Matchbox Janus membrane fog collector with highly efficient directional transport

Abstract

Coordinating the droplet capture, transport, and shedding processes during fog collection to achieve efficient fog collection is a major challenge. In this study, a copper mesh with different wettability was prepared by chemical etching and thiol modification. The Cu(OH)₂ needle structure on the surface of the samples was characterized by FE-SEM and EDS tests, and the surface of the samples was chemically analyzed by infrared and XPS analyses. A Janus membrane matchbox fog collector was thus designed and assembled with directional transport properties. While achieving directional transport of fog droplets on a grid, the fog capture efficiency was also improved. We built a fog collection test rig in the laboratory and tested the samples at a fog flow rate of 0.8 m s⁻¹, and the highest fog collection efficiency reached 6.9 g h⁻¹ cm⁻², enabling a long-term and efficient fog collection process even in dynamically changing fog environments. This study demonstrates a wide range of applications to achieve green, low-cost, and efficient fog collection strategies.

---

New concepts

In this study, we were inspired by the naturally different properties of the front and back sides of lotus leaves to design and develop a Janus membrane matchbox fog collector with directional transport properties. This matchbox consisted of different wettability grids that can achieve a maximum fog harvesting efficiency of 6.9 g cm⁻² h⁻¹ at a fog flow rate of 0.8 m s⁻¹. Our research focuses on the effective enhancement of the harvesting efficiency, while achieving transmembrane transport of droplets within the grids, when working in dynamically changing fog environments.

1. Introduction

Water scarcity has caused hundreds of millions of people to live in difficult situations,^1–3 where the acquisition of freshwater resources is a problem. Desalination^4–6 and recycling of domestic sewage^7–9 are two measures with intense requirements of operational conditions and costs for application in economically backward arid areas, making them environmentally unfavorable in such areas and difficult to achieve. In such regions, fog harvesting has the advantage of being independent of the influence of the geographic environment, and thus, it alleviates the current pressures on freshwater resources and provides effective solutions to the problem at hand.^10–12 Fog collectors comprise a large mesh prepared with a Raschel mesh.^13–16 For the functioning of these collectors, wind-driven droplets impact the mesh surface, and the mesh captures droplets through inertial collision. The droplets then merge and coalesce to reach a critical size, but they fall off under the action of gravity from the Raschel mesh; especially, in wind conditions, under the action of the elastic-plastic deformation, the surface stress and the air resistance coefficient increases, resulting in damage to the mesh, limiting the mesh surface for fog harvesting efficiency.¹⁶ In order to improve the efficiency and durability of the mesh, we were inspired by the opposite wettability of the upper and lower surfaces of the lotus leaf. The Janus membrane material has an asymmetric wettability, and it has been widely tested and applied; researchers have designed a large number of Janus membrane materials.^17–19 The unique wettability of the two sides of the membrane and the pore structure helps achieve the required mass-transfer behavior from these membranes. The hydrophobic side of the spontaneous drainage and the hydrophilic side of the capillary drive force is also achieved. The coordinated action helps achieve the trans-membrane transport of liquid and realize the application requirements in specific scenarios, which has a wide application prospect in fog harvesting.^20–22 Different combinations of Janus membrane materials have different wetting properties. In order to enhance the fog harvesting performance of these membrane materials, Liu et al.²³ used lasers to prepare a multi-mimetic double-interleaved wetting Janus surface (DIWJS), which could accelerate the overall fog harvesting process. Zeng et al.²⁴ prepared Janus membranes with asymmetric wettability by modifying a copper mesh, and they successfully applied it for liquid manipulation. These researchers further developed a sandwich fog collector consisting of a superhydrophilic (SHL) inner layer and a double superhydrophobic (SHB) outer layer. Currently, fog collectors prepared with Janus membranes are used in specific environments, where the direction of the fog flow is perpendicular to the sample surface; in such scenarios, the fog is captured and transported by the inertia of the fog flow impinging on the mesh surface. The direction of natural fog flow changes dynamically,^25–27 and bifacial membranes are extremely sensitive to the direction of fog flow. In this study, to better utilize the impact inertia on the Janus membrane during fog harvesting, we assembled a double-layer copper mesh and constructed a matchbox-structured fog collector.

