Desert beetle-inspired fog-harvesting surfaces integrating buckled microchannels and alternating wettability

Abstract

In this study, we present a novel fog-harvesting surface featuring buckled microstructures with alternating hydrophilic–hydrophobic regions, inspired by the elytra of desert beetles. The surface was fabricated by dispersing hydrophilic SiO₂ particles on hydrophobic polytetrafluoroethylene (PTFE) buckled patterns. The hydrophilic protrusions promote droplet condensation and capture, whereas the hydrophobic buckled surfaces, functioning similarly to microchannels, facilitate directional transport to enhance water collection. Compared to flat surfaces, buckled structures can improve water collection efficiency by approximately 30%. This design with alternating wettability was systematically compared with fully hydrophilic and fully hydrophobic surfaces. Various parameters, including the fog flow distance, hydrophilic particle composition, surface wettability distribution, and buckle alignment, were investigated. Furthermore, the geometric funnel-shaped growth pattern of bird's nest fern leaves was incorporated to guide condensed droplets toward the collection zone, forming a dual-biomimetic mechanism that integrates microscale desert beetle-inspired structures with macroscale fern-inspired geometry. Real-world field tests—specifically, overnight fog collection experiments conducted in a forest environment—further confirmed the excellent performance of our device. Notably, the entire fabrication process can be conducted under ambient conditions; high-vacuum techniques and costly equipment are not required. The developed high-efficiency fog-harvesting device offers a scalable, sustainable, and inexpensive solution to atmospheric water collection, contributing to improved water accessibility in water-scarce regions.

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1. Introduction

The problem of global water scarcity is receiving increasing attention. Factors such as rainfall instability due to climate change, a growing demand for water because of rapid population growth, and water pollution resulting from industrialization have increased the difficulty of obtaining and storing clean freshwater resources. Fog-harvesting technology has garnered considerable interest as a potential solution to this problem. In this technology, components with specialized structure or surface properties are employed to capture moisture from the air. The design of the components is often inspired by biological systems, with some of the most widely studied being desert beetles, cacti, spider silk, and Nepenthes alata (pitcher plants).^1,2 These organisms exhibit structural features that promote the collection and transport of water, inspiring the development of various fog-harvesting materials. For example, some studies have mimicked the patterned surfaces, with alternating hydrophilic and hydrophobic domains and granular protrusions, on the elytra of desert beetles;^3–10 others have replicated the needle-like leaves of cacti, the unique geometry of which causes water droplets to spontaneously migrate from the tip toward broader, larger radius regions.^11–14 Some researchers have imitated the spindle-shaped structures of spider silk, which guide water droplets toward spindle zones,^15–18 whereas others have modeled the lubricant-infused, low-sliding-angle surfaces on the peristome of pitcher plants, on which droplets are highly mobile, or the microgrooves with hierarchical structures that facilitate directional water transport in the plants.^19–22 In addition to mimicking individual organisms, many scholars have integrated multiple biological strategies to develop efficient fog-harvesting materials by combining complementary mechanisms.^23–28

The desert beetle was the first organism discovered to exhibit fog-harvesting behavior. To survive in extremely arid desert environments, it has evolved a unique elytral structure. When the ambient humidity increases, the beetle performs a specialized “fog-basking” behavior in which the body is tilted forward.²⁹ Parker et al. reported that the elytra of the desert beetle possess granular protrusions and exhibit a surface pattern of alternating hydrophilic and hydrophobic regions.³ This unique surface architecture, combined with the beetle assuming a wind-facing posture, enables the beetle to intercept fog droplets and rapidly direct the collected water toward its mouth along hydrophobic pathways. Inspired by the desert beetle, a variety of fog-harvesting devices have been developed. Surfaces composed of hydrophilic and hydrophobic regions have been widely utilized to enhance fog collection. For example, Lai et al. wove superhydrophilic and superhydrophobic fibers into a patterned surface with alternating wettability, and discovered that optimizing the hydrophilic-to-hydrophobic ratio maximized fog-harvesting performance.⁴ Wang et al. sprayed polydimethylsiloxane (PDMS) aerogel onto a substrate to create a hydrophobic layer, and then deposited platinum through a mask to create an alternating hydrophilic–hydrophobic patterned surface. This design had higher fog harvesting efficiency than did purely superhydrophobic or superhydrophilic surfaces.⁵ In addition to surface wettability, surface roughness is a key factor influencing fog-harvesting capacity.^7,30 Kim et al. fabricated devices with three-dimensional (3D) hydrophilic protrusions by using PDMS molding techniques and demonstrated that the fog harvesting efficiency resulting from 3D structures was considerably higher than that achieved using two-dimensional (2D) patterned designs.⁷

In general, surface roughness and the combination of hydrophilic with hydrophobic domains are both crucial to fog collection performance. However, some researchers have argued that surface roughness is the primary factor influencing fog collection, to the point where the effect of intrinsic wettability may be secondary or negligible. They claim that surfaces comprising hydrophobic granular protrusions can achieve high fog collection efficiency, even higher than those with alternating hydrophilic–hydrophobic patterns. For example, Guo et al. developed fog-harvesting surfaces by embossing hemispherical bumps onto copper sheets, and demonstrated that this granular-protrusion-based design outperformed a design featuring hydrophilic–hydrophobic protrusions.⁶ Although the desert-beetle-inspired water collection mechanism serves as a foundational model for fog-harvesting devices, certain aspects of the model remain under debate. In addition, researchers of this topic have employed differing experimental setups, measurement methods, and environmental conditions, rendering direct comparisons between studies difficult. Moreover, fog-harvesting devices are often fabricated using expensive equipment and high-vacuum processes such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), which hinders large-scale production and industrial applicability. Therefore, elucidating the mechanisms underlying fog harvesting and developing inexpensive, high-performance and large-area devices remain major challenges in this field.

In this study, we present a novel fog-harvesting surface featuring a buckled microstructure with alternating hydrophilic and hydrophobic regions, inspired by the elytra of desert beetles. The surface was fabricated by dispersing hydrophilic particles on a hydrophobic buckled pattern. The hydrophilic protrusions promote droplet condensation and capture, whereas the hydrophobic buckled surfaces, which function as microchannels, facilitate directional transport to enhance water collection. We systematically compared the alternating wettability design with fully hydrophilic and fully hydrophobic designs. To ensure a meaningful comparison, fog-harvesting parameters such as the flow distance, velocity, and rate were selected in accordance with those reported in other studies. Furthermore, by incorporating the funnel-shaped geometry of bird's nest fern leaves, a dual-biomimetic design was achieved that synergistically enhances fog interception and large-scale directional water transport. Real-world field tests confirmed the excellent performance of our device. Importantly, the entire process of fabricating the device can be conducted under ambient conditions; high-vacuum techniques such as CVD or PVD are not required. To the best of our knowledge, this is the first study to integrate buckled microchannel structures with alternating wettability, together with a fern-inspired macroscopic geometry, to enable high-efficiency fog harvesting over a large area.

