucleation and shedding, significantly enhancing the water harvesting efficiency. Consequently, this work introduces an effective approach for water collection materials suitable for fog/mist conditions and provides an effective solution for the foggy area with water scarcity, demonstrating significance for advancing research aimed at mitigating the worldwide water shortage.

Patterned Hybrid Surfaces for Efficient Dew Harvesting.

Dew harvesting, minimally influenced by climate and geographical locations, is an ideal method for addressing water shortage problems. Superhydrophilic surfaces, characterized by their highest affinity for water, are particularly attractive for this purpose as they can attract more water molecules via condensation. However, a significant challenge arises from the high surface capillary force that impedes water from sliding down and being effectively collected. The resulting water film on the superhydrophilic surface tends to stay around the edge of the water collection surface, leading to evaporation loss and reduced collection efficacy. To overcome this problem, triangular patterns with low surface adhesion to water were introduced at the edge of superhydrophilic surfaces. This modification, achieved through a wet chemical method and masked oxygen plasma treatment, has significantly improved the efficiency of water collection. Results indicate that the hybrid surface reduced the time for the first water droplet to slide down by half and increased water collection efficiency by 78% compared to uniform superhydrophilic surfaces and by 536% compared to uniform superhydrophobic surfaces under a relative humidity of 55% with a temperature difference of 15 °C. The underlying principles were elucidated through computational simulations, and the mechanisms driving the enhancement in collection efficiency were explained.

Triboelectrically Empowered Biomimetic Heterogeneous Wettability Surface for Efficient Fog Collection.

Biomimetic engineering surfaces featuring heterogeneous wettability are vital for atmospheric water harvesting applications. Existing research predominantly focuses on the coordinated regulation of surface wettability through structural and chemical modifications, often overlooking the prevalent triboelectric charge effect at the liquid-solid interface. In this work, we designed a heterogeneous wettability surface by strategic masking and activated its latent triboelectric charge using triboelectric brushes, thereby enhancing the removal and renewal of surface droplets. By examining the dynamic evolution of droplets, the mechanism of triboelectric enhancement in the water collection efficiency is elucidated. Leveraging this inherent triboelectric charge interaction, fog collection capacity can be augmented by 29% by activating the system for 5 s every 60 s. Consequently, the advancement of triboelectric charge-enhanced fog collection technology holds both theoretical and practical significance for overcoming the limitations of traditional surface wettability regulation.

Fog water harvesting with cylindrical brush

In this study, an efficient fog collector—type 1 and 2 cylindrical brush—has been introduced and its harvesting performance compared to other conventional collectors under field and laboratory conditions. Under field conditions, type 1 cylindrical brush with Raschel and aluminum mesh was installed in the inlet of the humidity front of the Caspian Sea to Ardabil plain in Abi-beyglu in the northwest of Iran. The results of the analysis of the collected water revealed that type 1 cylindrical brush collected 9.3% and aluminum mesh collected 4.3% more water than Raschel mesh. Under laboratory conditions, by applying different fog speeds, the performance of three conventional collectors and two proposed cylindrical brush were determined. The results indicated that type 2 cylindrical brush had the highest efficiency compared to others, and due to the lack of clogging, its efficiency did not diminish over time. Upon increasing the speed of the output fog, the amount of collected water and accordingly the efficiency of the collectors increased. Type 2 cylindrical brush showed the highest increase in efficiency at high fog speeds. Its maximum value reached about 50%, while the maximum efficiency of Raschel collector was equal to 28%. In general, it can be stated that cylindrical brush introduced in this study, in addition to the high performance of fog harvesting and the high efficiency of water collection, has a high resistance against strong winds. Therefore, it can be a suitable collector for water harvesting in foggy areas without any implementation problems.

