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.


Introduction
The basis of life is water. And although the oceans cover more than 70% of the Earth's surface, the global water crisis affects at least two-thirds of the human population living in areas that lack water. 1 Moreover, the problem worsens each year as global climate change takes place. 2 While collecting water from fog may sound revolutionary, it is a simple technique, which can be observed in nature. 3 For millions of years, nature has developed special mechanisms that allow organisms to harvest water from humid air, dew and fog. The best examples are Namib desert beetles (Stenocara gracilipes and Onymacris unguicularis) 4 or cactus (Opuntia microdasys), 5 which have created a hydrophobic-hydrophilic system for the water collection. 6 The Namib desert beetle is bioinspiration for surface functionalization to demonstrate hydrophobicity. [7][8][9] A lot of reptiles live in the desert regions such as a thorny devil lizard (Moloch horridus), which collects water from the air by its skin. 10 Their body created the specic mechanism with many micro-channel, which allow them to harvest the moisture from the environment. The example of natural fog collector is a spider web that combines the hydrophobic and hydrophilic properties, creating the natural Janus bers system. 11 Among many biomimetic strategies, Pinchasik et al. 12 indicated the three main aspects that are important in fog water harvesting such as condensation, adhesion and guided transport of water droplets. Water easily condenses on hydrophilic surfaces in opposition to hydrophobic surfaces with the minimum pinning of the water droplet. In the case of biphilic surfaces, the border between the hydrophobic and hydrophilic parts is the origin of water pinning observed in nature. 6 Indeed the wettability gradient on surfaces drives droplets motion necessary for water transportation in collecting systems. We can mimic the mechanisms that were developed by nature and apply those strategies in material and structure design to create systems collecting water from fog with extraordinary efficiency. 13,14 One of the most popular solutions are Fog Water Collectors (FWCs), 15,16 that use meshes with specialized weave stretched on the special stand. [17][18][19] FWCs collect water from droplets ranging from 1-30 mm, which collide with the bers of the mesh or other medium. 20 Many factors have signicant inuence to the efficiency of FWCs like: fog velocity, the diameter of the fog droplets and liquid water content in fog. 17,20 Real important are wetting and aerodynamic characteristics of the collecting medium and the relation between the diameter or width of the mesh bers or ribbons. [21][22][23] The fog passes through the mesh can be collect by the collecting medium such as bers, ribbons or wires. 22 Importantly, the efficiency of water collection strongly depends from the fog velocity 21 and the porosity of the mesh. 24 Therefore, we incorporated the hydrophilic polyamide 6 (PA6) nanobers in the existing Raschel mesh system, 25,26 to increase the water collection efficiency. For this purpose, we use electrospinning to deposit nanobers directly on the Raschel mesh. Electrospinning works by applying the electric eld to the solution, which causes it to form a polymer jet. [27][28][29] While the polymer jet is exposed to the environment, the solvent rapidly evaporates, which leads to deposition of polymer on the desired surface in form of bers. [30][31][32] The randomly electrospun bers form membranes with porosity above 90%. 33 The electrospun bers have a wide range of applications in such as ltration, [34][35][36][37] structured composites, 38-40 water 41,42 and energy harvesting, 36,[43][44][45] as well as medical applications, especially tissue engineering. [46][47][48] Our focus on water harvesting using large surface area created by nanobers gives the advantage of catching small water droplets. We have selected PA6 due to its mechanical properties of individual bers, 49 membranes. In addition, they provide a very good reinforcement in the composite structures 40,50,51 and are hydrophilic. 52,53 In this study we investigated the water collection rate in collecting water from fog on the Raschel meshes modied with electrospun PA6 nanobers. We focused on extreme condition, where the fog ow velocity is very low, below 1 m s À1 . The inspiration came from nature, where animals and plants harvest water from fog and humid air in the windless environmental conditions. Our system gives the possibility to increase the amount of collected water by our meshes in comparison to commercial mesh, which need the wind to collect water effectively.

