Unravelling the aerodynamic enhancement of water harvesting via dynamic liquid bumps

Abstract

Harvesting atmospheric water offers a sustainable solution to water scarcity in arid regions. While previous reports that proved the wettability of materials play a crucial role in the fog collection process, the underlying mechanism remains unclear. Despite the focus on convex-backed beetles, hydrophobic smooth-backed beetles like Onymacris unguicularis also efficiently harvest fog. Through comprehensive investigation, the enhancement of fog collection efficiency on hydrophobic surfaces was attributed to the in situ 3D patterning process of microdroplets. Hydrophobic surfaces form dynamic liquid bumps that disturb airflow, improving the capture of tiny fog droplets. With a harp-like collector configuration, the superhydrophobic surface further enhances efficiency by 57% compared to superhydrophilic collectors. COMSOL Multiphysics simulations show that surfaces with stronger hydrophobicity and lower contact angle hysteresis intercept fog droplets more effectively. This work provides insights into the aerodynamic role of wettability in fog harvesting and offers guidelines for developing high-performance, bioinspired fog collectors with optimized material properties.

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New concepts

Fog-basking beetles in the Namib desert were long thought to rely on back surface bumps for water collection, but the observations of Onymacris unguicularis reveal that efficient fog harvesting can also occur on completely smooth beetle backs, challenging the conventional understanding of biological water collection mechanisms. Here we explain how desert beetles with smooth backs can also achieve efficient fog collection, revealing the aerodynamic mechanism of water harvesting enhancement on hydrophobic surfaces. Smooth hydrophobic surfaces form dynamic liquid bumps that significantly enhance fog droplet interception which is similar to fixed physical 3D patterned bumps. Through comparative studies, we demonstrate that surfaces with stronger hydrophobicity and lower contact angle hysteresis exhibit superior fog collection performance, making superhydrophobic surfaces approximately 7.7 times more efficient than superhydrophilic surfaces. We successfully applied this concept to harp-like fog collector arrays, validating its effectiveness. This study provides a bioinspired strategy for designing high-efficiency fog harvesters, offering a sustainable solution to water scarcity in arid regions.

Introduction

Water scarcity remains a critical global challenge, particularly in arid and economically disadvantaged regions where access to clean drinking water is severely limited.^1,2 While advanced water purification systems^3–5 offer potential solutions, high energy requirements and substantial costs make such systems inaccessible to developing areas.^6–9 In contrast, atmospheric water harvesting, which captures moisture from both fog and low-concentration water vapor present in the air, presents a sustainable alternative for obtaining fresh water.^10–12 Fog is a natural phenomenon in many arid and semi-arid regions, offering a promising alternative for directly harvesting freshwater.^13–16 Fog collection is a clean, sustainable, and environmentally friendly solution, offering low equipment costs, simple operation, and no dependence on external energy sources.^17–20

Bio-inspired fog collectors have demonstrated remarkable potential by emulating efficient water-harvesting mechanisms found in nature.^21–24 These innovative designs draw inspiration from various biological adaptations,^25–27 including directional structured surfaces inspired by the rice leaves,²⁸ hydrophobic spines of cactus,^23,29–32 and the specialized fog-collecting fibers of spider silk.^33,34 By mimicking the natural structures, remarkable high-performance fog collectors have been developed that combine minimal energy requirements with superior collection efficiency, marking significant advances in sustainable water harvesting technology.^35–38

Fog-basking beetles in the Namib Desert have been thoroughly studied as natural water harvesters.^39,40 The hydrophobic–hydrophilic patterns and surface bumps serve as classical bionic models, which have been successfully applied in fog collection technologies.^41–46 Interestingly, within the same desert environment, another group of beetles with smooth backs also achieved remarkable fog collection efficiency.^47–49 These beetles exhibit hydrophobic smooth surfaces where convex-shaped droplets form during fog collection. The natural phenomenon provides additional insights into fog harvesting mechanisms, revealing that hydrophobic surfaces with characteristic microdroplet formation can achieve remarkable fog collection performance.

Based on biological observations, our research demonstrates that isolated droplets formed on smooth hydrophobic surfaces effectively enhance fog collection efficiency. The droplets maintain a spherical shape through dynamic processes of growth, coalescence, and detachment, forming “dynamic liquid bump structures” (DLibs) that function similarly to physical solid bumps. By intercepting the fog flow, DLibs significantly enhance the capture efficiency of tiny droplets. This mechanism reveals a distinct liquid–gas interface regulation, which differs from the physical bump structure commonly observed in other beetles and bionic fog collectors.

