Fog Harvesting of a Bioinspired Nanocone-Decorated 3D Fiber Network

Subscriber access provided by United Arab Emirates University | Libraries Deanship

Surfaces, Interfaces, and Applications

Fog Harvesting of Bioinspired Nanocones Decorated 3D Fiber Network Chang Li, Yufang Liu, Chunlei Gao, Xin Li, Yan Xing, and Yongmei Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15901 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Fog Harvesting of Bioinspired Nanocones Decorated 3D Fiber Network Chang Li, Yufang Liu, Chunlei Gao, Xin Li, Yan Xing, Yongmei Zheng* Key Laboratory of Bioinspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University (BUAA), Beijing, 100191 (P. R. China) KEYWORDS: fog harvesting, bioinspired, nanocones, 3D fiber network, Rayleigh instability ABSTRACT: The bioinspired nanocones decorated three-dimensional fiber network (N3D) can be fabricated, where original 3D web is designed and inspired by some newest research findings of spider web, and it is decorated with hydrophilic Zinc Oxide (ZnO) nanocones inspired by cactus spine. Multilevel high specific surface area exposure on fiber together with the hydrophilic decoration enables it to be more attractive to water molecules. These nanocones can capture fog droplet, generate coalesced droplet and accordingly make droplet transport efficiently due to Laplace pressure difference. Especially, a novel mechanism was revealed that after the nanocones decorated fiber was getting wetted, i.e., water film formed and immediately broke up into droplet owing to the force relating to Rayleigh instability. Consequent lower retention surface realizes the formation of fast continuous water flow, rather than the traditional intermittent course. Thus, outstanding fog-harvesting efficiency was achieved on N3D, e.g., probably reaching 865.1 kg/m2/day, where the mass of collected water within 2 hours can raise

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

up to over 240 times higher than the weight of original 3D web without nanocones. Such bioinspired ZnO nanocone-decorated 3D fiber network (i.e., N3D) have potential application to harvest fog-water for production or living, e.g. water re-condensation in cooling water tower, in agricultural irrigation system and even in water-deficient countries.

□INTRODUCTION Harvesting fog-water with high efficiency is a charming approach to relieve the menace of water deficiency.

1-3

Inspired by creatures in nature, bionic studies concerning functional

materials with water collecting capacity are in the ascendant. 3-5 Spider silk presents the periodic spindle-knot structure when affected with dampness, endowing the performance of moisture capture and tiny water gathering owing to a cooperation of curvature gradient and surface energy gradient. 5-7 More significantly, some nearest researches pointed that by weave the single fiber into artificial spider web, much more efficient water-harvesting was implemented.

8,9

It is

indicated that intersectional arrangement shall give rise to Laplace pressure difference on the tiny droplet, gaining an easier access to water transportation and combination. Another typical biomimetic pattern was inspired by spine system of cactus that can collect water in the arid desert.

10

The mystery lies in its multilevel and multiscale spiny structure, which enables

wettability gradient and exerts Laplace force on the droplet.

10-13

Two main courses exert

conclusive impact on the water collecting efficiency: fog-water capturing and water droplet transporting for collection. Both are firmly associated with the surface wettability of material. The fog harvesting behavior of (super-) hydrophilicity or (super-) hydrophobicity has been intensively study in the previous works.

13-18

However, it can be summarized that each of them

has superiority and drawback. For the hydrophobic, it is easy for tiny dew to shape, combine into


2

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


relatively big droplet and then move away from the material. Nevertheless, before these processes, such materials fail to quick capture and seize water from humid atmosphere, which is largely detrimental for the total efficiency of fog-water collecting. 7,18 As to hydrophilic materials, although fog-water capturing is rapid, what makes troubles is the fact that the spreading of tiny fog-water on the surface (and frequent formation of water membrane covered on the surface) hinder the formation of relatively big water drop and enhance the adherence of water on the material (towards horizontal placed 1D or 2D-plane materials). 13,19 In order to handle the dilemma, it may be cumbersome to improve the water-molecule catching course of hydrophobic materials. Hence, we focus on the solution that the formation of water membrane cuts off water collection or water stays unmoved on the hydrophilic material because of high normal adherence or tangential retention. Herein, we employed a 3D multiintersectional web and decorate it with hydrophilic ZnO nanocones, where the bioinspired nanocones decorated 3D fiber network (i.e., N3D) was achieved. The multi-intersectional network helps direct the transportation of water. Along with the combination of nanocones on the decorated 3D fiber network, accordingly, tiny water is likely to form continuous water flow rather than stay still on N3D owing to integrative cooperation of Laplace force, increasing weight of droplet, Rayleigh instability and consequent lower tangential retention force. Additionally, high specific surface area that exposed to moisture, conical structure together with the hydrophilic attraction enable the rapid seizing of water molecule on N3D, where both fog capturing and water gathering procedures were largely accelerated. Thus, N3D realizes the outstanding fog-harvesting capacity.