2. Experimental

2.1 Materials

Polytetrafluoroethylene (PTFE) particle solution (60 wt% aqueous dispersion), TERGITOL™ TMN-10, silicon dioxide (SiO₂) particles (diameters of 5 and 10 μm), zinc acetate, and hexamethylenetetramine (HMT) were obtained from Sigma-Aldrich. Tetraethoxysilane (TEOS, 98%) and methyltriethoxysilane (MtES, 98%) were purchased from Alfa Aesar and used without further purification. Ethanol was acquired from J. T. Baker. Ammonia solution (25%), potassium chloride (>98%), and sodium hydroxide (99.38%) were supplied by Choneye Pure Chemicals. Diatomaceous earth was sourced from First Chemical, and ethanol (95%) from Echo Chemical. The commercially available pre-strained PS shrink film (KSF50-C, Grafix), an amorphous polymer, was used as the substrate. The film undergoes significant shrinkage upon heating above its glass transition temperature (T_g). Commercially available copper, aluminum, silicon dioxide, titanium dioxide, polystyrene (PS), and Teflon sheets were used as substrates for comparison in fog harvesting efficiency tests.

2.2 Fabrication of buckled-engineered efficient texture for liquid extraction and scavenging (BEETLES) surfaces

Based on the method developed by Stöber et al., 1 μm SiO₂ particles were synthesized using the sol–gel process.³¹ The PTFE precursor solution was diluted with deionized water, and SiO₂ particles were added to the PTFE dispersion at various weight ratios. TMN-10 surfactant was introduced into the mixture to facilitate the formation of homogeneous PTFE micelles and SiO₂-blended dispersions. The BEETLES surface was fabricated by drop-casting 1 mL of a dispersion containing varying ratios of SiO₂ and PTFE particles onto a heat-shrinkable PS sheet (8 cm × 8 cm), after which spin-coating was performed at 3000 rpm for 30 seconds. The weight ratios of hydrophilic to hydrophobic particles in the dispersion varied as 1 : 50, 1 : 100, 1 : 200, 1 : 300, 1 : 400, and 1 : 500; the solid content was adjusted to 10, 20, and 30 wt%; and the SiO₂ particle sizes employed were 1, 5, and 10 μm. The heat-shrinkable PS sheets were cleaned with ethanol and deionized water, followed by drying under a high-purity nitrogen stream. To further remove organic contaminants and render the surface slightly hydrophilic, the PS sheets were treated with UV ozone for 10 minutes prior to coating. The T_g of the PS sheet was approximately 110 °C, and the melting temperature (T_m) was around 230 °C. To relieve internal stress in the PS sheets and to induce shrinkage, the PS sheets coated with SiO₂/PTFE dispersions were thermally treated in an oven at 140 °C for 10 minutes. The original size of each sample was 8 cm × 8 cm; after thermal treatment, the sample size shrank to approximately 3.2 cm × 3.2 cm due to the shrinkage effect. As a result, desert beetle-inspired surfaces with hierarchical micro-scale buckles and alternating hydrophilic SiO₂ and hydrophobic PTFE regions were successfully fabricated.

2.3 Fabrication of BEETLES surfaces using different materials

To extend the applicability of BEETLES for fog harvesting, silicon dioxide (SiO₂) particles were replaced with other hydrophilic materials. Diatomaceous earth, zinc oxide (ZnO), and calcium carbonate (CaCO₃) were selected as alternative fillers and processed using the optimized parameters previously established for SiO₂-based composites. Diatomaceous earth was commercially sourced and used without further purification. The calcium carbonate was regenerated from waste oyster shells and purified by calcination at 500 °C to remove impurities. The two solutions were prepared separately and then combined under vigorous magnetic stirring. Stirring was continued for 24 hours to promote the formation of the calcite phase of CaCO₃. ZnO particles were synthesized via a hydrothermal method.^32,33 In brief, 0.3 g of zinc acetate (Zn(CH₃COO)₂) and hexamethylenetetramine (HMT) were each dissolved in 27 mL of deionized (DI) water. Subsequently, 10 mL of 2 M sodium hydroxide (NaOH) solution was added to the mixture. The resulting solution was transferred to a hydrothermal autoclave and heated at 70 °C for 20 hours. After cooling to room temperature, the suspension was filtered and dried, yielding flower-like ZnO particles in powder form. Following the synthesis of diatomaceous earth, ZnO, and CaCO₃ particles, the fabrication of BEETLES surfaces was carried out using the same procedure as described for the SiO₂-based composites. Moreover, for comparison of the fog harvesting efficiency with BEETLES surfaces, the hydrophilic flat surface was fabricated by sputtering platinum onto a heat-shrinkable PS substrate, while the hydrophilic buckled surface was produced by subjecting the platinum-sputtered substrate to thermal treatment. The hydrophobic flat surface was prepared by spin-coating a 30 wt% micellar PTFE particle dispersion onto a heat-shrinkable PS substrate at 3000 rpm for 30 seconds. The hydrophobic buckled surface was then obtained by thermally treating the hydrophobic flat surface. Both the hydrophilic and hydrophobic buckled surfaces were thermally treated at 140 °C for 10 minutes.

2.4 Fabrication of fog-harvesting devices with aligned microbuckled structures

Following the same experimental procedure described previously, a particle-blended dispersion was spin-coated onto a UV ozone-treated PS sheet. The coated PS sheet was then placed on a glass substrate and mechanically clamped at both ends along a single axis, as illustrated in Fig. S4. The clamped sample was subsequently subjected to thermal treatment in an oven at 140 °C for 10 minutes. Upon cooling to room temperature, the clamped axis retained its original length, while the perpendicular, unconstrained axis shrank to approximately 3.2 cm due to heat-induced contraction. This uniaxial shrinkage resulted in the formation of aligned microbuckles on the surface.

2.5 Measurement of fog harvesting efficiency

Fog-harvesting performance was evaluated using a custom-built fog simulation system operated at 25 °C. The setup consisted of an ultrasonic humidifier housed within an acrylic chamber measuring 45 cm × 45 cm × 55 cm. Gaps on the side panels of the chamber were included to allow excess fog to escape, ensuring stable fog conditions. The ultrasonic humidifier generated fog at an air velocity of 1.2 m s⁻¹ and an air volume of 0.072 m³ min⁻¹. During testing, the samples were positioned vertically on an aluminum collection tray sealed with a membrane, leaving only a slit through which condensed water droplets could pass. The sample surfaces were oriented away from the direct fog stream to minimize the influence of high-speed airflow on collection performance. Each test was conducted for 1 hour. The weight of the aluminum tray was measured before and after testing by using an electronic balance to determine the amount of collected water. Each sample was tested three times, and the average fog harvesting efficiency along with the standard deviation was calculated.