In this study, copper meshes with SHL and SHB were prepared by chemical etching and thiol modification, respectively. These layers were overlapped into the Janus copper meshes (JCM) with asymmetric wettability and assembled into a matchbox Janus membrane fog collector (MJMFC). trans-Membrane directional transport of droplets inside the meshes was achieved through the capillarity of the micropores on the membrane surface. The process of directional transport of the droplets inside the membranes reduced the rate of water evaporation, leading to 147% and 133% fog harvesting efficiency with the SHL/SHL mesh and the SHB/SHL mesh, respectively. Considering that the direction of fog flow in nature changes dynamically, the effect of fog flow entering the mesh surface from different directions on the fog harvesting performance of the fog collector was investigated. The fog harvesting efficiency of the matchbox fog collector remained in the range of 4.8–6.2 g h⁻¹ cm⁻² upon changing the direction of the fog flow. A continuous and effective fog harvesting process was realized, when the fog flow was flushed to the surface of the mesh from any angle.

2. Results and discussion

2.1. Sample design and preparation

The MJMFC had asymmetric wettability that enabled directional transport of the droplets. In addition, the Janus membrane was sensitive to the direction of fog flow, and its three-dimensional structure ensured that the sample was not affected by the direction of the fog flow during the fog harvesting process. The preparation process is shown in Fig. 1, where the process of achieving asymmetric wettability of the samples is divided into three main steps, i.e., chemical etching, thiol modification, and overlapping of the copper mesh with asymmetric wettability (Fig. 1a). The process is shown in Fig. S1 (ESI†). Firstly, the copper mesh was cut into rectangles of 8 × 4 cm, which were then ultrasonically cleaned with 0.1 M dilute hydrochloric acid solution, anhydrous ethanol, and deionized water for 30 minutes to remove the oxidized layer and impurities on the surface of the copper mesh; the copper mesh thus obtained is shown in Fig. S1 (ESI†) that had a contact angle of 118°. In the second step, the cleaned copper mesh was chemically etched using a mixture of 2.5 M NaOH and 0.15 M (NH₄)₂S₂O₈ for a reaction time of 14 min. A homogeneous SHL copper mesh was prepared (shown in Fig. S1, ESI†). The SHL copper mesh was modified by immersing it into 0.01 M octadecanethiol (ODT) solution with a reaction time of 80 min, which yielded an SHB copper mesh (shown in Fig. S1, ESI†) with a surface contact angle of 152°. The SHL copper meshes prepared via chemical etching may regain their hydrophobicity if they are allowed to rest for a long time. To investigate this phenomenon, we placed the SHL copper mesh in water and air, respectively, and tested the change in its contact angle over time. The results showed that the surface of the samples remained SHL after three days (as shown in Fig. S2, ESI†). Therefore, all tests in this paper were performed within 3 days after sample preparation to ensure that the results were not affected by the recovery of hydrophobicity. The preparation of the cubic structure of the sample was also divided into three steps, as shown in Fig. 1b. These steps were cutting and folding of the copper mesh, the combination and fixation of the copper mesh and thin copper strips, and the construction and molding of the cubic structure. The copper mesh was cut into a rectangle of 4 × 8 cm. A 0.01 mm thick copper sheet was cut into 0.5 × 8 cm copper strips, as shown in Fig. 1b, to prepare a 1 × 3 × 4 cm matchbox structure for the fog collector.

Fig. 1 (a) Preparation of a copper mesh with asymmetric wettability. (b) Preparation of a double-layer mesh that was overlapped and assembled to form a matchbox fog collector.

