Water Collection Rate Research Articles

How Different Are Fog Collection and Dew Water Harvesting on Surfaces with Different Wetting Behaviors?

As the clean water shortage becomes a serious problem for mankind, atmospheric water harvesting has emerged as a viable solution. Two main approaches to collect water from the atmosphere exist: the first is to capture it from fog, whereas the second is through condensation of vapor on surfaces with a temperature below the dew point. The water collection mechanism in these two modes is completely different. In this work, we develop a deeper understanding of the effect of surface wettability on gravity-assisted atmospheric water harvesting and a comparative study of the two collection modes (fog and dew). First, we present theoretical estimates for the maximum water mass available in each mode and introduce an efficiency factor η which enables the direct comparison among surfaces in different setups and modes. Then we fabricate a series of micronanostructured surfaces with different surface wetting properties from hydrophilic to superhydrophobic. Our results demonstrate that drop mobility, derived from the surface superhydrophobic properties and micronanotopography, is the most important factor affecting fog collection: superhydrophobic surfaces show 40-65% higher fog collection rates compared to flat hydrophilic surfaces, with the more mobile among superhydrophobic surfaces (hysteresis 2°, and air-liquid fraction fA-L > 0.9) showing higher water collection. On the other hand, dew harvesting efficiency depends on the combination of drop mobility and nucleation rate, with superhydrophobic surfaces exhibiting 40% higher water collection rate compared to the flat hydrophilic or hydrophobic surfaces due to their low hysteresis as well as high surface area available for nucleation.

A Novel Design of Hybrid Hydrophilic-Superhydrophobic Surfaces for Fog Harvesting

The continuous growth in the human population and climate change exacerbates the problems related to water scarcity. Harvesting the atmospheric water can mitigate the water scarcity in many regions around the globe. Fog collection using hybrid hydrophilic-superhydrophobic surfaces has the capacity to achieve a higher water collection rate. In this paper, a new method and materials are introduced to create the hybrid surfaces. The method includes additive manufacturing- to make sheets with holes-, mixing and casting polymeric matrix composite, and a controlled spray coating mechanism. The materials comprised of hydrophobic coating on top of the acrylic printed sheet and hydrophilic composite. The ratios of the pitches to diameters of the hydrophilic regions varied during the experiments to obtain the best water generation. The water collection rate for the sample with diameters of 583 um and a pitch of 1600 um has achieved 57% more than the untreated hydrophilic sample. The contrast in wettability accomplished by this novel method has the potential to be implemented on a large scale for atmospheric water harvesting.

Wettability Contrast in the Hexagonally Patterned Gold Substrate of Distinct Morphologies for Enhanced Fog Harvesting.

Inspired by the Stenocara beetle's hydrophobic-hydrophilic surface, we fabricated hexagonally patterned hydrophobic-hydrophilic surfaces consisting of silicon and gold regions using colloidal lithography and selective surface functionalization. We investigated surface wettability for different patterns (hexagonally ordered nanotriangles and nanoholes) and the influence of surface functionalization (octadecanethiol and 16-mercaptohexadecanoic acid/octadecyltrichlorosilane (MHA/OTS)). The as-prepared patterned substrates exhibit hydrophilicity, which transforms to hydrophobicity after surface functionalization. The MHA/OTS functionalization results in maximum enhancement in the contact angle (114 ± 0.4°) with the least contact angle hysteresis (19 ± 2°). Fog harvesting studies show that the patterned substrate has a higher water collection rate, a factor of 1.32, than the nonpatterned substrates. A further enhancement in water collection (almost twice) is observed with selective functionalization. The patterned (nanohole) and functionalized (MHA/OTS) substrate facilitates rapid falling of droplets at a frequency of 20 mHz and an average droplet mass of 15 ± 2 mg/cm2. Furthermore, it yielded a maximum water collection rate of 1051 ± 132 mg cm-2 h-1. This work provides valuable insights into the influence of surface wettability and morphology for fog harvesting applications.