2.6 Characterization

The microstructure morphology of the surfaces was examined using a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM) operated at an accelerating voltage of 3 kV. Prior to imaging, the samples were sputter-coated with a thin layer of platinum under vacuum to enhance surface conductivity. Energy-dispersive X-ray spectroscopy (EDS) analysis was performed using a ZEISS ULTRA PLUS scanning electron microscope at an accelerating voltage of 7 kV to obtain elemental distribution maps. Bright-field transmission electron microscopy (TEM) was employed to analyze the distribution of micellar PTFE particles, using a JEOL JEM-2010 TEM operated at 200 kV. The static contact angle and sliding angle of water droplets on the sample surfaces were measured with an SEO Phoenix 300 contact angle goniometer. The static water contact angle was determined with a 2 μL droplet of deionized water. The sliding angle was measured by placing a 5 μL droplet of deionized water on the sample surface, tilting the sample stage of the goniometer, and monitoring the droplet movement with a camera to record the angle at which sliding occurred. Particle size distribution was characterized using a dynamic light scattering (DLS) instrument (nanoSAQLA, Otsuka Electronics). The condensation and flow behavior of water droplets on a surface was monitored using a Dino-Lite AM8917MT8 handheld digital microscope.

3. Results and discussion

3.1 Fabrication of BEETLES surfaces

To imitate the surface structure of desert beetles, which consists of bumpy structures with alternating hydrophilic–hydrophobic surfaces, PTFE and SiO₂ particles were employed as core hydrophobic and hydrophilic materials, respectively. PTFE was selected because of its intrinsic properties, such as its low surface energy, high hydrophobicity, and good chemical resistance resulting from multiple C–F functional groups. On the other hand, SiO₂ is widely occurring in nature, has favorable hydrophilic properties, is non-toxic, and is easily synthesized. As illustrated in Scheme 1a, mixed dispersion solution comprising hydrophilic and hydrophobic particles was prepared by mixing PTFE particles, SiO₂ particles, deionized water, and the nonionic surfactant Triton X-10 (TMN-10), after which the solution was stirred thoroughly. TMN-10 is one of the commonly used alkylphenoxy polyethoxyethanol surfactants and is a branched C₁₂–C₁₄ secondary alcohol ethoxylate. TMN-10, as a surfactant, surrounded the PTFE particles to aid in the formation of PTFE micelles, facilitating dispersion. Thermal retractable PS sheets, which were used as substrates in the buckle formation process, were cleaned with ethanol and then exposed to ultraviolet (UV)-ozone irradiation to remove organic pollutants from the surface, thereby enhancing their hydrophilic characteristics. Subsequently, the mixed particle dispersion was spin-coated onto the pre-cleaned PS sheet to form a thin layer of PTFE@TMN-10/SiO₂ (Scheme 1b). After the sheet was thermally treated at 140 °C, the pre-existing stress in the shrinkable PS sheets was released, resulting in approximately 60% side length shrinkage and the spontaneous formation of micrometer-sized buckles on the surfaces of the PS sheets. The thermal treatment also facilitated thermally driven motion of the TMN-10 surfactant, exposing the PTFE particle surfaces and thereby forming stable superhydrophobic PTFE aggregates. Simultaneously, SiO₂ particles were also exposed (i.e., hydrophilic regions). This process yielded the fog-harvesting surfaces that had hierarchical micrometer-scale buckles coated with alternating hydrophilic SiO₂ and hydrophobic PTFE, as illustrated in Scheme 1c. In this study, the novel buckled fog-harvesting device is referred to as “BEETLES” (Scheme 1d).

Scheme 1 Illustration of the fabrication of desert beetle-inspired, high-efficiency fog-harvesting water collectors, which consist of granular protrusions and alternating hydrophilic–hydrophobic surfaces: (a) spin-coating a dispersion mixture of hydrophilic SiO₂ particles, hydrophobic PTFE particles, and nonionic surfactant TMN-10 onto a commercially available thermally-shrinkable PS sheet; (b) formation of a PTFE@TMN-10/SiO₂ thin layer on the PS sheet. Gray, yellow, white, and green represent the thermally-shrinkable PS sheet, SiO₂ particles, PTFE particles, and TMN-10, respectively; (c) formation of desert beetle-inspired surfaces after thermally driven the motion of the TMN-10 surfactant and spontaneous buckling of the PS sheet, generating hierarchical micrometer-sized buckles covered with alternating hydrophilic SiO₂–hydrophobic PTFE regions; (d) the fabricated BEETLES surface presents an innovative method for fog harvesting.

As mentioned, the elytra of the desert beetle have granular protrusions and a patterned surface with alternating hydrophilic–hydrophobic areas, giving the beetle the ability to harvest atmospheric water (Fig. 1a). Inspired by the body surface of desert beetles, we created a hierarchically buckled nanoporous PTFE film with superhydrophobic properties, randomly decorated the film with hydrophilic protruding SiO₂ particles to enable fog harvesting. Fig. 1b presents field emission scanning electron microscope (FE-SEM) images of SiO₂ particles, which constitute the hydrophilic areas of the desert beetle-inspired surfaces. To enable systematic comparison, SiO₂ particles of various sizes were employed, including SiO₂ particles with a diameter of 1 μm synthesized through the sol–gel method in our laboratory, as well as commercial SiO₂ particles (5 μm and 10 μm in diameter). On the other hand, the bioinspired hydrophobic areas were created through aggregation of PTFE@TMN-10 micelles. To investigate the morphology of the micellized PTFE@TMN-10 colloids, samples were examined without staining using transmission electron microscopy (TEM), as depicted in Fig. 1c. Well-dispersed, dark, spherical objects of similar sizes (i.e., PTFE particles, marked with dashed white circles) surrounded by a gray medium (i.e., TMN-10 surfactant, marked with white arrows) were observed, indicating the formation of micellized PTFE@TMN-10 colloids. Compared with the TMN-10 surfactant, which comprises carbon, hydrogen, and oxygen, the micellized PTFE is composed of fluorine and carbon, resulting in the dark appearance of the microdomains in the TEM images due to mass-thickness contrast. PTFE particles with an average diameter of 200 nm were surrounded by a TMN-10 surfactant layer with an average thickness of 25 nm.