□EXPERIMENTAL SECTION


3


Page 4 of 24

Preparation of materials. The original web was designed and weaved as 27×37×14 mm with 0.300 g multi-intersectional lightweight Nylon fiber (Figure S1). Specified textile machine can make the web in larger scale (e.g., sectional area more than 0.5 m2). ZnO nanocones decoration. (1) Sample pretreatment: The original fabric web was first dipped into and cleaned by 1:1 Petroleum Ether (Beijing Chemical Works, China)/ Isopropanol (Beijing Chemical Works, China). After successive Ethanol (Beijing Chemical Works, China) washing, deionized water washing, forced air drying and Plasma (PDG-32G, Harrick Plasma, American) etching, it was submerged into 30% H2O2 aqueous solution (Beijing Chemical Works, China) for 10 min and then vacuum drying. (2) Crystal-seed layer decoration: The first step of germinating ZnO nanoparticle was to prepare seed solution by dissolving 1.32 g Zinc Acetate Dihydrate (Shanghai Macklin Biochemical Co., Ltd., China) together with 0.72g NaOH (Beijing Chemical Works, China) into 200 ml methanol (Beijing Sinopharm Chemical Reagent Co., Ltd., China), which was then strong magnetic stirred for 40 min at 333.15 K to become colloidal solution. Subsequent, pretreated web sample was soaked uniformly in the colloidal seed solution for 15 min, and in the meantime, the sample was kneaded and squeezed from multiple dimensions. Afterwards, it was drawn out softly from the solution and desiccated at 338.15 K. The procedure from soaking to desiccation was repeated four times before the as-treated sample was finally heated at 463.15 K for 45 min. (3) Hydrothermal growth: The web with crystal-seed was vertically placed into growth solution, which consists of 0.60 g Zinc Nitrate Hexahydrate (Shanghai Macklin Biochemical Co., Ltd., China) together with 0.28 g Hexamethylenetetramine (Shanghai Macklin Biochemical Co., Ltd., China) aqueous solution in 100 mL Teflon reactor. Hydrothermal reaction lasted for 4 hours at 368.15 K. Ultimately, it was washed with plenty of water. Thus, N3D was achieved.


4

Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanostructure characterization. Morphologies of the original web and N3D were observed via Field Emission Scanning Electron Microscope (JSM-7500F, JEOL, Japan) and Environmental Scanning Electron Microscope (Quanta 250 FEG, FEI, America). Elements on the surface of the samples were analyzed by Energy Dispersive Spectrometer (X-Maxn, Oxford, Britain; connected with ESEM). The crystal form of decorated nano ZnO was characterized via X-ray powder diffractometer (XRD-6000, Shimadzu, Japan). Mechanical property of ZnO nanocones was measured by Nano Mechanical Test Instrument (PA85, Hysitron, America) and observed via ESEM at the same time, where a sensor is directly connected with the testing probe to simultaneously record the pressuring force and displacement, with a sensitivity of 1.4 nm or 1 μN. Characterization of wettability and fog-collection. All fog flows were sprayed via a same controllable ultrasonic humidifier (JSQ107-J18, Chigo, China), where the practical flow rate was measured using a digital anemometer (AS8336, Smart Sensor, China). The test of one single horizontal-placed fiber was observed by Optical Contact Angle Measuring Device (OCA40Micro, Dataphyxics, Germany) while the tests of two-crossing-fiber and whole 3D webs were recorded via Charge Coupled Device (V9.1, Phantom, America) and Digital Single Lens Reflex (705D, Canon, Japan). The formation and breaking-up into droplets of water film on the hydrophilic nanocones were observed via Laser Scanning Confocal Microscopy (FV1000-IX81, Olympus, Japan). The retention of water droplet motion on fiber (f

, where

is roll

angle) was measured by Dynamic Contact Angle Measuring Device (DCAT21, Dataphysics, Germany) with an uncertainty less than 2%. The temperature of 273.15K or 280.65K is obtained in household refrigerator. Other temperatures were controlled by room air conditioner. All


5


Page 6 of 24

recorded temperature values were measured and calibrated by air thermometer. The mass of water collection was measured by an analytical balance (CP114, Ohaus, America).