As shown in Fig. 1, the mesh surface relies on the capture of droplets by inertial collision, therefore the impact of vertical fog flow through the surface of the mesh with different specifications and the inflow of the streamline may produce deviations, resulting in different aerodynamic efficiency. When the mesh porosity is small, the fog droplets block the mesh after merging.^11,28,29 At this time, the mesh plane becomes equivalent to a “plate”,³⁰ and the fog flow vertically impacts the “plate” surface and detours, forming a “bypass flow” (as shown in Fig. S3, ESI†). This causes the fog harvesting ability via inertial collision with the mesh plane to decline and present obvious defects. When the mesh porosity is large, the number of hydrophilic sites on the mesh surface that can capture fog droplets decreases for the same surface area, while the airflow resistance also reduces; therefore, water droplets are more likely to coalesce and be deposited on the mesh. In order to ensure that the fog harvesting performance of the mesh is aerodynamically efficient, we tested the fog harvesting efficiency of copper mesh with different specifications under the same environment and operational conditions, as shown in Fig. 1. Among the samples tested, the 60-mesh copper mesh had the highest fog water-harvesting performance. By further optimizing the mesh specifications, this study indicated the 60-mesh copper mesh to be the research object here for characterization (Fig. S4, ESI†). To maximize the fog harvesting performance of the Janus membrane, this study used a double-layer copper mesh with different wettability to assemble an MJMFC; here, droplets blocked the mesh holes and the micropores on the surface of the mesh, therefore, capillary suction was used to drive droplets to the hydrophilic side. The MJMFC not only makes up for the defects of the traditional mesh surface that may be blocked by the droplets, but it also ensures that there are enough sites to harvest water on the surface of the mesh to have excellent fog harvesting performance. The MJMFC not only compensates for the defects in the droplet blockage on the surface of the traditional mesh, but it also ensures that there are enough points to harvest water points on the surface of the mesh to ensure excellent fog harvesting performance. Fig. S3 (ESI†) shows a schematic for the fog harvesting process with a sample (mesh) surface with a vertical fog outlet and a copper mesh surface; fog flow collision occurs to capture droplets, and the small droplets adhere to the surface of the copper mesh, while the smaller droplets merge to expand into larger droplets. Some of the droplets reached the critical size under the gravitational force and suffered the effect of shedding. The other fraction of the intercepted droplets was transported directly from the hydrophobic surface to the hydrophilic surface under the directional transport through the Janus membrane. Finally, these droplets fell off at the lower end of the mesh, allowing the advantages of directional transport and timely drainage of fog droplets.

As shown in Fig. 2, the Janus membrane transports droplets by capillary action to suck droplets through the micropores; the droplets then spontaneously permeate to the hydrophilic side along the wettability gradient. The capillary force can be calculated using the Laplace's formula:^31,32

where γ is the surface tension; r is the pore radius, and θ is the water contact angle. This equation shows that the capillary force is inversely proportional to the mesh pore diameter. A small pore diameter r or a strong capillary pressure effect causes a viscosity-induced resistance to the absorption of water by the tiny pores slowing down the movement of water. The transmembrane transport of water droplets across the membrane surface, when the hydrophilic/hydrophobic sides of the Janus membrane are exposed to a fog stream, is depicted in Fig. 2. When the hydrophilic side of the Janus membrane is exposed to the fog flow, the droplets come in contact with the hydrophilic mesh, where they are absorbed and spread on the mesh surface in the form of a water film; no transmembrane movement occurs due to the transverse capillary force induced by the wetting gradient, hindering the penetration of the water film to the hydrophobic side. In contrast, when the hydrophobic surface is exposed to the fog flow, the droplets initially stay on the hydrophobic surface until they are squeezed into the pores of the mesh under the action of an external force, penetrating along the wettability gradient to the hydrophilic side under the action of the capillary force. Afterward, the droplets are immersed parallel to the surface of the membrane once they arrive in the hydrophilic region, and finally, they detach from the lower end of the mesh.³³ This structure helps achieve directional transport of droplets across the membrane inside the Janus membrane, accelerating fog circulation and improving the fog harvest operation.