Fig. 1 (a) Artificial-intelligence-generated image of a desert beetle. (b) FE-SEM images of SiO₂ particles with various sizes. (c) TEM image of PTFE colloids surrounded by TMN-10 surfactant. The white arrow marks the TMN-10 surfactant, and the dashed circle indicates the PTFE colloids. (d and e) FE-SEM images of test sample surfaces composed of micellized PTFE/TMN-10 particles and SiO₂ particles before and after thermal treatment, respectively. Insets show enlarged areas, presenting dim and clearly nanoporous surfaces, respectively. (f) FE-SEM-EDS elemental mapping of C, F, O, and Si for the desert beetle-inspired surface, showing hierarchical buckled surfaces with randomly distributed hydrophilic SiO₂ and hydrophobic PTFE particles.

Fig. 1d displays FE-SEM images of micellized PTFE@TMN-10 and SiO₂ particles closely packed and dispersed on thermally-shrinkable PS sheets before thermal treatment. The PTFE particles were clearly fully covered with TMN-10 surfactant, forming a smooth and dim composite layer (inset of Fig. 1d). Additionally, larger SiO₂ particles were randomly dispersed among the micellized PTFE@TMN-10, specifically on the smooth surface. After thermal treatment was applied, which caused the TMN-10 to flow down onto the PS sheet, the PTFE surfaces were exposed, which imparted hydrophobic properties. Meanwhile, micro-level buckles (with a wavelength of ∼25 μm) formed spontaneously and were distributed across the PS sheets due to the sheet's thermal contraction (Fig. 1e). In contrast, due to the flow of TMN-10, the surface exhibited clear nanoporous buckled morphology (i.e., distinct aggregation of PTFE particles) rather than a dim morphology (see the inset of Fig. 1e). The resulting surface displayed hierarchical geometric characteristics: micro-level buckles were homogeneously covered with submicron-level wavelength buckles (∼2 μm), which comprised closely packed PTFE particles. These structures are interconnected in three dimensions, forming nanoporous buckled surfaces. Buckled structures, due to their large surface area and groove-like features, tend to have localized vortices that facilitate the adsorption and capture of surface materials.^32,34 The surfaces composed of SiO₂ and PTFE particles exhibited nanoporous structural features (pore size: around 50–150 nm), which helped trap air and enhanced the efficiency of water collection. Energy-dispersive X-ray spectroscopy (EDS) mapping images were obtained and are shown in Fig. 1f. Clearly, the multi-scale buckled regions comprising PTFE particles contained carbon and fluorine, while silicon and oxygen were clearly distributed on the larger particles' surfaces, indirectly confirming that they were silica particles. We thus concluded that hierarchical buckled surfaces with randomly distributed hydrophilic SiO₂ and hydrophobic PTFE particles—BEETLES surfaces, resembling the surface of a desert beetle—had been successfully fabricated.

3.2 Effects of particle composition and solution parameters on BEETLES surface performance

To investigate how the weight ratio of hydrophilic SiO₂ to hydrophobic PTFE particles in a dispersion affected fog harvesting efficiency, mixed hydrophilic–hydrophobic particle dispersions with varying weight ratios were prepared. The dispersions exhibited consistent particle sizes, and the solid content of the particle solution was initially set to 30 wt%. Under this condition, solutions for the BEETLES surface were prepared using SiO₂ and PTFE particles in weight ratios of 1 : 50, 1 : 100, 1 : 200, 1 : 300, 1 : 400, and 1 : 500. Despite the varying mixing ratios, the samples had similar hierarchical buckled surfaces, as shown in Fig. 2a. Before the samples' fog-harvesting performance was tested, their surface wetting properties were assessed. In the measurement of water contact angle, hierarchical buckled surfaces composed solely of PTFE particles (without SiO₂) were used for comparison, as shown in Fig. S1. Obviously, surfaces composed of higher hydrophilic particle ratios exhibited lower water contact angles and higher sliding angles, indicating increased hydrophilicity. Nevertheless, these surfaces had noticeable hydrophobic properties due to the predominant distribution of hydrophobic PTFE particles as the base material. Since fog-harvesting performance is determined by the synergistic interaction between hydrophilic and hydrophobic regions, both excessive and insufficient hydrophilic coverage can negatively affect the efficiency of water collection. Although a 1 : 50 ratio resulted in more hydrophilic characteristics, the samples with a hydrophilic-to-hydrophobic particle weight ratio of 1 : 300 achieved the highest fog harvesting efficiency (Fig. 2a, right bar chart). To further investigate the effects of upper film thickness on fog harvesting efficiency, a weight ratio of 1 : 300 for hydrophilic–hydrophobic particles was used in the subsequent experiments.

Fig. 2 FE-SEM images showing surface morphologies and corresponding fog harvesting efficiency under different control conditions: (a) buckled surfaces covered with SiO₂ and PTFE particles at weight ratios of 1 : 50, 1 : 100, 1 : 200, 1 : 300, 1 : 400, and 1 : 500; (b) buckled surfaces covered with SiO₂ and PTFE particles from solution concentrations of 10, 20, and 30 wt%; and (c) buckled SiO₂/PTFE surfaces using SiO₂ particles with diameters of 1, 5, and 10 μm. The white arrow indicates the 1 μm SiO₂ particles. The inset shows the profile of the corresponding water droplet under different experimental conditions. (d) Fog harvesting efficiency of flat and buckled surfaces covered with SiO₂/PTFE, PTFE, and Pt particles. (e) Comparison of fog harvesting efficiency between the optimized desert beetle-inspired buckled surfaces and flat sheets made of other materials.

Particle solutions with solid content of 10, 20, and 30 wt% were prepared and used to fabricate fog-harvesting surfaces. Cross-sectional images of the prepared PS surfaces with different solid contents are shown in Fig. S2a. The upper film thickness clearly increased with higher dispersion concentration. The thickness was 374.0, 552.7, and 1002.0 nm for concentrations of increased from 10, 20, and 30 wt%, respectively. As shown in Fig. S2b, hydrophobicity was enhanced as the solid content was increased. Surfaces with good hydrophobicity can accelerate the transport of water droplets, thus increasing fog harvesting efficiency. However, when the dispersion concentration was 10 wt%, parts of the PS substrate surface remained smooth (Fig. 2b). In the enlarged FE-SEM images (see inset), we can observe that some areas of the surface were even exposed, revealing the underlying PS substrates. It indicated poor and incomplete coverage with PTFE particles due to the lower solid content, which was also reflected in the increased sliding angle. As expected, the surfaces created with the 20 and 30 wt% particle solutions exhibited hierarchical buckled patterns. However, an overly thick upper layer (e.g., for the surfaces created with the 30 wt% particle solution) caused aggregation of PTFE particles and peeling of these particles from the PS substrate, resulting in a fragmented surface morphology (indicated by a white arrow). These surface characteristics directly affected fog harvesting efficiency. As shown in Fig. 2b (right bar chart), the surfaces made with the 20 wt% particle solutions showed comparatively better fog-harvesting performance than did those made with the other two particle solutions.