□RESULTS AND DISCUSSION Morphology of N3D. Figure 1a is optical image of N3D. After ZnO nanocones decoration, N3D takes on much whiter color than the original one (Figure S2a) due to the coverings of ZnO nanocones.

20-22

Figure 1b-d are the ESEM image and corresponding EDS image of fibers on

N3D, where the element Zn covers intensively and uniformly on every fiber (Figure 1b from frame I to frame II, and also see Figure S3). Figure 1c and 1d are the detailed analysis of a cluster of nanocones. It can be estimated that ZnO coverage fraction is more than 80% on N3D (Figure S2c). Mechanical property of ZnO nanocones on fibers. To verify the mechanical property of the ZnO nanocones decoration, a testing probe (1 μm in diameter) was used to press the ZnO nanocones. As is shown in Figure 2a-b, a cluster of nanocones were pressed downward for more than 700 nm and suffering 180 μN load in the meantime. After the probe was withdrawn, the ZnO nanocones were little changed: i.e., the nanocones recovered into initial state, without any fracture, deformation or falling off from the web, which implies the durability of ZnO nanocones decoration. Furthermore, single nanocone was observed by ESEM (Figure 2c): it is 2~3 μm in height and 200~300 nm in maximum diameter, which demonstrates a conical shape. Additionally, via grinding off these ZnO from treated web and then vacuum drying, we analyzed the synthetic nano ZnO via X-ray powder diffraction (XRD) and found (100), (002), (101), (102), (110), (103), (200), (112), (201) peaks, as is shown in Figure 2d. The location of peaks and their relative intensity ratio are perfectly consistent with the JCPDS 36-1451, indicating the purity of the assynthetic monocrystalline and the Wurtzite structure of nano ZnO.


6

Page 7 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wettability of ZnO nanocone-decorated fiber surface. The decoration of ZnO nanocones largely reduced the contact angle (CA) with water, where the surface wettability transformed from hydrophobic (CA=130.0°, Figure S4a) to hydrophilic (CA=64.9°, Figure S4c). Hydrophilic surface is much more attractive to water molecules and can be wetted spontaneously, as the following formula indicates,

23

(1) where

represents the work of wetting process,

is surface tension of water, and

is CA

(similarly hereinafter). When it comes to collecting water, hydrophilicity is favorable to rapid seize fog-water from the humid atmosphere, which accelerates the water capturing process. As shown in Figure S4b, d, tiny water drop can be observed on fiber of N3D at 138 s (Figure S4d), while the droplet did not clearly shape on orginal fiber without ZnO nanocone until 1809 s (Figure S4b). Mechanism in fog-collection. To investigate the advantage of crossing architecture cooperated with hydrophilic ZnO nanocones for collecting water (Figure 3, Movie S1, S2), twointersectional-fiber (as a whole) was sniped from N3D. The fog-harvesting performance was examined under a humid environment (Figure 3a, for the whole test see Movie S2). As to crossing fiber from N3D, tiny water droplets fast formed before 66 s due to capture of water molecules by the hydrophilic ZnO nanocones. After 140 s, tiny water droplets tended to directly flow into the intersection and then slid to the crossing location (see square and rightdown arrow). At the same time, as-grown droplets slid and also coalesced to form a larger droplet at the crossing location. The first droplet (i.e., critical droplet) dropped off from the fiber at 340 s, i.e., the first collecting circle. As water continuously flowed, the collecting rate became faster. The second collecting cycle completed at 566 s, which took only 226 s. On the contrary, the water


