FULL PAPER www.advmatinterfaces.de The Design of Hydrophilic Nanochannel-Macrostripe Fog Collector: Enabling Wicking-Assisted Vertical Liquid Delivery for the Enhancement in Fog Collection Efficiency Jonggyu Lee, Jinyoung So, Won-Gyu Bae, and Yoonjin Won* regions due to its potential to collect desal- The design of water collectors that capture atmospheric fogs has been inated freshwater naturally without energy challenging due to the reliability issue. In particular, the small openings of consumption or expensive infrastructures. collecting surfaces are often clogged by large-size droplets. The additional The enhancement of fog harvesting effi- ciency has been intensively studied in hydraulic resistances added to the collecting surfaces prevent the fog path- regard to the collector design and surface ways and further lower the overall performance coefficients by decreasing the properties such as theoretical modeling of shade coefficient of the collectors. The clogging issues can be addressed by the harvesting efficiency and modification modulating the fog harvesters’ surface chemistry and surface structures to of surface wettability.[5–7] deliver water droplets efficiently and to prevent re-entrainment of droplets to Fog collectors with superhydrophobic surfaces provide a low contact angle hys- the openings. Herein, the novel design of fog collectors that consist of both teresis with minimum adhesion force nanochannels and macrostripes is demonstrated. The combination of those between droplets and surface.[8,9] One two structures provides the anisotropic wettability and thereby directional example of hydrophobic fog collec- spreading in the longitudinal direction to the nanochannels. The directional tors such as a stainless steel mesh (i.e., liquid spreading facilitates continuous liquid transport through the formation MIT-14 mesh) outperforms the Raschel of a thin liquid film. As a result, the fog collector successfully enhances the mesh by 12% due to the efficient droplet removal.[10,11] Recently, the novel collector fog harvesting performance by 50% compared to conventional mesh-like fog design such as fog collectors incorporating collectors. hydrophilic/hydrophobic patterns or harp- like design has shown better fog collecting performances by achieving continuous 1. Introduction directional liquid transport.[12–19] In addition, new surfaces have significantly improved fog collection performances by mim- No creature on Earth can survive without a sufficient water icking characteristics of species in nature.[20–22] The common supply. Although water covers two-thirds of the Earth, many examples of the inspiration for the fog collector design include regions have still suffered from water scarcity due to the lack of the Namib desert beetles’ shell,[23,24] spiky leaves of cacti,[25,26] freshwater.[1] According to a recent report, the World Bank and and mesh-like spider webs’ knots.[27,28] Fog collectors with United Nations claimed that 40% of the world’s population suf- patterned or gradient wettability have shown the enhance- fers from water scarcity. The shortage of freshwater is expected ments in water harvesting performance by facilitating droplet to become a more serious issue in the next decades,[2] which removal.[29–31] Also, fog collectors with a gradient Laplace pres- makes it important to develop efficient water harvesting tech- sure caused by gradient conical geometry inspired from the nologies such as water desalination, dew harvesting, or fog har- cacti allow droplets to be transported by the surface energy vesting with the aim of hunting freshwater.[1,3,4] In particular, released in droplet coalescence.[25,26,32] Despite recent intensive fog harvesting technology has drawn great attention in arid research, none of the studies could overcome clogging issues caused by droplet re-entrainment to the openings, which makes the fog harvesting system fail as the surfaces are exposed to J. Lee, Prof. Y. Won the operating conditions for a long time. In addition, there Department of Mechanical and Aerospace Engineering have been no studies that have obtained both efficient droplet University of California Irvine, CA 92697, USA removal and prevention of clogging at the same time by taking E-mail: [email protected] advantage of surfaces’ anisotropic wettability. J. So, Prof. W.-G. Bae Herein, we first present the fog collector inspired by grass Department of Electrical Engineering leaves consisting of two different structures—nanochannels Soongsil University and macrostripes—enabling spontaneous droplet removal and Seoul 156–763, Republic of Korea preventing re-entrainment of the droplets (Figure 1a).