Furthermore, SiO₂ particles with diameters ranging from 1 μm to 10 μm were used to investigate their effect on fog-harvesting efficiency. As shown in Fig. S3a, the water contact angle was smaller when the SiO₂ particles were larger, which was attributable to the larger hydrophilic surface area. By contrast, the sliding angle increased as the particle size was increased, likely due to the increased surface roughness. Nevertheless, the fog harvesting efficiency increased with particle size, with the 10 μm SiO₂ particles demonstrating the highest efficiency as shown in Fig. 2c (right bar chart). Note that the average size of water droplets in fog is approximately 5–15 μm.³⁵ As a result, the 1 μm SiO₂ particles were too small to effectively intercept and adsorb water droplets, leading to significantly lower fog harvesting efficiency compared to samples with 5 μm and 10 μm particles. Conversely, particles larger than 15 μm tended to peel from the PTFE substrate because their larger volume prevented them from adequately embedding within aggregated PTFE particles, as shown in Fig. S3b. In summary, the ideal parameters selected in this study were 10-μm SiO₂ particles mixed with PTFE particles, a weight ratio of 1 : 300, and solid content of 20%; this combination resulted in fog harvesting efficiency of 983.55 mg cm⁻² h⁻¹. To investigate the impacts of surface wettability and buckled structural characteristics on fog harvesting efficiency, flat and buckled surfaces covered with entirely hydrophilic particles (platinum, Pt), entirely hydrophobic particles (PTFE), and hydrophilic–hydrophobic particles (SiO₂/PTFE) were prepared for fog-harvesting testing as shown in Fig. 2d. Regardless of the intrinsic surface wetting tendency of a surface, the fog harvesting efficiency was higher for buckled surfaces than for flat surfaces. This was attributable to the buckled surfaces providing a larger surface area, facilitating easier contact between the surface and fog. Also, as mentioned, the buckled surfaces formed channels that aided in the transport of water droplets, thereby enhancing the fog harvesting efficiency. Compared to flat surfaces, buckled structures can improve water collection efficiency by approximately 30%. Moreover, the surfaces coated with SiO₂/PTFE exhibited better fog harvesting efficiency than did the surfaces coated with only Pt or PTFE particles, regardless of whether the surface was flat or buckled. Although the buckled surface composed entirely of PTFE particles was hydrophobic, exhibiting a water contact angle of 155.75° (Fig. S1a), it did not demonstrate effective fog-harvesting behavior, as it was unable to capture water droplets from the air. In contrast, while the hydrophilic Pt surface could readily condense water, it was ineffective in transporting the collected droplets away (a more detailed discussion is provided in Fig. 4). In addition, flat sheets composed of various materials, including copper, aluminum, silicon dioxide, titanium dioxide, PS, and Teflon, were prepared for fog-harvesting tests and were compared with the desert beetle-inspired buckled surfaces. Notably, our desert beetle-inspired buckled surfaces exhibited excellent fog-harvesting capability (Fig. 2e). This result demonstrates that the surfaces alternating hydrophilic–hydrophobic characteristics, similar to the elytra of desert beetles, had high fog-harvesting capability.

3.3 Influences of particle shape and curvature-driven mechanisms on fog harvesting efficiency

We explored hydrophilic materials other than SiO₂ particles and with additional functions for the BEETLES surfaces—a natural material (diatomaceous earth), a circular economy material (calcium carbonate, CaCO₃), and an eco-friendly material (zinc oxide, ZnO). Notably, CaCO₃, SiO₂, and ZnO are intrinsically highly hydrophilic materials. Diatomaceous earth is composed of SiO₂. Because diatomaceous earth is a natural material, its particle size and shape are inhomogeneous, exhibiting a variety of morphologies. Typically, the size of particles ranges from 1 to 20 μm. The calcium carbonate employed in the study was regenerated from waste oyster shells and had a cubic form with a particle size of approximately 10 μm. The ZnO particles used were multibranched flower-like structures, synthesized through an eco-friendly hydrothermal process in our laboratory. The size of the multibranched flower-like ZnO particles was approximately 5 μm, and it possesses remarkable antimicrobial and photodegradation properties.^33,36Fig. 3a–c show the size and morphologies of the diatomaceous earth, CaCO₃, and flower-like ZnO particles, respectively. The three types of hydrophilic particles were prepared as particle dispersion solutions by using the same optimal parameters as those used for the SiO₂ particles: a hydrophilic–hydrophobic particle weight ratio of 1 : 300 and total solid content of 20 wt%. The generated BEETLES surfaces obtained using diatomaceous earth, CaCO₃, and multibranched flower-like ZnO particles are shown in Fig. 3d–f, respectively. Overall, the hydrophilic particles were embedded in the buckled surface formed by the hydrophobic PTFE particles.

Fig. 3 FE-SEM images of (a) diatomaceous earth (SiO₂), (b) calcium carbonate (calcite, CaCO₃), (c) multibranched flower-like zinc oxide (ZnO) particles, and BEETLES surfaces with (d) diatomaceous earth, (e) calcium carbonate, and (f) multibranched flower-like ZnO particles. (g) The fog harvesting efficiency of devices made using SiO₂ particles, diatomaceous earth, CaCO₃, and multibranched flower-like ZnO particles. (h) Schematic illustration of the water condensation and collection mechanism on the BEETLES surface. (i) Fog harvesting efficiencies of devices with randomly oriented buckles and single-orientation buckles, oriented parallel or perpendicular to the direction of water droplet movement. The inset shows the aligned PTFE buckled patterns embedded with artificial SiO₂ particles.