7


Page 8 of 24

collection observed on the corssing fibers without ZnO nanocones (Figure S5, for the total test see Movie S1), tiny water droplets were formed before 127 s, and then coalesced into relative big droplet and finally dropping off at 732 s. Even if water droplet had skimmed over the fiber time after time, the hydrophobicity prevented the fiber from being wetted. As a result, the subsequent water-collecting cycles still take a long time. Based on the observation above, the formation of water flow should be important for efficient water collection, which was further investigated by observation in detials via CCD and LSCM. As is shown in Figure 3b, water film formed at wetted fiber before 135 s and immediately broke up at 137 s. From 137 s to 141 s, tiny water dew piled up and then slipped off. After 141 s, the fiber was much more wetted and the consequent lower retention surface realized the fast water flow. As Figure 3c exhibits, N3D enjoyed about 8/9 retention deduction after getting wetted (from ~2090 μN to ~230 μN) while the retention of orginal fiber without ZnO nanocones had no notable transformation during harvesting fog (from ~820 μN to ~660 μN). It implies that there is a novel mechanism for high efficiency of water harvesting on N3D, which has little been reported so far. As for the improvement of water capturing, the bio-inspired nanocones decoration are deemed to play a crucial role. The size effect of nanostructure makes the material possess high specific surface area. 24,25 As shown in Figure 4a, when employed for harvesting fog, it means more contact area exposed to foggy air, which may increase the chance for fog-water to be captured and thus adsorbed much water, as the following equation shows,

26,27

(2) where

represents the most mass of adsorbed water per unit mass of material,

adsorption constant (which is linked to the temperature),


is the

is the vapor pressure (which is

8

Page 9 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


associated with the humidity condition), area of a water molecule and

is the mass of a water molecule,

is the occupied

is the valid specific surface area of materials. On the other hand,

the conical structure of such nanomaterial contributes to a Laplace pressure difference ( Figure 4b) exerted on tiny fog-water, which can be expressed as follow,

, see

10,28

(3) where R, R1 and R2 indicate the local radius of the cones, R0 is the radius of the tiny drop, represents half-apex angle of the cone, and

is the incremental radius of the cone. Such acting

force made the tiny fog-water rapidly move from the tip to the base side along the nanocones, expediting the wetting process of material surface. As for the efficiency of water collection, it is deemed to be attribute to the water film formation and breaking to form water flow on the hydrophilic nanocones. Due to the existence of water film, the captured water droplet can spread on the surface of fiber. Once the water film reaches a certain thickness higher than the nanocones, it propels to break up into droplets due to Rayleigh instability

6,29-32

and coalecense of droplet on the crossing fiber from N3D, which is

distinguished from that on the one horizontal hydrophilic fiber: Initially (Figure 4c), along the normal direction, water in the nanocones was motionless mainly owing to the balance of Laplace pressure difference (

, formula 3) and Capillary pressure (

, r is the interval between

nanocones). 28,33 As the fog-water capturing continued, the balance faded away and difference in diameter of the water film appeared. Then, along the tangential direction, water film transformed into droplets immediately owing to Rayleigh instability breaking-up force (detailed force analysis see supporting information). Eventually, most of the droplets became continuous fast water flow directly (Figure 4d) along with the formation of surface lubricating with much lower retention. 34


9


The total exerted driving force on a droplet ( described as

Page 10 of 24

near the intersection of N3D can be

35,36

(4) where l is the length of contact line of droplet inside the fibers (which is related to , volume of water droplet),

indicates the half angle of intersection, and

represents the Laplace force caused by the intersection. And decomposition of the droplet,

and the

is tangential gravity

is tangential retention of the surface. After getting wetted, the

tangential retention of fiber is reduced by 88%, which is beneficial to water fast transportation for coalescence (Figure 3b, 3c). Ultimately, the water drop detached off from the N3D due to fast increasing weight (Figure 4e). The course of fog-water capturing as well as water transporting for collection has been promoted, uncovering a brand-new mechanism and contributing to the high efficiency of fog-harvesting function (Figure 4f). In addition, a single fiber from N3D was deeply investigated to study the impact of inclined angle on fog collecting rate (Figure S6 and Table S1). It is evident that maximum collecting rate took place at inclined angle about 30° under the experiment condition. In this process, increasing inclined angle from 0 to 90° may lead to lower exposure area to fog flow (lower S0 to cause lower speed of fog capturing, see formula 2) but higher driving force (higher to cause higher rate of water transportation, see formula 4), which had totally distinct effect on fog harvesting. Hence, a moderate inclined angle and the crossing fiber part of N3D is essential for highly efficient water collection. Effect of fog collecting. Control experiment of water harvesting was done on the fibers and webs with or without ZnO nanocone decoration (Figure 5, some data of the fog-harvesting test was seen in Table S1,S2,S3). As shown in Figure 5a, water gathered at the velocity of ~0.04