[33–36] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201902150. First, nanoscale grooves (i.e., “nanochannels”) of polymer sub- strates are prepared via nanoimprinting lithography (NIL), DOI: 10.1002/admi.201902150 which is suggested for low-cost, scalable fabrication.[37,38] Adv. Mater. Interfaces 2020, 1902150 1902150 (1 of 9) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advmatinterfaces.de Figure 1. Design of nanochannel-macrostripe fog collector inspired by grass leaves. a) Photograph and microscopic images of the grass leaves show longitudinally aligned nanochannels to guide the water to their roots. b) Top illustration and photos show the proposed fog collector that employs the longitudinally aligned nanochannels. Bottom illustrations show the direct liquid transport in the vertical direction. Macrostripes are then patterned through laser cutting to pro- The fabrication follows two steps: 1) drum-to-drum nano- vide desired openings. In this work, the combination of two imprinting lithography to delineate nanochannels that enables features is proposed to demonstrate the directional wicking- the directional capillary wicking (Figure 2a) and 2) laser cut- assisted vertical liquid delivery while preventing the clogging ting to define the macrostripes’ geometry that determines the (Figure 1b). To prove this concept, we examine contact angles aerodynamic efficiency of the fog harvesting (see Supporting and droplet spreading mechanisms on the collecting surfaces Information S2 for the details) with varying pitch p and diam- by taking microscopic optical images from the side and top eter (for mesh-like collector) or width (for stripe-like collector) d views. Then, the fog harvesting performance of our fog col- (Figure 2d). Schematic illustrations show the detailed fabrica- lector is compared to the commercialized mesh-like fog collec- tion process to prepare NFC (see experimental section for the tors by measuring total water collected. details). The light reflection (Figure 2b) indicates the crystallinity of nanochannel structures (Figure 2c). Figures 2e,f confirms that the nanochannel morphology is preserved after the laser cut- 2. Results and Discussion ting. The detailed geometry of NFC is suggested as d = 1.0 mm, p = 2.7 mm (NFC1⊥ and NFC1//), d = 1.0 mm, p = 2.0 mm (NFC2//), 2.1. Wetting Requirements of Nanochannel-Macrostripe and d = 0.7 mm, p = 2.0 mm (NFC3//) to obtain various shade Collector coefficients (SC) where d is the width of stripes and p is the pitch between vertical macrostripes. The NFCs with the longi- For the design of efficient fog collectors, it is important to pre- tudinally and transversely aligned nanochannels are denoted as cisely engineer surfaces with proper wetting property that can NFC// and NFC⊥, respectively. The SC of the collector is 0.35, improve liquid delivery in the vertical direction. For example, 0.5, and 0.65 for the three designs where the SC is defined hydrophilic surfaces are more favorable for fog droplets to as SC = d/p (see Figure S2, Supporting Information, for the stick onto the collector, enabling the formation of a thin liquid details). Table 1 lists the details of fog collector parameters. film.[29,39] However, the higher adhesion force on hydrophilic Since the intrinsic wettability of the as-prepared nanochan- surfaces causes the liquid film to block the fog pathway, which nels is hydrophobic where the static contact angle of polyure- is hard to be removed. On the other hand, hydrophobic sur- thane acrylate (PUA) surface is θs= 91.2°, we employ an oxygen faces result in low adhesion force between droplets and col- plasma treatment for 10 s in order to modify the surface wet- lector surface. However, the spherical shapes of droplets on the ting property and enhance the capillary action. As a result, the hydrophobic surfaces might be blown away by the wind during static contact angle of the PUA surface decreases to θs = 50.8°, the transport to the gutter due to its low adhesion force. This which is hydrophilic. The nanochannel morphology and sur- motivates us to demonstrate new types of water collecting sys- face chemical details before and after the plasma treatment tems that can overcome the drawbacks of using either hydro- are characterized by using a scanning electron microscope philic or hydrophobic surfaces. (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier- The design of nanochannel-macostripe fog collectors transform infrared spectroscopy (FT-IR), as shown in Figure 3. (NFC) consisting of two distinct unidirectional morpholo- SEM images in Figure 3a,b confirms that the nanochannel gies—nanochannels and macrostripes—is fabricated by using morphology remains the same after the oxygen plasma treat- both nanochannels fabrication and laser cutting in series.