A high-performing single-stage invert-structured solar water purifier through enhanced absorption and condensation

<h2>Summary</h2> Interfacial solar vapor generation is an emerging technology for producing purified water with a minimized carbon footprint. While the evaporation rate has been significantly increased, the rate of water collection has been limited because of ineffective heat transfer during condensation and compromised light absorption in the conventionally designed solar still. In this work, an invert-structured, single-stage solar water purifier (ISWP) is elaborately designed and fabricated and consists of a top selective absorber and a honeycombed bottom water condenser based on hydrophobic nanostructured copper. This inverted structure not only avoids optical loss due to the vapor condensation but also realizes enhanced heat transfer and condensation. As a result, it can achieve a high water-collection rate of 1.063 kg m⁻² h⁻¹ and an overall efficiency of solar collected water of ∼70% for single-stage solar purification systems.

Surface Potential Driven Water Harvesting from Fog.

Access to clean water is a global challenge, and fog collectors are a promising solution. Polycarbonate (PC) fibers have been used in fog collectors but with limited efficiency. In this study, we show that controlling voltage polarity and humidity during the electrospinning of PC fibers improves their surface properties for water collection capability. We experimentally measured the effect of both the surface morphology and the chemistry of PC fiber on their surface potential and mechanical properties in relation to the water collection efficiency from fog. PC fibers produced at high humidity and with negative voltage polarity show a superior water collection rate combined with the highest tensile strength. We proved that electric potential on surface and morphology are crucial, as often designed by nature, for enhancing the water collection capabilities via the single-step production of fibers without any postprocessing needs.

Copper Oxide Microtufts on Natural Fractals for Efficient Water Harvesting.

Hierarchical surfaces that aid in the droplet nucleation, growth, and removal is highly desirable for fog and moisture harvesting applications. Taking inspiration from the unique architecture of leaf skeletons, we present a multiscale surface capable of rapidly nucleating, growing, and directional transport of the water droplets. Copper oxide microtufts were fabricated onto the Ficus religiosa leaf skeletons via electroplating and chemical oxidation techniques. The fabricated surfaces with microtufts had high wettability and very good fog harvesting ability. CuO surfaces tend to become hydrophobic over time because of the adsorption of the airborne species. The surfaces were efficient in fog harvesting even when the hydrophobic coating is present. The overall water collection efficiencies were determined, and the role of the microtufts, fractal structures, and the orientation of leaf veins was investigated. Compared to the planar control surfaces, the noncoated and hydrophobic layer-coated copper oxide microtufts on the leaf skeletons displayed a significant increase in the fog harvesting efficiency. For superhydrophilic skeleton surfaces, the water collection rate was also observed to slightly vary with the vein orientation. The CuO microtufts along with high surface area fractals allowed an effective and sustainable way to capture and transport water. The study is expected to provide valuable insights into the design and fabrication of sustainable and efficient fog harvesting systems.

Electrospun Janus fabrics with directional water transport property for efficient water collection

Collection of clean water from humid air has attracted immense attention in recent years. Janus fabrics (JFs) have been widely employed to harvest fog droplets. Herein, a double-layer hydrophilic/hydrophobic Janus fabric was prepared by depositing polystyrene (PS) layers of different thicknesses on hydrophilic cotton fabric via one-step electrospinning technology. The thickness of the PS layer and the distance from the fog outlet to the fabric surface affected the water collection rate (WCR). The water collection rate of the JFs initially increased and then decreased as the thickness of the PS layer was increased. Benefiting from the timely drainage and directional droplet delivery, the JFs showed a 147% enhancement in the WCR compared with hydrophilic cotton fabric. This work may provide insight into the design and fabrication of fog collection systems with high efficiency to alleviate the freshwater crisis.

Reed leaf-inspired anisotropic slippery lubricant-infused surface for water collection and bubble transportation

Anisotropic slippery lubricant-infused surfaces with favorable sliding behaviors are highly desirable for water collection and directional bubble transportation. Inspired by natural reed leaves with anisotropic wettability along two directions of groove geometry, a soft lithographic approach of replica molding was implemented to successfully replicate the surface microstructures of reed leaves using polydimethylsiloxane (PDMS). Subsequently, various lubricants were infused to fabricate anisotropic hydrophobic or hydrophilic slippery surfaces. The as-prepared anisotropic hydrophilic lubricant-infused surface (A-HH-LIS) exhibited remarkable droplet nucleation and removal during water collection. The results demonstrated that the surfaces maintained stable water collection rate even under high shear force rate up to 7000 rpm and exhibited favorable liquid repellency at lower or higher temperature. Furthermore, the directional transportations of water (in air) and air bubbles (underwater) on anisotropic hydrophobic lubricant-infused surface (A-HB-LIS) were achieved with outstanding performance. Moreover, the effects of tilted angle β (the angle between sample and plumb line) and rotational angle α (the angle between groove direction and horizontal line) on sliding behavior were explored. The reported anisotropic slippery lubricant-infused surfaces have great potential in water condensation, bubble transportation and marine anti-fouling.