Fig. 2 Mechanism of the action of the capillary tube force on the Janus membrane surface.

2.2. Material characterization

The test results in Fig. 3a show that after 12 min of etching with ammonia, the surface of the sample exhibits super-hydrophilicity. A light blue color appears on the surface of the copper mesh, and at 14 min of etching and afterward, the surface of the sample exhibits a uniform light blue color with more stable hydrophilicity. Subsequently, an ODT solution was used to modify the samples which caused the surface of the samples to show superhydrophobicity with an etching time of 80 min; the samples remained SHB with increasing etching time (Fig. 3b). In order to maintain the accuracy of the sample preparation process and save necessary time, the ammonia etching time for the preparation of the samples in this study was kept uniformly at 14 min, and the modification time was maintained at 80 min. As shown in Fig. 3c, as compared with the pristine copper mesh, the surface of the SHB mesh presented a lower adhesive force, and the kinetic energy of the water droplets on the surface was higher than those on the original copper mesh surface, which helps shed droplets.

Fig. 3 (a) Etching time-versus-contact angle of the copper mesh. (b) Modification time-versus-contact angle of the copper mesh. (c) A schematic representation of the adhesion behavior on the surface of the SHB copper mesh.

As shown in Fig. 4a–c, the surface of the acid-washed copper mesh is relatively flat, but ammonia etching generates Cu(OH)₂ with a micro-nano-needle structure on the surface of the copper wire, inducing the mesh to acquire SHL. The surface of the linear micro-nano-needle-like structure obtained after modification with ODT is rougher, and ODT is grafted onto Cu(OH)₂. At this time, the copper mesh gains SHB. The transformation equation for a chemical reaction is shown in Fig. S5 (ESI†). Analyzing the EDS images of the copper mesh (Fig. 4d–f) and the normalized images of the EDS element in the local area of the copper mesh (Fig. S6a and b, ESI†) reveals that the O content in the surface of the original copper mesh (OS) after ammonia etching increased significantly, where the surface was composed of uniformly distributed micro-nanoporous structures of Cu(OH)₂. Furthermore, the carbon content on the surface of the SHL copper mesh after hydrophobic modification also increased, and the uniform distribution of C and S elements can also be observed on the surface of the copper mesh in the EDS images (Fig. 4d–f). These observations confirm that thiol has successfully grafted onto the surface of Cu(OH)₂, successfully preparing the SHB copper mesh. According to the Fourier infrared spectroscopy analysis in Fig. 5a, the characteristic peaks of –OH appeared on the surface of the copper mesh at 3301 cm⁻¹ and 3335 cm⁻¹ after etching with ammonia alkali, proving that Cu(OH)₂ was generated on the surface of the copper mesh by chemical etching, producing a copper mesh with superhydrophilicity. The SHB copper mesh obtained by ODT modification was characterized, and the peaks of –CH₂ appeared in the infrared spectra at 2616 cm⁻¹ and 2850 cm⁻¹, indicating that ODT was successfully grafted onto the surface of Cu(OH)₂, yielding an SHB copper mesh with successful modification. The full XPS spectrum in Fig. 5b shows that the C content on the surface of the thiol-modified sample increased significantly; after fitting the C 1s peak (Fig. 5c), a C–C bond appeared at 284.8 eV, and a C–S bond appeared at 285.4 eV, indicating that ODT was successfully grafted onto the surface of Cu(OH)₂. In addition, peak fitting for elemental Cu in the OS and SHL samples (Fig. 5d) showed that the Cu 2p_1/2 peak appeared at 954.5 eV and the Cu 2p_3/2 peak appeared at 932.6 eV after ammonia etching. Therefore, a double peak appeared for the strong Cu²⁺ satellites, which, in combination with the previous FTIR analyses, suggests that the surface of the copper mesh was generated with Cu(OH)₂ after ammonia etching.

Fig. 4 (a)–(c) SEM images of OS, SHL, and SHB copper mesh surfaces. (d)–(f) EDS images of Cu, O, C, and S on the OS, SHL, and SHB copper mesh surfaces.