Multi-scale modeling of fog harvesting using thin-fiber grids – Towards new design rubrics

Fog collectors with fibers thinner than 1 mm harvest fog droplets more effectively at a low airflow velocity around 1 m s-1 than thicker fibers. However, existing models and design rubrics are restricted to thick fibers at higher velocity. This paper reports a multi-scale numerical model for fog harvesting at airflow velocity of 1 m s-1 using thin-fiber grids with different fiber spacing and diameter. The numerical model can simulate fog harvesting at two extreme length scales that are comparable to collector scale at the large end and fiber scale at the small end. The results confirm two new and important effects of fiber grid geometries on water collection efficiency, which are not included in existing theories and design rubrics. First, dense thin-fiber grids negatively influence the collection efficiency because of wall effect caused by viscous boundary layers. Second, the sparse thin-fiber grids can benefit from isolated clogging waterdrops and maintain relatively high efficiency when clogging blocks multiple grid openings. The two identified effects are then included to develop a new performance map for fog collectors, thereby shaping new design rubrics for fog harvesting. This work extends the existing theories of fog harvesting and sheds light on the design of novel fog collectors.

Nanoscale Drag Reduction Effect Enables Efficient Fog Harvesting

AbstractSurfaces with macro‐scale hydrophilic/hydrophobic regions (conventional Janus mode) are often considered as prime candidates for fog harvesting. However, the water collection efficiency of such surfaces frequently fails to meet the requirements of practical applications due to the occurrence of surface flooding caused by water droplet pinning. Here, an in situ molecular confined modification strategy is proposed to construct upgraded Janus mode for achieving nanoscale drag reduction effect, thereby improving the fog collection efficiency. Specifically, nanoscale hydrophilic sites are designed to effectively reduce the critical volume size required for water droplets to slide off, thus avoiding the surface flooding. Fibers with upgraded Janus mode (PAN‐TO) realize a water collection rate of 4035 mg cm−2 h−1, markedly exceeding that of most materials with conventional Janus mode. Moreover, the high interface bonding strength between hydrophilic sites and the material surface can withstand the impact of water and sand flushing, ensuring the long‐term usability of the material. The applications of PAN‐TO fibers in irrigation and dyeing are demonstrated. This work shows a nanoscale surface engineering proposal for building a next‐generation fibrous fog harvesting device.

Fog water collection of harp-like single smooth and texturized filaments under field conditions

Harp-like arrangements of filaments have been proposed as a more efficient alternative to cross-linked screens for fog water collection. We investigate the fog capture capabilities of both smooth and texturized single vertical nylon threads under field conditions. Redefining the Stokes number in terms of the wetted string and the pinning drop geometry allowed the average fog water collection efficiency to be described using a physically based droplet impaction model with acceptable fit (NSE = 0.614; RMSE = 0.093). In addition, rugosity of the filaments was found to affect the long-term fog water yield (up to 41 % lower), while parallel grooves along the filaments maximised the fog collection, with 51 % improvement, under real field conditions.

Janus membrane with heterogeneous wettability top surface for fog harvesting

To alleviate the global water scarcity crisis, fog harvesting has seen considerable development as a promising strategy. However, the current main challenge lies in achieving efficient fog deposition, timely transport of water droplets, and storage. Here, inspired by clusters of cactus trichomes and sponges, a Janus membrane with heterogeneous wettability top surface (JM-HWTS) is proposed. The JM-HWTS was obtained by spraying paraffin wax particles (hydrophobic) on the copper mesh top surface (superhydrophilic). The relationship between the loading amount of paraffin particles and the surface morphology, water permeability, and one-way penetration ability of the sample was clarified. Compared to fog collection surfaces with single hydrophilic or hydrophobic properties, the proposed JM-HWTS system significantly increased fog collection efficiency. For JM-HWTS, the fog will first deposit into water droplets on the hydrophobic front side, then grow up, and finally be sucked into the hydrophilic back side. This unique cyclic fog collection method is the key to efficient fog collection. The JM-HWTS exhibits excellent water-collection efficiency, enabling efficient capture of fog and rapid directional delivery of droplets. This work will significantly complement the fog collection and water droplet storage fields.