Fig. 3g demonstrates the fog harvesting efficiency of devices fabricated using SiO₂ particles, diatomaceous earth, CaCO₃, and multibranched flower-like ZnO particles. Although all of these hydrophilic additives resulted in remarkable water collection capabilities, the BEETLES surface incorporating artificial SiO₂ particles exhibited the highest performance (983.55 mg cm⁻² h⁻¹). This superior efficiency was attributable to the particles' uniform shape, size, and distribution. The significant enhancement of water vapor capture by SiO₂ particles is attributed to the Laplace effect and the particles' spherical structure. The Laplace pressure, caused by the curvature of a droplet's surface, generates a pressure difference between the interior and exterior of the droplet. This pressure difference influences the condensation process by affecting the stability and growth of newly formed droplets on the surface. As shown in Fig. 3h(I), water droplets of various sizes initially condensed on the surface of the SiO₂ spheres. Smaller droplets, which have greater curvature, experience greater Laplace pressure, which destabilizes them and renders them more likely to evaporate. By contrast, larger droplets have smaller curvature, resulting in lower Laplace pressure, higher stability, and the promotion of further condensation. In other words, water vapor is driven by the pressure gradient, migrating from high-pressure regions (i.e., the surface of small droplets) to low-pressure regions (i.e., the surface of larger droplets), where it recondenses. This pressure-driven mass transfer leads to water being concentrated in low-curvature areas and facilitates stable droplet growth. From a macroscopic perspective, the smaller droplets gradually disappear as they feed the growth of the larger droplets on the SiO₂ spheres—this is the classic Ostwald ripening process (Fig. 3h(II)). Subsequently, due to the curved geometry of the spheres, the growing droplets become spindle-shaped under the influence of gravity (Fig. 3h(III)). Once the gravitational force acting on the droplet exceeds the interfacial friction (i.e., when θ₁ ≫ θ₂), the droplet begins to slide off due to the geometric shape effect (the curved triangular region indicated by the red line in Fig. 3h(IV), resembling a slide-like structure). Finally, the sliding droplets are directed into the hydrophobic buckled channels formed by PTFE particles, which completes the water-harvesting process.

Compared with the SiO₂ particles, which had spherical morphology and inherent hydrophilicity, the cubic CaCO₃ and polygonal diatomaceous earth were associated with lower fog-harvesting performance. The fog harvesting efficiencies of devices fabricated using diatomaceous earth, CaCO₃, and flower-like ZnO particles were 953.93, 928.72, and 894.54 mg cm⁻² h⁻¹, respectively. Nonetheless, the devices incorporating these alternative hydrophilic materials exhibited commendable water collection performance. Notably, both diatomaceous earth and CaCO₃ derived from waste oyster shells have significant potential for future industrial applications, particularly those in alignment with environmental sustainability and circular economy principles. To further enhance fog harvesting efficiency, unidirectionally aligned buckled patterns were fabricated that could function similarly to microchannels. Unlike randomly distributed buckles formed via uncontrolled shrinkage, directional alignment during the thermal process of PS sheet shrinkage restricted the release of stress along one axis, resulting in uniformly oriented buckled patterns (see inset of Fig. 3i and S4). These aligned structures are expected to improve water droplet transport efficiency, thereby enhancing fog-harvesting performance. Fig. 3i presents a comparison of the fog harvesting efficiencies of devices with randomly oriented buckles and those with single-orientation buckles aligned either parallel or perpendicular to the direction of droplet movement. When the buckles were aligned with the direction of droplet flow, they acted as microchannels, generating capillary forces that facilitated linear transport and significantly improved fog harvesting efficiency (1069.02 mg cm⁻² h⁻¹). By contrast, when the buckles were perpendicular to the droplet movement, they acted as barriers to transport, resulting in a substantial decrease in fog harvesting efficiency (475.54 mg cm⁻² h⁻¹). The device with randomly oriented buckles exhibited performance that fell between those of these two configurations.

3.4 Influence of device placement and surface wettability on fog harvesting efficiency

As fog-harvesting technology has advanced, various methods for simulating fog environments and measuring fog collection efficiency have been developed. In general, testing samples are fixed and placed within a closed chamber, where a fog outlet faces the samples and emits water mist at controlled velocities and flow rates. However, differences in measurement techniques between research groups make it challenging to objectively compare the fog collection efficiency of different fog-harvesting surfaces. For example, some research groups position samples directly in front of the fog outlet at a controlled distance, while others place the samples in open air to capture naturally floating water droplets. Factors such as the angle and position of the sample, temperature, mist velocity and flow rate, as well as the size and shape of the nozzle, can all significantly influence measurements of fog collection efficiency. To comprehensively compare the fog-harvesting performance of the desert beetle-inspired buckled surfaces with that of other reported surfaces, fog-harvesting measurements were conducted using various fog collection systems commonly employed by different research groups. As shown in Fig. 4a, the setups involved front-facing positions at 10 and 20 cm from the fog outlet, as well as a non-front-facing position at 20 cm from the fog outlet, which was designed to capture naturally floating water droplets. A photograph of the actual fog-harvesting device is provided in Fig. S5. Additionally, three types of samples—the BEETLES surfaces, hydrophobic buckled surfaces (i.e., buckled PTFE particle surfaces), and hydrophilic buckled surfaces (i.e., buckled Pt surfaces) were arranged in the three configurations to investigate the effects of surface wettability. The corresponding surface morphologies and water contact angles are shown in Fig. S6.

Fig. 4 (a) Schematic illustration of fog-harvesting measurements performed under three configurations: front-facing positions located 10 and 20 cm from the fog outlet, and a non-front-facing position located 20 cm from the fog outlet, capturing naturally floating water droplets. (b) Fog harvesting efficiencies of hydrophilic buckled, hydrophobic buckled, and BEETLES surfaces under the three placement conditions described in (a). (c) Optical microscope images showing the condensation behavior of water droplets on the BEETLES, hydrophobic buckled, and hydrophilic buckled surfaces over a period of 180 seconds.