Study on the enhancing water collection efficiency of cactus- and beetle-like biomimetic structure using UV-induced controllable diffusion method and 3D printing technology.

Collecting water from fog flow has emerged as a promising strategy for the relief of water shortage problems. Herein, using a UV-induced (ultraviolet light induced) controllable diffusion method combined with technology of three-dimensional (3D) printing, we fabricate biomimetic materials incorporating beetle-like hydrophobic–hydrophilic character and cactus-like cone arrays with various structure parameters, and then systematically study their fog-harvesting performance. The UV-induced controllable diffusion method can break away from the photomask to regulate the hybrid wettability. Moreover, employing 3D printing technology can flexibly control the structure parameters to improve the water collection efficiency. It is found that the water collection rate (WCR) can be optimized by controlling the hybrid wettability of the sample surface and cone distance and using substrates with printed holes, which lead to a 109% increase of WCR.

Highly salt-resistant and all-weather solar-driven interfacial evaporators with photothermal and electrothermal effects based on Janus graphene@silicone sponges

Abstract Solar-driven interfacial evaporators are very promising for obtaining clean water, but suffer from serious performance degradation due to salt-fouling, low evaporation rate under weak illumination and low clean water collection rate. Here, we report highly salt-resistant and all-weather evaporators with photothermal and electrothermal effects based on the Janus graphene@silicone sponges with opposing wettability. The evaporators achieve a remarkable high evaporation rate of 6.53 kg m−2 h−1 for 3.5 wt% NaCl solution under 1 sun illumination with a 5 V solar cell as compensation owing to their high solar absorption, low thermal conductivity, unique Janus structure and synergetic photothermal and electrothermal effects. Even in gloomy and dark environments, the evaporators could still generate vapor (1.51 kg m−2 h−1). Moreover, the evaporators feature long-term excellent salt-resistance, e.g., > 10 d continuous evaporation in 10 wt% NaCl solution without performance degradation and salt precipitation, because of ultrafast water supply and salt diffusion in the macroporous superhydrophilic shell. Furthermore, the evaporators show high clean water collection rates of 21.92 kg m−2 d−1 (1 sun-9 h/0 sun-15 h + 5 V, indoor) and 9.65 kg m−2 d−1 (natural sun light + 5 V, outdoor). This study offers a new approach for efficiently obtaining clean water via solar desalination.

Harvesting water from air using adsorption material – Prototype and experimental results

Drinking water resources have always been limited in the gulf region of the Middle East and other desert regions around the world. In attempt to provide viable supplement, a device that harvests clean drinking water from air is designed, built and tested. The operation of the device is based on harvesting water naturally from air using adsorption materials. The prototype of this device consists of sorbent (silica gel is used in this study) exposed to radiant flux, water sorbent unit, condenser and reflector. Experimental studies of production of fresh water from air in controlled indoor environment have been carried out using the prototype. Several experimental tests were conducted under the conditions of 22 °C ambient temperature, a range of relative humidity (RH) from 30 to 60%, a range of silica gel thickness from 25 to 35 mm, surface area to volume ratio from 0.29 to 0.4 and radiant heat flux range from 509 to 556 W/m2. The prototype was able to produce up to 159 g of water per 1 kg of silica gel in a 12 h cycle when exposed to 556 W/m2 radiant flux. In terms of per one day (24 h), the harvester can produce 800 mL of water with an overall efficiency of 50% for 25 mm silica layer thickness. Increasing the relative humidity speeds up the adsorption cycle and increases the water capture, release and collection rates. The system can be improved by adding multiple layers of sorbent stacked on top of each other and by using sorbents with improved adsorption and desorption properties.

All-Day Freshwater Harvesting through Combined Solar-Driven Interfacial Desalination and Passive Radiative Cooling.