Three-dimensional flow around and through a porous screen

We investigate the three-dimensional (3-D) flow around and through a porous screen for various porosities at high Reynolds number $Re = {O}(10^4)$ . Historically, the study of this problem has been focused on two-dimensional cases and for screens spanning completely or partially a channel. Since many recent problems have involved a porous object in a 3-D free flow, we present a 3-D model initially based on Koo & James (J. Fluid Mech., vol. 60, 1973, pp. 513–538) and Steiros & Hultmark (J. Fluid Mech., vol. 853, 2018 pp. 1–11) for screens of arbitrary shapes. In addition, we include an empirical viscous correction factor accounting for viscous effects in the vicinity of the screen. We characterize experimentally the aerodynamic drag coefficient for a porous square screen composed of fibres, immersed in a laminar air flow with various solidities and different angles of attack. We test various fibre diameters to explore the effect of the space between the pores on the drag force. Using PIV and hot wire probe measurements, we visualize the flow around and through the screen, and in particular measure the proportion of fluid that is deviated around the screen. The predictions from the model for drag coefficient, flow velocities and streamlines are in good agreement with our experimental results. In particular, we show that local viscous effects are important: at the same solidity and with the same air flow, the drag coefficient and the flow deviations strongly depend on the Reynolds number based on the fibre diameter. The model, taking into account 3-D effects and the shape of the porous screen, and including an empirical viscous correction factor that is valid for fibrous screens may have many applications including the prediction of water collection efficiency for fog harvesters.

Novel mushroom-like cotton-based evaporator for simultaneous freshwater production and salt harvesting via edge-preferential crystallization

Solar-driven interfacial evaporation (SDIE) has emerged as a promising technology to alleviate the global freshwater scarcity. However, the simultaneous collection of water and salt as resources while maintaining stable evaporation rate without salt contamination at the evaporation interface remains a huge challenge. Herein, we report a novel mushroom-like cotton-based evaporator, inspired by the vertical stem and inclined caps of mushrooms, for simultaneous freshwater production and salt harvesting. By depositing polyaniline (PANI) and carbon nanotubes (CNTs) onto the cotton fabric (CF), an evaporation rate of 1.88 kg m−2h−1 and a high solar evaporation efficiency of 99.4 % can be achieved in 3.5 wt% NaCl solution under one solar irradiation. Furthermore, a significant water collection efficiency of 70.2 % and a high salt harvesting rate of 42.1 g m−2h−1 can be derived. Even exposed to high-concentration (15 wt%) brine, our design still effectively prevents salt contamination. The distinctive edge-preferential crystallization not only avoids the salt contamination but also promotes the salt harvesting, due to the capillary force, the criss-cross water channels from the nature of cotton cloth, as well as the gravity-assisted water transport of the “mushroom cap” during evaporation. In addition, the low thermal conductivity (0.163 W m−1 K−1) of the photothermal layer with the integration of PANI and CNTs, as well as the vertical water upward transport via the “mushroom stem” effectively reduce the heat loss. This research holds great potential for seawater desalination applications with the comprehensive utilization of seawater resources.

A new Janus mesh membrane with ultrafast directional water transportation and improved fog collection

Fog collection materials have been intensively investigated and can potentially be used to alleviate freshwater shortages. However, developing highly efficient fog collection materials remains challenging. Requisite improvements in fog collection technology are the ultrafast transportation of captured water and the effective regeneration of fresh regions on the surfaces of fog collection materials. Herein, a novel Janus mesh membrane (JMM) with an alternating hydrophobic–superhydrophilic-patterned hydrophobic surface and asymmetric wettability interface is prepared by a selective liquid-phase surface oxidation strategy. The unique structure and high-contrast wettability of the JMM enable ultrafast directional transportation of tiny water droplets (< 0.1 µL), thereby achieving an excellent water collection efficiency of 15.28 g h−1 cm−2, which is much higher than those of conventional Janus membranes. The JMM functioned as a versatile fog collector and maintained continuous fog collection regardless of the fog flow direction. This study offers a novel approach for preparing innovative fog collection materials and establishes a foundation for developing advanced fog collection systems suitable for diverse applications.