Materials and electrospinning
The polyamide 6 (PA6; BASF, Germany; M w ¼ 24 000 g mol À1 ), dried to constant weight at a temperature of 40 C for 3 h, was dissolved in a mixture of formic and acetic acids with ratio 1 : 1 (vol.) (99.5%, Avantor, Poland). The PA6 solution with a concentration of 12 wt% was stirred for 4 h at an ambient temperature of 25 C and at a constant speed of 500 rpm (IKA RCT basic, Germany). The electrospun bers were obtained by electrospinning technique which is presented schematically in Fig. 1a. The chamber with environmental climate control (IME Technologies, The Netherlands) provided the constant temperature of 25 C and humidity of 40% during the manufacturing process. The bers were electrospun with the constant voltage of 16 kV applied between the stainless needle and the grounded collector. The polymer ow rate and used distance were set to 0.1 ml h À1 and 15 cm, respectively. The PA6 bers were electrospun directly on the Raschel mesh, which was placed on the slowly rotating collector (10 rpm). The potential difference had to be increased to 17 kV, because of the insulating properties of Raschel mesh. The PA6 bers were deposited on Raschel for 30 min, while the PA6 membranes by itself were electrospun for 3 h. We produced 4 types of samples with PA6 nanobers by itself: Raschel mesh with PA6 nanobers (R + PA6), double Raschel with PA6 nanobers 2Â(R + PA6) and one thick layer of PA6 nanobers produced and measured separately (PA6) and then placed between two Raschel meshes (R-PA6-R).

Microscopy analysis
The samples were coated with 5 nm of gold (rotary-pump sputter coater Q150RS, Quorum Technologies, Laughton, U.K.) before the analysis by scanning electron microscope (SEM, Merlin Gemini II, ZEISS, Germany). The bers morphology was investigated using the accelerating voltage, current and working distance of 3 kV, 150 pA and 5-8 mm respectively. The average PA6 ber diameter was measured similar way as described in the previous study. 53,54 The wetting experiments on the Raschel mesh and its combination with PA6 bers was carried out using environmental SEM (ESEM, Versa 3D, FEI, USA). The samples were analyzed together with the same environmental conditions such as the pressure in the chamber 100 Pa. The accelerating voltage and current were set to 5 kV and 4 nA.

Fog collection experiments
The fog collection setup is presented in Fig. 1b, where the conventional humidier (Beurer GmbH, Germany) was used to produce fog. The equipment performance in the fog production is 400 ml h À1 and its velocity reaches up to 0.19 m s À1 . The meshes with the area of 100 cm 2 were placed on the specially designed stand. The outlet of a fog feeder was set at an angle of 90 and with a distance of 6 cm from the mesh. The humidity in the stream of fog was above 95%. 53 The water captured by meshes was running down to the glass beaker, which was weighted every 30 min over a 3 h experiments. The mesh aer Fig. 1 The schematics of the experimental setups for (a) electrospinning; (b) fog collection. the water collection experiment was weighted to calculated the water retained inside it. The obtained water collection rate was calculated per the mesh area and the time of 1 h. The six types of samples were measured and were described by the symbols listened below: R -Raschel mesh; 2Â Ra double layer of Raschel mesh; PA6 -PA6 nanobers mesh; R + PA6 -Raschel mesh with PA6 nanobers; 2Â(R + PA6)a double layer of Raschel mesh with PA6 nanobers; R-PA6-Rseparately produced PA6 nanobers placed between two Raschel layers.