Obviously, the closer the fog-harvesting device was to the fog outlet, the higher was the fog collection efficiency (Fig. 4b). For the BEETLES surfaces, the harvesting efficiency at distances of 20 and 10 cm was respectively approximately 2.2 times and 5.9 times that for the non-front-facing configuration. The harvesting efficiencies at distances of 10 cm and 20 cm were 5770.77 and 2148.92 mg cm⁻² h⁻¹, respectively. The higher efficiency for the front-facing configurations was attributable to the higher droplet density and kinetic energy near the fog outlet, which promoted the collection of droplets. However, conditions such as high droplet concentration and high wind speeds (which are common in controlled laboratory settings) are rarely present during foggy weather. In other words, directly testing samples with their front side facing the fog flow for efficiency measurement does not reflect efficiency in real-world settings. A more accurate approach to measurement would be positioning samples farther from the fog outlet to capture naturally floating water droplets in the air. To observe water droplet condensation on device surfaces, a digital microscope was used to monitor the in situ fog collection process on the BEETLES, hydrophobic buckled, and hydrophilic buckled surfaces, as shown in Fig. 4c. The optical images of water droplets on the surfaces were adjusted to enhance their contrast and brightness, making the condensed droplets could be more easily distinguished (Fig. S7). All surfaces began to capture water droplets within 30 seconds, and over time, all three surfaces accumulated a high number of droplets. On the hydrophilic buckled surfaces, water droplets condensed easily due to their intrinsic hydrophilicity. However, the condensed droplets tended to adhere to the surface rather than slide off. By contrast, on the hydrophobic buckled surfaces, while droplets detached immediately after condensing, but the intrinsic hydrophobicity rendered it less effective at attracting water vapor for condensation, resulting in a lower condensation rate and the formation of smaller droplets. As a result, the hydrophilic and hydrophobic buckled surfaces both exhibited relatively low fog collection efficiency. The fog harvesting efficiency for capturing naturally floating water droplets in the air was 634.67 mg cm⁻² h⁻¹ for the hydrophilic buckled surface and 663.52 mg cm⁻² h⁻¹ for the hydrophobic buckled surface (Fig. 4b). By contrast, the BEETLES surface demonstrated excellent fog collection efficiency; it induced water droplet condensation through hydrophilic SiO₂ zones, and the large-area hydrophobic PTFE buckles promoted the coalescence and rapid detachment of droplets. The BEETLES surface's efficiency in capturing water droplets in the air was 983.55 mg cm⁻² h⁻¹. The detachment of water droplets from the surface was clearly observed (Fig. S8). Moreover, to evaluate experimental reliability, we added the results of a 10-day fog-harvesting cycling test on the BEETLES surface. In this experiment, the device was placed for 10 consecutive days, and one fog-harvesting test was performed each day, simulating the natural frequency of fog occurrence. The average fog-harvesting efficiency of the BEETLES surface showed no significant variation throughout the 10-day period (Fig. S9), confirming the reproducibility and durability of the device.

3.5 Fabrication and field application of a bioinspired device based on the dual mechanisms of the bird's nest fern and desert beetle

In addition to desert beetles, the bird's nest fern (Asplenium nidus) also provides valuable inspiration for fog collection.³⁷ The leaves of ferns exhibit a three-dimensional spatial architecture, and their multidirectional growth enables interception of fog droplets from various angles. In particular, the funnel-shaped leaves of the bird's nest fern naturally capture mist and direct condensed water droplets toward the center of the plant, ensuring efficient water transport and uptake in humid environments. Inspired by this geometry, we combined beetle-inspired microscale surfaces with fern-inspired macroscale structures to further enhance efficiency under real-world conditions (Fig. 5a). This dual-biomimetic design integrates droplet capture, coalescence, and directional transport across multiple length scales. The bird's nest fern, which thrives in tropical and subtropical regions, exhibits a natural funnel-like leaf pattern (Fig. 5b) that facilitates efficient fog capture and rapid transport of collected droplets toward the plant's center. This mechanism enables the roots—often suspended high in tree canopies—to absorb sufficient moisture for sustained growth. To mimic the leaf shape of the bird's nest fern, heat-shrinkable PS sheets were cut into fern-like geometries, approximately 15 cm in length and up to 4.5 cm in maximum width after shrinkage. The sheets were then coated with the previously described mixed dispersions of SiO₂ and PTFE particles. For industrial-scale fabrication, the fog-harvesting device can be produced using brush-coating or spray-coating methods. These approaches allow for the preparation of large-area surfaces with irregular shapes. The resulting surface morphology was comparable to that obtained via spin-coating (see Fig. S10), and did not significantly affect the fog-harvesting performance. Although brush-coating may lead to slight variations in film thickness, it considerably enhances manufacturing efficiency. After being brush-coated and thermally treated, the leaf-shaped PS sheets were bundled together using iron wire. The bioinspired PS leaf bundle was then inserted into a glass bottle, as shown in Fig. 5c, which collected water droplets accumulated. To facilitate visualization of the water level, blue dye was added to the bottle, causing the collected water to appear blue. Additionally, a control sample was prepared using leaf-shaped PS sheets coated with PTFE micelles only, without the inclusion of SiO₂ particles (i.e., buckled PS surfaces covered solely with hydrophobic PTFE components and lacking hydrophilic SiO₂), for comparative evaluation of fog harvesting efficiency.

Fig. 5 Bioinspired fog-harvesting system integrating BEETLES surfaces with the bird's nest fern geometry. (a) Illustration scheme of the leaf-shaped BEETLES fog-harvesting device. (b) Photographs of a real Asplenium nidus (bird's nest fern), illustrating its natural funnel-shaped leaf structure. (c) Fabricated Asplenium nidus-like fog-harvesting devices made from BEETLES surfaces, which were assembled into leaf-shaped bundles, and inserted into glass bottles. (d) Time-sequenced photographs of fog-harvesting experiments conducted overnight in Huisun Forest (Nantou, Taiwan). The collected water, visualized with blue dye, accumulated in the bottles positioned below each device. The BEETLES-based device (right) exhibited significantly higher fog harvesting efficiency compared to the control device did (left), as highlighted in the enlarged view after 11 hours.

To evaluate fog-harvesting performance under natural conditions, the two fog-harvesting devices were placed overnight in Huisun Forest (Nantou, Taiwan), as shown in Fig. 5d. The device on the left consisted of PS sheets coated with only PTFE micelles (control group), while the device on the right featured BEETLES surfaces. Due to the presence of SiO₂ particles, the BEETLES surfaces exhibited a light green appearance. Fig. 5d shows a time-lapse sequence, with images captured at 0, 4, 8, 9, and 11 hours. The entire fog collection process is also presented in Movie S1. Initially, the density of fog in the forest was low, and limited water was observed in the collection bottles. Nevertheless, fog formation became noticeable after approximately 4 hours, and water visibly accumulated in both bottles. The blurriness of the 8-hour photograph was caused by dense fog enveloping the setup. After 11 hours, the BEETLES-based device (right) had collected substantially more water than the control device had (left). Following overnight exposure (approximately 12 hours), the bioinspired BEETLES surfaces shaped like bird's nest fern leaves had harvested approximately 125 grams of water from the ambient air, roughly twice the amount collected by the control sample (see enlarged inset of Fig. 5d after 11 hours). Although fog-harvesting performance is inherently influenced by environmental factors such as temperature, humidity, and wind speed, the BEETLES surfaces were clearly demonstrated superior efficiency under real-world conditions.