Solar-driven interfacial evaporation has received increasing research attention for clean water generation, but freshwater harvesting often occurs only during daytime. Herein, we report a strategy for all-day freshwater harvesting through combining solar-driven interfacial desalination and dew water generation. Through spray-coating carbon nanotubes onto a flexible substrate that has high-infrared emittance in the atmosphere window, a dual functional film was prepared to efficiently absorb solar energy for desalination at daytime and passively cool down the surface temperature to collect dew water at night. By integrating the dual functional film within a three-stage membrane distillation device, the home-made system achieved a drinkable freshwater collection efficiency of 71.1% when desalinating seawater under 1 sun illumination and achieved a dew water collection rate of 0.1 L m-2 day-1 at night. The readily available low-cost raw materials, simple fabrication process of the dual functional films, and the resultant all-day freshwater collection systems make the combined solar-thermal interfacial desalination and dew water collection a promising solution to alleviate the freshwater shortage in many underdeveloped regions, arid areas, and islands.

Biomimetic Fabrication of Janus Fabric with Asymmetric Wettability for Water Purification and Hydrophobic/Hydrophilic Patterned Surfaces for Fog Harvesting.

The long-term shortage of freshwater resources has drawn increasing research attention for water purification and collection. This work reports a facile method to prepare Janus fabrics with asymmetric wettability for on-demand oil/water separation and hydrophobic/hydrophilic patterned fabrics for efficient fog harvesting. Here, the superhydrophobic fabric was prepared by in situ polymerization of polydivinylbenzene (PDVB) on cotton fabric. By regulating the polymerization time, the PDVB polymer content was changed, thereby achieving the regulation of the surface structure and wettability of the prepared fabric. Meanwhile, the superhydrophobic fabric exhibited excellent self-cleaning and antifouling performance, mechanical abrasion and chemical resistance, and environmental durability. Moreover, the photocatalytic degradation properties of PDVB were utilized to prepare the Janus fabric with asymmetric wettability. Water droplets could spontaneously penetrate from the hydrophobic side to the hydrophilic side, while not vice versa, achieving unidirectional transport of water. In addition, the prepared Janus fabric could be used for on-demand oil/water separation, including the heavy oil/water mixture and light oil/water mixture. The separation efficiency and collected oil purity of each mixture were higher than 99.00 and 99.94%, respectively. Furthermore, the hydrophobic/hydrophilic patterned fabrics were prepared by using the lithographic masks with different apertures under UV light irradiation. Based on the fog-capturing ability of the hydrophilic areas and the water transport performance of the hydrophobic regions, efficient fog harvesting was achieved. For the patterned fabric with larger hydrophobic/hydrophilic areas, the water collection rate reached 224.7 mg cm-2 h-1. Therefore, this simple strategy to achieve controllable gradient wettability by adjusting the surface structure and chemical composition of the fabric shows great potential in the filtration of purification of oily sewage and the efficient condensed collection of water.

Heat transfer enhancement on tube surfaces with biphilic nanomorphology

The heat transfer performance of heat exchangers is extremely restricted due to the considerable thermal resistance of condensing droplets. By adopting the coalescence-induced droplet jumping phenomenon on a superhydrophobic coated surface, the condensing droplets can be spontaneously removed from the heat exchanger, and the heat transfer performance can be enhanced. However, under unfavorable conditions, the flooding effect, which restricts the droplet jumping rate and volume, occurs on heat exchangers and results in the degradation of heat transfer performance. In this study, novel heat transfer tubes with biphilic nanomorphology were fabricated and adopted to address the flooding issue and improve the heat transfer performance of heat exchangers. Through experiments we demonstrated that the droplet jumping effect could be sustained on a biphilic tube due to the formation of droplets in Cassie state during condensation. Because of the high droplet jumping and self-removal rate, the convective heat transfer coefficient was improved by 29% and 38% on a biphilic tube as compared with a typical copper tube and superhydrophobic tube, respectively. Besides, the condensation performance (i.e. water collection rate) on a biphilic heat exchanger prototype was also enhanced by 123% as compared with that on a copper heat exchanger prototype. This study not only enhances the condensation and convective heat transfer performance of heat exchangers by addressing the flooding issue, but also provides a practical strategy to improve the energy conversion efficiency of the thermal components with condensation effect in various thermal applications.

Optimal Design of a Fog Collector: Unidirectional Water Transport on a System Integrated by Conical Copper Needles with Gradient Wettability and Hydrophilic Slippery Rough Surfaces.