Fig. 5 (a) FTIR analysis profiles of pristine, SHL, and SHB copper mesh surfaces. (b) XPS full spectrum of OS, SHL, and SHB sample surfaces. (c) C 1s peaks fitted to the surface of the SHB sample. (d) Cu bifurcation spectra from XPS analysis before and after etching.

In order to reveal the shedding mechanism of the droplets on the surface of the Janus membrane, this study investigated the nucleation process of the droplets on different surfaces as shown in Fig. 6. The small droplets on the surface of the pristine copper mesh (Fig. 6a) merged slowly, and the droplets on the surface of the superhydrophilic copper mesh (Fig. 6b) were captured at a faster speed, where the fog droplets were captured and merged to form a water film on the surface of the copper mesh. Droplets on the surface of the SHB copper mesh (Fig. 6c) expanded into larger droplets that rapidly dislodged from the surface, with a short start-up time and a fast surface refreshment rate. The hydrophobic surface of the double-layer mesh (Fig. 6d) was exposed to the fog, and the large droplets on the surface of the hydrophobic mesh encountered the hydrophilic mesh. Under the action of the wettability gradient, directional transport to the hydrophilic side of the mesh was carried out.

Fig. 6 (a)–(d) Droplet behavior on the surface of OS, SHL, SHB, SHL/SHB samples.

2.3. Fog harvesting performance test

As shown in Fig. 7a, it can be seen that gaps are expected in the middle of the Janus membrane structure which is composed of a copper mesh. This study assembled samples with different gaps (s = 0, 1, 2, 3 mm) and tested their water harvesting performances. The results showed that the copper meshes with different wettability were separated from each other, therefore, the samples lost the ability to allow directional transport, and the fog harvesting efficiency was substantially reduced. To further verify the water harvesting advantage of the Janus membrane structure, the water harvesting performance of SHL/SHL, SHB/SHL, and SHL/SHB samples was tested under the fog flow rate of 0.8 m s⁻¹. The water harvesting volume of the samples was recorded once every 10 minutes 10 times for a test duration of 100 min; the results are shown in Fig. 7d to show that the samples with the Janus membrane structure had 1.47 and 1.33 times higher fog harvesting efficiencies than SHL/SHL and SHB/SHL, respectively, indicating that directional transport plays an important role in the fog harvesting process.

Fig. 7 (a) Effect of spacing on the fog harvesting performance of samples. (b) Fog harvesting performance of SHL/SHL, SHB/SHL, and SHL/SHB samples. (c) Comparison of fog harvesting efficiencies of OS, SHL, SHB, SHB, SHB/SHL, and SHL/SHB samples. (d) Different stages of the samples and the fog harvesting process. (e) and (f) UV and acid/alkali resistance tests on the SHB mesh surfaces. (g) 10 fog cycles. (h) Effect of fog flow direction on the fog harvesting performance of samples. (i) 10 fog cycles. (j) Effect of fog flow direction on the fog harvesting performance of samples (k) fog harvesting performance of samples. (l) Fog harvesting performance of the SHB mesh surface for acid and alkali resistance. (g) 10 fog cycles. (h) Effect of the fog flow direction on the fog harvesting performance.

The fog harvesting test environment was kept unchanged to test the fog harvesting performance of the OS, SHL, SHB, SHB/SHL, and SHL/SHB samples, and the results are shown in Fig. 7c. The fog harvesting ability of the SHL/SHB sample was 1.58 and 1.22 times higher than that of the OS and SHB/SHL samples, respectively, indicating that both the chemical modification and asymmetric wettability-induced intermembrane orientation and transportation positively enhanced sample performance. It positively enhanced the fog harvesting performance of the samples and the chemical modification and asymmetric wettability-induced intermembrane orientation and transportation.