4. Conclusions

In this study, a low-cost and easily manufacturable biomimetic fog-harvesting device was developed. By coating hydrophilic–hydrophobic particle suspensions onto commercial PS substrates, a surface mimicking that of the desert beetle was successfully fabricated. The employed coating techniques—spin coating, brush coating, and spray coating—render the process convenient and scalable, making it highly suitable for industrial applications. Importantly, our design integrates micro/nanoscale strategies, where buckled microchannels are combined with alternating hydrophilic–hydrophobic regions to enable both efficient fog interception and curvature-guided directional transport. Our experimental results demonstrate that both the patterned alternation of hydrophilic–hydrophobic regions and the presence of particulate surface protrusions are critical for efficient fog harvesting; neither component can be omitted. The hydrophilic regions promote the condensation and capture of airborne water droplets onto the solid surface, while the hydrophilic protrusions further enhance the fog collection efficiency through their surface morphology. On the other hand, the hydrophobic regions facilitate droplet transport, preventing droplet accumulation. Therefore, the distribution and size of both the hydrophilic and hydrophobic regions must be carefully optimized to maximize the device's fog-harvesting performance. In addition, compared to flat surfaces, buckled structures can improve water collection efficiency by approximately 30%. In other words, surfaces that are purely hydrophilic or purely hydrophobic without topographic features, or those that possess only surface topographies without a well-defined hydrophilic–hydrophobic distribution, are not ideal for efficient fog collection.

Beyond SiO₂, we also demonstrated the use of alternative hydrophilic fillers such as CaCO₃ derived from waste oyster shells, diatomaceous earth, and ZnO, thereby highlighting the material versatility and sustainability of this approach. Furthermore, this study introduced a dual-biomimetic design, in which the desert beetle-inspired micro/nanostructure was integrated with the bird's nest fern-inspired macroscopic geometry, significantly enhancing fog collection efficiency across multiple length scales. Scholars have continued to debate the relative importance of surface chemistry (i.e., hydrophilic–hydrophobic properties) versus physical structure (micro- and nanoscale topographies) in the design of fog-harvesting surfaces, as discussed in the introduction. Moreover, a standardized methodology for measuring fog collection efficiency is still lacking across research groups, making it difficult to perform direct comparisons between different fog-harvesting surfaces. Nevertheless, we attempted to organize and compare a variety of reported biomimetic fog-harvesting surfaces. As shown in Table 1, taking into account differences in fog flow distance, velocity, and flow rate, we can conclude that our BEETLES device exhibits outstanding water collection performance. Its low-cost, scalable fabrication, integration of dual biomimetic strategies, material sustainability, and comprehensive validation imbue it with significant industrial competitiveness, indicating high potential for wide-scale applications. In future work, we aim to engineer gradient buckled structures in the hydrophobic regions, where the buckles will function as microchannels with gradually increasing wavelengths. In addition, our future work will focus on integrating macroscopic, nature-inspired structural designs—such as cactus-like geometries—with the BEETLES framework to further enhance the performance of composite fog-harvesting devices.

Table 1 Comparison of fog harvesting efficiency of fog-harvesting device with other studies

Substrate	Method	Bio-inspired mechanism	Fog flow distance (cm)	Fog flow velocity (m s^−1)	Fog flow rate (L h^−1)	Fog harvesting efficiency (mg cm^−2 h^−1)	Publication year	Ref.
Copper wire	Surface modification	Cactus/spider silks/Nepenthes/blueberry	10–20	1.39–2.37	N/A	30 162	2024	38
Aluminum sheet	3D printing	Cactus/spider silk/camphor leaves/desert beetles/lizards	15	1.83–2.24	N/A	27 720	2025	39
Copper sheet	Laser printing	Desert beetle/nephrolepis cordifolia	10	2	N/A	19 200	2024	26
PS sheet	Spin coating	Namib desert beetle	10	1.2	2.2	5770.77		This work
PS sheet	Spin coating	Namib desert beetle	20	1.2	2.2	2148.92		This work
PP plate	Surface modification	N/A	10	1	0.23	2130	2024	40
APTES/PDMS	Grafting	Porcupinefish	10	0.2	N/A	1700	2023	41
Zinc sheet	Surface modification/grafting	N/A	15	0.25	N/A	1442	2023	30
A SHB/SHL fabric	Textile weaving	Namib desert beetle	20	N/A	0.3	1432.7	2021	4
PS sheet	Spin coating	Namib desert beetle	20 (non-directly)	1.2	2.2	983.55		This work
Stainless steel sheet	Laser structuring/surface oxidation	Cactus spine/leaf vein/desert beetles	20	N/A	1.5	560	2024	42
PET film	Surface oxidation	Leaf vein	10	0.5	0.35	467.3	2018	43
Copper sheet	Surface modification	Namib desert beetle	12	0.5	N/A	430	2018	6

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Author contributions

The design of experiment, analysis of data, and the draft of the manuscript were done by T.-Y. Xu. The overnight fog-harvesting experiment and the measurement of surface morphology were performed by C.-C. Hung. The review of the draft of the manuscript and FE-SEM data were performed by C.-Y. Juan and P.-C. Tseng. H.-Y. Hsueh provided the research concept, suggestions for data analysis, edited and reviewed of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

All data are included in the manuscript and/or the SI. Supplementary information: water contact and sliding angles of buckled surfaces fabricated with SiO₂/PTFE at different weight ratios, and all-PTFE surfaces (Fig. S1); cross-sectional FE-SEM images and water contact angles of buckled surfaces fabricated with mixed particle solutions at different concentrations (Fig. S2); water contact angles of buckled SiO₂/PTFE surfaces with various SiO₂ particle sizes, and FE-SEM image of buckled structures formed by PTFE after SiO₂ particle detachment (Fig. S3); schematic of unidirectional buckled pattern formation via thermal shrinkage, and FE-SEM images of aligned PTFE patterns with embedded SiO₂ particles (Fig. S4); photograph of the fog-harvesting setup, including an ultrasonic humidifier, acrylic chamber, and samples mounted on an aluminum tray for water collection (Fig. S5). FE-SEM images of buckled PTFE particle and platinum surfaces, and water contact angles of BEETLES, buckled PTFE particle, and platinum surfaces (Fig. S6); digital microscope images of the BEETLES surface before and after contrast/brightness adjustment to enhance droplet visibility (Fig. S7); handheld digital microscope image of the BEETLES surface during the roll-off process of water droplets (Fig. S8); fog-harvesting efficiency of the BEETLES surface throughout the 10-day period (Fig. S9). FE-SEM images of BEETLES surfaces fabricated by spin-coating, brush-coating, and spray-coating methods, and cross-sectional FE-SEM images of micellized PTFE/TMN-10 and SiO₂ thin layers (Fig. S10). See DOI: https://doi.org/10.1039/d5ta05905d.

Acknowledgements

We thank the National Science and Technology Council of Taiwan (R.O.C.) for financially supporting this study under the grant number NSTC 112-2221-E-005-002-MY3 and NSTC 114-2223-E-005-001-MY3. This study was also partially supported by the Innovation and Development Center of Sustainable Agriculture and the Innovative Center on Sustainable Negative-Carbon Resources under the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project of the Ministry of Education in Taiwan.

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