Inclined solar interface evaporation system with downward steam generation for efficient desalination and salt crystallization positioning

Solar-driven Interfacial Vapor Generation (SIVG) has been recognized as a potentially effective method of converting seawater into potable freshwater resources. In this work, we employed a straightforward approach to cultivate CuS and polypyrrole on copper foam as an solar absorber, while sterilized face towels served as water an evaporation interface and water transport sites. An inclined steam blocking evaporator with a salt-out part that attracts salt aggregation was ingeniously designed to improve the salt-resistant performance. Even in 15 wt% brine, the evaporation equipment maintained a stable and high evaporation rate, with an average evaporation rate of about 1.588 kg m−2 h−1 under 1 solar illumination. More particularly, in order to improve the water collection rate, a beveled condensers placed at the bottom of the steam blocking evaporator was customized, so as to shorten the steam condensation path and achieve internal water collection. Therefore, the water collection rate of the evaporator is as high as 2.238 kg m−2 h−1 and water collection efficiency is 85.90 %. Additionally, the evaporation device can effectively purify heavy metal ions and organic matter. This study can provide a new idea for the design and performance improvement of solar interfacial vapor generation and water collection device.

A conical shaped self-healing slippery film for enhanced fog harvesting in windy environment

Wind presents both advantages and disadvantages for water collection. It can bring an abundant fog resource to arid areas for water harvesting, however, some of the captured water droplets are inevitably blown away in a windy environment because of the easy-sliding property of the material surface. Herein, a conical-shaped, self-healing, liquid-infused film with tunable cone angles based on candle soot, thermoplastic poly (ethylene-co-vinyl acetate) (EVA), and hydrophobic silicone oil has been developed using a simple template method and traditional Chinese origami. Benefiting from the inward force induced by the conical structure, the condensed droplets prefer to gather toward the top of the conical structure in windy environments rather than splashing out, which usually occurs on a plane surface. This advantage can greatly ensure the high water collection efficiency of materials in windy regions. In addition, owing to the unique photothermal property and thermoplasticity of the designed conical structure, surface scratches during application can be quickly repaired under solar illumination without extra fossil energy consumption, and long-term stable application of the materials can be guaranteed. Therefore, this study finely synergized structure design and performance enhancement and established a new paradigm for effective and reliable fog harvesting applications.

Active Surface Area-Dependent Water Harvesting of Desert Beetle-Inspired Hybrid Wetting Surfaces.

The increasing frequency of water scarcity is an acute worldwide problem. Nature-inspired water harvesting from fog is an important method to obtain freshwater in arid areas. Existing literature reports varied and diversified results in water harvesting capacity by employing a biphilic surface with control over hydrophilic and hydrophobic patterns. In this study, we first demonstrate a facile and scalable method to fabricate a biphilic surface using a simple electroless etching and desilanization technique. Considering the nucleation, growth, and transport of condensate, biphilic surfaces with controlled active surface area of hydrophilic spots were given special attention. We studied the water collection performance of pattern shape with its associated active surface area and further evaluated the critical surface area beyond which the water collection efficiency decreases. A high water collection capacity of 2050 mg cm-2 h-1 was achieved, and the hydrophilic active area-engineered surface retained its efficiency even after 50 test cycles. We further demonstrate high collection efficiency with a square pattern compared to a triangular path-like-patterned surface. The observations and surface engineering strategies reported in this study can provide insights into efficient and sustainable water harvesting devices.

Research on New Whitening and Water-Saving Technology Based on Industrial Equipment

Energy conservation and consumption reduction have always been the goals pursued by the power industry. Based on these goals, this study explored a new type of whitening and water-saving technology for industrial equipment. Unlike existing direct heating and condensation heating technologies, the main innovation of this work lies in not changing the saturated wet flue gas before it is discharged into the atmospheric environment. A series of experiments were conducted on electrode plates with different wind speeds, supersaturation levels, and porosities using three principles, namely droplet electrostatic adsorption, ionic-wind-enhanced condensation, and droplet dipole deflection, through the construction of a parameter-adjustable and -controllable enthalpy and humidity chamber and a pilot development platform with a wind volume of 10,000 m3/h. In addition, the collection efficiency was calculated using thermodynamic laws. The results showed that, under the working conditions of white mist supersaturation of 5.77 g/kg, a hole opening rate of 70%, and a wind speed of 3 m/s, the water collection efficiency was the highest—close to 60%—verifying the feasibility of this technology. This technology not only eliminates white smoke but also saves water resources and has certain economic benefits, providing support for the development of industrial equipment for smoke removal in the future.