The durability of the fog collector was also an important factor we considered. In order to test the long-term effective use of the samples in harsh environments, the SHB samples were exposed to the UV lamp at 10 cm for 12 h, and the contact angle was tested every 2 h. The results are shown in Fig. 7e, where the contact angle of the samples was almost unchanged, indicating that the SHB samples had good UV resistance. To examine the durability of the samples and to effectively characterize the damage to their surface properties, we performed pH tests and 10 fog cycle tests on the SHL/SHB samples. At the end of each test, the contact angle of the SHB copper mesh was tested, and the change in it was used to characterize the degradation of the sample's performance. As shown in Fig. 7f, the sample exhibits excellent acid and alkali resistance, and Fig. 7g illustrates that after the sample underwent 10 fog cycles, the fog harvesting efficiency exhibited a gentle downward trend, but maintained a high level of performance. The fog harvesting efficiency decreased by 6.7% after 10 cycles, and reached a value of 5.4 g h⁻¹ cm⁻². After 10 fog cycles, the contact angle of the SHB copper mesh loses about 6.8°, while remaining above 144° with good hydrophobicity, indicating that the samples have good durability and weathering resistance. It also shows that the samples can realize long-term effective use in harsh environments. The special matchbox structure ensures an effective fog harvesting process, when the fog flow is blown to the sample surface from all directions. In this paper, the direction of the fog flow was changed by varying the tilt angle of the sample (0, 15, 30, 45, and 60°). The efficiency of fog harvesting by the SHB samples in the same fog flow environment was in the range of 4.8–6.2 g h⁻¹ cm⁻², maintaining it as a highly efficient fog and water harvesting process. Compared with recent literature, the three-dimensional fog collector prepared in this study has excellent fog water collection performance (Fig. S7, ESI†).

3. Conclusion

The mesh with asymmetric wettability achieved the trans-membrane transport of droplets inside the mesh through capillary action on the membrane surface, which helped realize directional transport and timely drainage of water droplets. The internal transport process of the membrane reduced the evaporation rate of water. The three-dimensional structure of the matchbox ensured effective fog collection by the sample material even in dynamically changing fog flow environments. An efficient fog collection process was achieved by continuous interception and directional transport of the fog droplets, with fog collection efficiencies ranging from 4.8 to 6.2 g h⁻¹ cm⁻². MJMFC achieved a fog harvesting efficiency of 6.9 g h⁻¹ cm⁻², which was 1.58 times higher than that of the original mesh. The SHB sample maintained a water harvesting efficiency of 5.4 g h⁻¹ cm⁻² after 10 fog cycles, which is an indication of its excellent durability that can enable the realization of a long-term, continuous, and effective fog harvesting process.

Data availability

The authors declare that all data in this manuscript are available upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (no. 52442507).