Inspired by a cactus spine and pitcher plant slippery surface, a strategy is proposed to design a superhydrophobic-hydrophilic conical copper needle (SHB-HL CCN) and hydrophilic slippery rough surface (SRS) integrative system. In this strategy, the SHB-HL CCN was inserted vertically on the hydrophilic SRS, and such a hydrophilic SRS + SHB-HL CCN system exhibited a high-efficiency cycle in droplet capture-coalescence (supply)-transport during the fog collection process. Even with a single SHB-HL CCN or hydrophilic SRS, the water collection rate is much higher than that of the usual materials (original copper needle, superhydrophobic substrate, hydrophobic SRS, etc.). It is demonstrated that a newly enhanced fog harvesting mechanism and higher fog collection rate can be realized due to the synergy between the Laplace pressure difference from the conical needle, wettability force of wettability difference in the conical copper needles, and released surface energy in droplet coalescence in addition to the attracting force from water bridges formed between needles and substrate. Compared with a single SHB-HL CCN and hydrophilic SRS, the water collection rate of the hydrophilic SRS + SHB-HL CCN system increased by approximately 328 and 152%, respectively. This fog collector provides direction to design water harvesting systems, which has important promotion significance for water collection application engineering in industry, aerospace, and other fields.

Evaluation of Zn- and Fe- rich organic coatings for corrosion protection and condensation performance on waste heat recovery surfaces

Abstract The corrosion protection performance of commercial organic coatings over Stainless Steel (304 L) was investigated in highly acidic media. Three coatings were applied by air spraying and subsequently evaluated for their performance against chemical corrosion by concentrated H2SO4, HNO3, and HCl in terms of % mass loss. Open current potential (OCP) and potentiodynamic polarization (PD) measurements were performed. The water condensation properties of the coatings were monitored for their evaluation as potential protective coatings in condensing economizer applications. ZN691 coating exhibited better protection than ANORZINC and TROPIS versus time in contact with 40% of H2SO4, HNO3 and 37% HCl. Electrochemical results confirmed the superior performance of ZN691 over ANORZINC and TROPIS with corrosion rates at 0.017, 0.098 and 0.097 mpy respectively. Condensation experiments for ZN691 showed a water collection rate of 1.487 ml m−2 h−1 at 40 °C and 100% RH. According to the results, the Zn rich epoxy-based coatings are promising for corrosion protection of condensing economizer elements.

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.

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.

Fog Collection on a Bio-inspired Topological Alloy Net with Micro-/Nanostructures.

Because of the scarcity of freshwater resources, fog collection as one of the effective methods to solve this issue has attracted widespread concern. Inspired by several natural creatures with the capability to collect water from fog, the bio-inspired water-harvesting materials have aroused considerable attention and been widely developed. Inspired by the directional water droplets transportation to the apex on both shorebirds beaks and wheat awns, the bio-inspired topological alloy net with a V-shaped asymmetric geometry in its mesh was designed for fog collecting. Then, micro-/nano-hierarchical structures were modified on the surface of the netting wire via the cathodic electrodeposition method. Thus, the bio-inspired topological alloy net with micro/nanostructures was fabricated successfully. Through the integration of topological geometry and surface microstructure, not only the water-collection rate is improved by efficient drainage along the designated pathways, but also the issue of mesh clogging is resolved. In addition, a theoretical model was constructed to reveal the mechanism, especially the resultant force arising from the V-shaped structure. This work provides insight into the development of novel fog-collecting materials, which has potential applications in other fields, such as liquid transportation, microfluidics, and interface science.

Multistep Wettability Gradient on Bioinspired Conical Surfaces for Water Collection from Fog.

Water scarcity is on the rise globally. In severely arid deserts, nature has developed different mechanisms to transport water collected from fog and condensation before it is evaporated. This water is either stored or consumed. Some of the studied mechanisms are Laplace pressure gradient, grooves, and gravity. Another way to transport water is through heterogeneity in wettability, in which a droplet moves from a water repelling (hydrophobic) region to a water loving (hydrophilic) region. A combination of multistep heterogeneity on a bioinspired conical surface is studied to provide fast movement of water droplets. A representative cone was selected, and its water collection rates at different wettabilities were measured.