Morphology and wetting of meshes
The images and SEM micrographs of the Raschel mesh and PA6 nanobers are shown in Fig. 2. The Raschel ribbons were characterized in a previous study and their average width reached 1.61 AE 0.12 mm and the space between ribbons in Raschel is ranging from 1.65 AE 0.22 to 3.71 AE 0.23 mm, which we managed to cover with the electrospun PA6 nanobers. The PA6 nanobers have the average ber diameters of 110 AE 27 nm and 118 AE 23 nm for the PA6 deposited between ribbons and on the ribbons of the Raschel mesh, respectively. The histograms of PA6 nanobers with their diameter distributions were reported in our previous studies. 53,54 The wetting of Raschel mesh and PA6 nanobers was investigated with the ESEM, see Fig. 3. The droplets that remain on the surface of Raschel take on a characteristic oval shape, Fig. 3b. In the case of the connection of Raschel with PA6 bers the water enters and stays between the PA6 bers, where it freezes due to decreased pressure in the ESEM chamber. The PA6 nanobers are able to catch small water droplets on bers and also between pores due to its hydrophilicity as showed in Fig. 3c and d. Additionally, the investigation with ESEM showed the hydrophobic character of Raschel mesh and hydrophilic of PA6 nanobers, where droplets are spread between bers and also being collected on the bers, see Fig. 3. The addition of PA6 nanobers increases the number of droplets collected on the mesh due to increased surface area. Also, the water collection process is accelerated by the hydrophilic nanobers. 55,56 As imaged with ESEM, Fig. 3, some water droplets were accumulated between ribbons in Raschel mesh.