References

1. N. S. Wigginton, Gauging vulnerability to water shortages, Science, 2016, 351(6275), 828.
2. W. Musie and G. Gonfa, Fresh water resource, scarcity, water salinity challenges and possible remedies: A review, Heliyon, 2023, 9(8), e18685,  DOI:10.1016/j.heliyon.2023.e18685.
3. M. Gavrilescu, Water, soil, and plants interactions in a threatened environment, Water, 2021, 13(19), 2746,  DOI:10.3390/w13192746.
4. J. D. Hunt, N. D. Weber, B. Zakeri, A. T. Diaby, P. Byrne, W. Leal Filho and P. S. Schneider, Deep seawater cooling and desalination: Combining seawater air conditioning and desalination, Sustainable Cities Soc., 2021, 74, 103257,  DOI:10.1016/j.scs.2021.103257.
5. X. Wang, M. Liang, J. Zhang, X. Chen, M. Zaw, T. Z. Oo, N. W. Lwin, S. H. Aung, Y. Chen and F. Chen, Double-photoelectrode redox desalination of seawater, Water Res., 2023, 239, 120051,  DOI:10.1016/j.watres.2023.120051.
6. J. Di, H. Wang, L. Zhang, Z. Chen, Y. Zhang, B. B. Mamba, M. Qiu, J. Guo and L. Shao, Recent progress in advanced polyamide nanofiltration membranes via interfacial polymerization for desalination and beyond, Desalination, 2024, 592, 118167,  DOI:10.1016/j.desal.2024.118167.
7. Y. Hu, Y. Yang, S. Yu, X. C. Wang and J. Tang, Psychrophilic anaerobic dynamic membrane bioreactor for domestic wastewater treatment: Effects of organic loading and sludge recycling, Bioresour. Technol., 2018, 270, 62–69,  DOI:10.1016/j.biortech.2018.08.128.
8. N. Yuan, Z. Li, Q. Shang, X. Liu, C. Deng and C. Wang, High efficiency of drinking water treatment residual-based sintered ceramsite in biofilter for domestic wastewater treatment, J. Environ. Manage., 2024, 354, 120401,  DOI:10.1016/j.jenvman.2024.120401.
9. G. Gu, X. Yang, Y. Li, J. Guo, J. Huang, E. N. Nxumalo, B. B. Mamba and L. Shao, Advanced zwitterionic polymeric membranes for diverse applications beyond antifouling, Sep. Purif. Technol., 2025, 356, 129848,  DOI:10.1016/j.seppur.2024.129848.
10. Z.-H. Liang, R. Feng, J.-M. Wu, D. Li, F. Wang, X.-L. Wang, Y.-Z. Wang and F. Song, Temperature-gradient-induced enhanced fog collection on polymer brush surfaces, Chem. Eng. J., 2023, 455, 140785,  DOI:10.1016/j.cej.2022.140785.
11. Y. Huang, Y. Zhang, Y. Li and Z. Tan, Waterdrop-assisted efficient fog collection on micro-fiber grids, Chem. Eng. J., 2024, 481, 148423,  DOI:10.1016/j.cej.2023.148423.
12. D. Yan, Y. Chen, J. Liu and J. Song, Super-fast fog collector based on self-driven jet of mini fog droplets, Small, 2023, 19(36), e2301745,  DOI:10.1002/smll.202301745.
13. M. Rajaram, X. Heng, M. Oza and C. Luo, Enhancement of fog-collection efficiency of a Raschel mesh using surface coatings and local geometric changes, Colloids Surf., A, 2016, 508, 218–229,  DOI:10.1016/j.colsurfa.2016.08.034.
14. J. de Dios Rivera and D. Lopez-Garcia, Mechanical characteristics of Raschel mesh and their application to the design of large fog collectors, Atmos. Res., 2015, 151, 250–258,  DOI:10.1016/j.atmosres.2014.06.011.
15. E. S. Shanyengana, J. R. Henschel, M. K. Seely and R. D. Sanderson, Exploring fog as a supplementary water source in Namibia, Atmos. Res., 2002, 64, 251–259.
16. R. Holmes, J. de Dios Rivera and E. de la Jara, Large fog collectors: New strategies for collection efficiency and structural response to wind pressure, Atmos. Res., 2015, 151, 236–249,  DOI:10.1016/j.atmosres.2014.06.005.
17. T. Deng, Y. Chen, Y. Liu, Z. Shang and J. Gong, Constructing Janus microsphere membranes for particulate matter filtration, directional water vapor transfer, and high-efficiency broad-spectrum sterilization, Small, 2022, 18(51), e2205010,  DOI:10.1002/smll.202205010.