10

Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mm3/s on one intersection of original fiber without ZnO nanocones. As for fiber with ZnO nanocones on N3D, such velocity rised to ~0.20 mm3/s in the first drop cycle, indicating the decoration is valid. While in the second cycle, the velocity further raised to ~0.27 mm3/s. The 2nd water droplet formed in a larger size than that of 1st droplet within the same collecting time, because the wetted fiber surface favors the droplet transport. Water droplet growth rate on intersection, as well as water transporting velocity on fiber conforms to the sequence: 2nd drop cycle of fiber with ZnO nanocones＞1st drop cycle of fiber with ZnO nanocones ≫ drop cycle of original fiber without ZnO nanocones, which is in good accordance with the results of retention test (Figure 3c) as well as the above mechanism analysis. Furthermore, fog-harvesting efficiency of whole N3D was examined. As is shown in Figure S7, under the same foggy flow (e.g., 0.75 m/s), the first water-drop from original web without ZnO nanocones (Figure S7b-c) was in 215 s and the N3D was in 164 s (Figure S7d-e). It can also be explicitly observed that the volume (Vmax) of the drop from N3D is 1.5~3 times that from original web without ZnO nanocones (see Movie S3 and Movie S4). Figure 5b establishes a comparation of water collecting efficiency between original web and N3D under the fog flow is 0.15 m/s or 0.75 m/s. It is evident from the diagram that the mass of collected water on N3D was doubled or tripled that on original web, i.e., ratio of N3D to original web in water collecting efficiency is 2~3. Figure 5c shows differences of water collecting efficiency for orginal web (i.e., web without ZnO nanocones) and N3D (i.e., web with ZnO nanocones) under fog flow rate ranges of 0.1-0.8 m/s. The faster fog flow induces the more as-collected water.7 There was an approximate linear relation between water collectiong effiency (η) and fog flow rate (x) when 0.15≤x≤0.75. As for original web, ηno-ZnO=10.24x. As to N3D, ηZnO-nanocone=24.62x. A higher slope also reveals the


11


Page 12 of 24

superior fog harvesting ability of N3D. Figure 5d shows the relationship between  and ambient temperature ranged in 273.15~303.15K, where visibly enhancement occured as ambient temperature decreases before freeze. Temperature may exert considerable influence on the fog harvesting efficiency, which has seldom been studied and analyzed before. Herein, the phenomenon can be explained: moisture preferred to condense on the material owing to lower saturated vapor pressure under lower temperature, i.e., lower temperature means a higher K value and consequent a higher

value (see formula 2), which promotes moisture capturing.

Stability of fog-collection. Water collecting property is stable for more than 2 months. All the fog-collecting tests can be repeated for more than 10 times, which indicates N3D is durable (Table S4) for potential industrial application. For one thing, the substrate material, Nylon, are durable. Rather than the metal, there is no need to worry about the rust and corrosion when getting wet. For another, the nano decoration are durable. Nano mechanical test (Figure 2) had demonstrated that the nanocones are resistant to external press. Forthermore, it is estimated that the mass of collected water using N3D within 2 hours (~72 g) can raise up to more than 240 times higher than the weight of the web sample (0.300 g). As for ZnO-nanocone, its value reached 36.07 g/h under 0.75 m/s fog flow at ambient temperature 273.15 K, which makes it possible to harvest fog in large scale (taking sectional area into consideration, the value of such ZnO-nanocone can be simply converse to 865 kg/m2/day). When comparing it with some other already reported fog collector sample (Table S5), 7,8,13,36,37 the efficiency of N3D is relatively higher. More significantly, it is found that the decoration of ZnO nanocones collected water faster than ZnO nanorods or ZnO cones in micrometer (Figure S8), which can be attribute to the superiority of nanometer size (Formula 2) together with conicial shape (Formula 3) cited above.


12

Page 13 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Moisture-capturing test under quasi-static humid atmosphere (Figure S9) was done to simulate the condition of industrial cooling water tower. Within one hour, 0.26 g and 1.10 g water was captured on original web and N3D, respectively. Both weight increased to 0.83 g (for N3D) and 2.71 g (for original web) in 2 hours, which indicates that N3D geenrates the highly efficient fog-capturing, where estimated ηZnO-nanocone can reach up to 32 kg/m2/day. Apparently, all above high performance is linked to the integration with nanocone-induced hydrophilic surface, Rayleigh instability break-up into droplet, and much lower retention surface, which hasten both fog-water capturing and water transportation for collection in efficiency.