Fog collection experiments
The shape of water droplets on the Raschel meshes is shown in Fig. 4a. The droplets deposited on the ribbons run down under their own weight and gravity to the beaker below. Oen the water is trapped between the ribbons and limits the ow of fog throughout the mesh, what reduced its efficacy in collecting water. 24 The mechanism of water harvesting on the PA6 mesh is quite different from the processes taking place on Raschel. The geometry of PA6 nanobers provides an unique mechanism to drainage the water from the mesh in another way than the gravity. 53 The ultra-small size of PA6 bers allow to faster water removal from the mesh. The nanober meshes are characterized, with very high porosity reaching 96%, with the typical distance between bers 1.7 mm, 40 what allows the free ow of fog. Fig. 4b shows that the water is collected between nanobers as they are hydrophilic, however the water is spread and we do not observe any large droplets sticking out of the mesh.
This hydrophilic behavior of PA6 nanober meshes was also conrmed with ESEM observations indicated in Fig. 3c. Therefore, we combine the already commercially used Raschel mesh with PA6 nanobers to add the hydrophilic part to increase their water collection rate, as shown in Fig. 4c and d. The water droplets condense on both ribbons and nanobers. Importantly, the PA6 nanobers increase signicantly the effective area of catching the droplets. From the other hand the water saturated between nanober may reduce the wind passage reducing collected water. This effect is strongly depended on the wind speed as the high winds reaching even the speed from 10 to 70 m s À1 , may be able to shake the captured water between nanobers. In this study we performed experiments in so called low wind conditions, with the fog ow velocity of 0.19 m s À1 . 53 The meshes with the larger ber diameter, approximately 5 mm, are characterized with the increased space between bers, as we observed in case of electrospun PS bers, however the collected water from fog was lower in the laboratory conditions. 53 In Fig. 5, we compared the water collection rate between commercial Raschel and its combination with our PA6 nano-bers. We notice that the best water collection is for one layer of Raschel with PA6 nanobers (R + PA6), where the ribbons in the Raschel mesh are also covered with the nanobers as showed Fig. 2d. The R + PA6 mesh better drained water into the beaker compared to the double Raschel meshes (2Â Raschel), 2 layers of Raschel with PA6 nanobers (2Â(R + PA6)) and one layer of separately produced PA6 nanobers placed between double Raschel (R-PA6-R), see Fig. 5. These results conrm, that increase of the water collecting area is very important, however, the extra layers reduce possibility to pass the fog through the fog collecting system. In case of separately produced PA6 nano-bers, the membrane was deposited for longer to obtain easy to handle sample with higher thickness, which was controlled by the time of electrospinning. The samples R + PA6 mesh, where the PA6 nanobers were directly electrospun on the Raschel meshes were easier to handle and perform the water harvesting experiments. Importantly, the addition of hydrophilic PA6 nanobers increased the water collection rate by 3 times in comparison to Raschel mesh. The mesh from double Raschel allows to obtain a similar result for water collected in beaker like in the case of PA6 mesh. However, the geometry of Raschel obstructed the drainage system for water, what decreases water collection rate by 29%, see Fig. 5.
In the previous study, we investigated the wetting behavior on the hydrophobic PS microbers and hydrophilic PA6 nano-bers, which allow to obtain the hierarchical composites to water collection. 53 We conrmed that hydrophilic material retained accumulated water for longer, what gives the water more time to drain into the beaker. As indicated by Park et al., 24 water retained in the mesh decreases the collection efficiency in fog harvesting in the atmospheric conditions. In case of the laboratory experiments, the fog velocity is low and oen the droplet size distribution of the nebulizer used is smaller than the size distribution of atmospheric fogs observed in fog harvesting. However, electrospun bers able to improve water drainage, and increasing collection rates. These solutions are oen found in nature, which adapts organisms to live in an extremely hostile environment without freshwater reservoirs. 3 The key element in proposed modication of the commercially used Raschel meshes are PA6 nanobers that have been proved very stable mechanically as individual bers 49 and in meshes, 53,57 for composite constructions 40 and also in the cryo environment. 33 Our previous study conrmed the good mechanical properties of randomly oriented PA6 nanober  meshes, where the average maximum stress reached 1.24 MPa. 53 Importantly, the deposited PA6 nanobers on the Raschel mesh modify the surface properties of ribbons by increasing its roughness and the hierarchical elements to mesh as their sizes are in the range of 100 nm. Azad et al. 58 provided evidence of how important is the surface structure of the system used to harvest water effectively. The best results showed the structure based on the bers with round, oval and rectangular shapes in their cross sections.
Particular importance in the fog harvesting has the combination of hydrophobic and hydrophilic surface properties. Lee et al. 59 proposed the construction inspired by the cactus stem, which based on the superhydrophilic-superhydrophobic system. Their cylindrical double structural system reached of 209 mg cm À2 h À1 of water recovery. Also Cao et al. 60 proposed a hydrophobic-hydrophilic Janus System based on the hydrophobic copper mesh and hydrophilic cotton absorbent. This system with the area of 2 Â 2 cm 2 reached of 0.31 AE 0.03 g of water per piece. The performance of water harvesting from fog depends on environmental conditions like fog ow velocity and ambient humidity and also from the FWC construction. In both cases the fog ow velocity was 3.5 times faster from our system and reached z0.7 m s À1 . Our model of Raschel with PA6 nanobers has lower water collection rate due to differences in the experimental conditions, however, it shows the desired ability to collect water in regions with the low fog ow velocity. The water collection rate reaching 64 mg cm À2 h À1 is a great achievement in such laboratory conditions. Additionally, we show a possibility to modify Raschel meshes at a low cost by stable electrospinning of PA6 nanobers. The technology for production of high amounts of electrospun nanobers is already present in the eld of air ltration. 61

Conclusion
In conclusion the water collection rate of single layer of commercial Raschel mesh can be increased by 300% in a very simple way by incorporation hydrophilic PA6 nanobers layer in a single step manufacturing method. It is possible thanks to increasing the water collection area and improving the water drainage mechanism. This solution allows to collect water in more effective way in the windless or low wind speed conditions. This solution shows the new approach and future development path of creating more efficient FWC constructions. In terms of water harvesting, we need to keep in mind that water harvesting innovations must be coupled with better water management. People need to nd new ways of storing or capturing water in places that are becoming scarcer with this vital resource. The water crisis is a big social problem and it requires the material scientists working together with applied environmental units to be able to bring innovation to the fog collectors.

Data availability
The data supporting this article are found within the text. Any additional data and the data that support the plots within this paper are available from the corresponding author upon reasonable request.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.