18. H. Zhou, M. Zhang, C. Li, C. Gao and Y. Zheng, Excellent fog-droplets collector via integrative Janus membrane and vonical spine with micro/nanostructures, Small, 2018, 14(27), e1801335,  DOI:10.1002/smll.201801335.
19. L. Chen, W. Li, Z. Gan, Y. Zhou, M. Chen, D. Cui, H. Ge, P. K. L. Chan, L. Wang and W.-D. Li, Ultrathin metal-mesh Janus membranes with nanostructure-enhanced hydrophobicity for high-efficiency fog harvesting, J. Cleaner Prod., 2022, 363, 132444,  DOI:10.1016/j.jclepro.2022.132444.
20. S. Song, Y. Zhang, T. Yu and J. Yang, A new Janus mesh membrane with ultrafast directional water transportation and improved fog collection, J. Mater. Sci. Technol., 2024, 202, 129–139,  DOI:10.1016/j.jmst.2024.03.030.
21. L. J. Zheng, M. L. Wang, D. H. Kang, N. K. Kim, K. Gao, W. Lee and H. W. Kang, Janus membrane with heterogeneous wettability top surface for fog harvesting, Appl. Surf. Sci., 2024, 669, 160508,  DOI:10.1016/j.apsusc.2024.160508.
22. Y. Su, S. Cai, T. Wu, C. Li, Z. Huang, Y. Zhang, H. Wu, K. Hu, C. Chen, J. Li, Y. Hu, S. Zhu and D. Wu, Smart stretchable Janus membranes with tunable collection rate for fog harvesting, Adv. Mater. Interfaces, 2019, 6(22), 1901465,  DOI:10.1002/admi.201901465.
23. H. Gao, H. Zhao, S. Chang, Z. Meng, Z. Han and Y. Liu, Multi-biomimetic double interlaced wetting Janus surface for efficient fog collection, Nano Lett., 2024, 24(25), 7774–7782,  DOI:10.1021/acs.nanolett.4c01918.
24. W. Zhou, C. Zhou, H. Yang, J. Wang, J. Du, L. Chen, H. Shen, L. Tan, L. Dong and X. Zeng, Janus copper mesh with asymmetric wettability for on-demand oil/water separation and direction-independent fog collection, J. Environ. Chem. Eng., 2021, 9(5), 105899,  DOI:10.1016/j.jece.2021.105899.
25. J. Y. Kim, J. H. Kang, J. W. Moon and S. Y. Jung, Improvement of water harvesting performance through collector modification in industrial cooling tower, Sci. Rep., 2022, 12(1), 4658,  DOI:10.1038/s41598-022-08701-3.
26. J. A. Johnstone and T. E. Dawson, Climatic context and ecological implications of summer fog decline in the coast redwood region, Proc. Natl. Acad. Sci. U. S. A., 2010, 107(10), 4533,  DOI:10.1073/pnas.0915062107.
27. R. Vautard, P. Yiou and G. J. van Oldenborgh, Decline of fog, mist and haze in Europe over the past 30 years, Nat. Geosci., 2009, 2(2), 115–119,  DOI:10.1038/ngeo414.
28. A. Mukhopadhyay, A. Datta, P. S. Dutta, A. Datta and R. Ganguly, Ranjan Ganguly, Droplet morphology-based wettability tuning and design of fog harvesting mesh to minimize mesh-clogging, Langmuir, 2024, 40(15), 8094–8107.
29. J. Li, W. Li, X. Han and L. Wang, Sandwiched nets for efficient direction-independent fog collection, J. Colloid Interface Sci., 2021, 581, 545–551,  DOI:10.1016/j.jcis.2020.07.153.
30. J. de Dios Rivera, Aerodynamic collection efficiency of fog water collectors, Atmos. Res., 2011, 102(3), 335–342,  DOI:10.1016/j.atmosres.2011.08.005.
31. N. Mao and S. J. Russell, Capillary pressure and liquid wicking in three-dimensional nonwoven materials, J. Appl. Phys., 2008, 104(3), 034911,  DOI:10.1063/1.2965188.
32. G.-Y. He, H.-K. Tsao and Y.-J. Sheng, Capillary flow in nanoporous media: effective Laplace pressure, Colloids Surf., A, 2024, 699, 134499,  DOI:10.1016/j.colsurfa.2024.134499.
33. J. H. Lee, Y. J. Lee, H. Y. Kim, M. W. Moon and S. J. Kim, Unclogged Janus Mesh for Fog Harvesting, ACS Appl. Mater. Interfaces, 2022, 14(18), 21713–21726,  DOI:10.1021/acsami.2c03419.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nh00458b

‡ The authors contributed equally to this work.

---