□ CONCLUSIONS In conclusion, we employed N3D composed of a brand-new lightweight multi-intersectional web and decorated with durable hydrophilic ZnO nanocones. The N3D enables water collecting in much larger scale while the nanocones decoration shows much higher water collecting efficiency, thus which realizes a new style of fog capture and water transport to promote fog harvesting. Material design and nanostructure decoration herein may offer serviceable suggestions concerning the research fields of fluidics, catalyst and battery. It is safe to draw the conclusion that this N3D are most likely to have potential application to harvest fog water in large scale for production or living, such as water re-condensation in cooling water tower, in agricultural irrigation system and even in water-deficient countries.


13


Page 14 of 24

Figure 1. Morphological observation of N3D. a) Optical photograph of the N3D. Width: 27 mm, Length: 37 mm, Height: 14 mm. b) FESEM image and corresponding EDS image of single tows cut down from N3D, Diameter: ~150 μm. c) EDS spectrogram of single tows from N3D. Atomic percentage: 17.86% C, 49.43% O, 32.71% Zn. d) ESEM images and corresponding EDS image of the ZnO nanocones.


14

Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Characterization and Mechanical testing of the ZnO nanocones on fiber of N3D. a-b) Nanomechanical testing images and corresponding force-depth diagram, I, probe contact with nanocones, where the pressuring force is 0 μN and the depth is 0 nm, II, probe pressing the nanocones for 700 nm where the pressuring force is 180 μN, III, probe withdrawing and the nanocones recovery. c) ESEM image of single ZnO nanocone, height: 2~3 μm, underside diameter: 200~300 nm. d) XRD spectrum of the lapping powder of ZnO nanocones, peak (100), (002), (101), (102), (110), (103), (200), (112), (201) according with Wurtzite ZnO (JCPDS 361451).


15


Page 16 of 24

Figure 3. Characterization of fog harvesting and the formation of fast water flow on fibers of N3D. a) In-situ observation on process of fog collecting on location of two crossing fiber from N3D via microscopic CCD. Water dew shapes at 66 s, fast water flow forms at 140 s, the first water droplet at 340 s, the second water droplet at 566 s. b) In-situ observation by CCD and corresponding LSCM of water film formation at 135 s, breaking up at 137 s, getting wetted and form low retention surface at 141 s. c) Comparation on retention of water motion for original web (without ZnO nanocones) and web with ZnO nanocones (i.e., N3D). There is the retention ( ): original dry and wet fiber: ~820 μN and ~660 μN on average, respectively. The dry fiber with ZnO nanocones has retention force of

= ~2090 μN. The wet fiber with ZnO nanocones has much lower

= ~230 μN. Sample direction: vertical to the ground; Sprayed steam flow:

0.15 m/s, 289.15 K, vertical (to the ground) sprayed and 0.15 m away from the sample; Ambient:


16

Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


298.65 K, 101.4 kPa.

Figure 4. Illustration of water-collecting mechanism of bioinspired N3D. a) The bioinspired nanocone-decorated 3D fiber network (i.e., N3D) with multilevel high specific surface area exposure and high speed to seize water from moisture. b) Force analysis of water on a single cone,

and

: Local radius of the cones,

: Half-apex angle of the cone,

: Laplace

pressure difference caused by cones. c) Water film formation on a cluster of nanocones,

:

Capillary pressure caused by the interval of the cones. d) Fast water flow forming on an oblique fiber,

: Gravity decomposition of the droplet. e) Water drops on two-crossing-fiber,

caused by dynamic Rayleigh Instability,

: Force

: Gravity of a droplet under dropping off. f) The

bioinspired N3D with high efficiency water collection.


17


Page 18 of 24

Figure 5. Statistics of the fog collecting test. a) Maximum volume (Vmax) of droplet with time (T) on different droplet circles. Water growth rate on an intersection (water transport velocity): 2nd drop cycle of fiber with ZnO nanocones from N3D＞1st drop cycle of fiber with ZnO nanocones from N3D ≫ drop cycle of original fiber (without ZnO nanocones), T: water collecting time, Vmax: the volume of water droplet on the crossing. Sprayed steam flow: 0.15 m/s, 289.15 K, vertical sprayed and 0.15 m away from the sample. Ambient: 298.65 K, 101.4 kPa. b) Harvesting efficiency comparation of orignial web and N3D under fog flow of 0.15 m/s and 0.75 m/s. Collecting efficiency: ηno-ZnO

Fog Harvesting of a Bioinspired Nanocone-Decorated 3D Fiber Network

Recommend Documents