Comparable to the fog-basking beetles with smooth backs, our observations confirm that relying solely on the hydrophobic surface can yield highly effective fog collection. For primary planar fog collectors with various wettabilities, superhydrophobic (SHB) surfaces demonstrated a 7.7 times increase in fog collection efficiency compared to superhydrophilic (SHL) surfaces. COMSOL Multiphysics simulations were employed to explore the interactions between DLibs of varying convexities and spacings. The results indicate that surfaces with stronger hydrophobicity generate DLibs with more violent fog collisions. Moreover, rapid droplet drainage facilitated by low contact angle hysteresis increases the spacing between DLibs, thereby significantly enhancing fog flow disturbance. These findings were extended to harp-shaped fog collectors, where the aerodynamic benefits of DLibs on superhydrophobic surfaces were further validated. In this configuration, superhydrophobic surfaces improved the fog collection efficiency by 57% compared to superhydrophilic surfaces. This study underscores the critical role of hydrophobic surfaces in enhancing fog collection efficiency and reveals the underlying aerodynamic mechanisms. These insights provide a valuable foundation for designing high-performance fog collectors, offering innovative strategies to address water scarcity in arid and resource-limited regions.

Results and discussion

Comparison between physical bumps and dynamic liquid bumps

Traditional bio-inspired desert beetle surfaces employ static physical bumps for fog droplet interception. In this conventional approach, fog droplets are captured by these physical surface bumps, accumulating until sufficient mass is attained for gravitational removal. Throughout this process, the physical bump structure remains static and unchanged (Fig. 1a). However, a potential limitation emerges as departing droplets move along the intervening grooves, compromising the fog collection efficiency of the entire surface. Additionally, these physical bump structures require complex fabrication processes, increasing production costs and limiting their widespread application in resource-constrained environments.

Fig. 1 Comparative analysis of static physical and dynamic liquid bumps in biological fog collection mechanisms. (a) Fog collection on desert beetles with static physical bumps. The surface features convex structures or wettability patterns, with liquid being captured directly on artificial hydrophilic bumps. The preparation of bumps and wettability treatment leads to high costs. (b) Fog collecting process on desert beetles with smooth backs, the collected droplets form dynamic liquid bumps which have a distinct fog collecting mechanism. The liquid bumps can intercept tiny droplets, and the inherent superhydrophilicity of liquid bumps is also attributed to high efficiency, which leads to higher collection efficiency and lower cost.

In contrast, dynamic liquid bumps that spontaneously form on hydrophobic surfaces exhibit distinct advantages. Upon fog deposition on a hydrophobic surface, surface tension drives the formation of isolated liquid bumps that function as dynamic collection sites. The liquid bumps effectively mimic the structural features observed on desert beetle backs, demonstrating superior fog interception capabilities. The inherent liquid nature provides superior wettability compared to physical bumps, facilitating improved capture of tiny fog droplets. It is important to note that such fog collection effects are exclusively achievable on hydrophobic surfaces. Although hydrophilic surfaces demonstrate superior initial tiny droplet capture, the lack of stable liquid bump structures consequently limits sustained fog harvesting efficiency.

The dynamic characteristics of liquid bumps represent a significantly different fog collecting mechanism compared to conventional physical bumps. Throughout the collection process, the droplet bumps continuously grow, coalesce, and capture additional fog droplets. In contrast to static physical bumps, where droplet departure can generate efficiency-reducing regions, the liquid bumps maintain their presence on the hydrophobic surface even as larger droplets are gravitationally removed, ensuring a self-sustaining cycle of high fog capture capability (Fig. 1b). This unique behavior exhibited by liquid bumps has been conceptualized as “dynamic liquid bump structures” (DLibs) to describe these liquid formations that spontaneously develop on hydrophobic surfaces during the fog collection process.

Different from the static physical bumps on desert beetle backs, DLibs demonstrate several critical advantages over traditional static physical bumps: (1) enhanced super-hydrophilic behavior at the liquid–air interface enabling superior microscopic droplet capture and retention; (2) cost-effective application on simple hydrophobic surfaces without requiring complex micro/nanofabrication procedures, making them particularly valuable for resource-constrained applications; (3) efficient water collection through spontaneous detachment and self-renewal; and (4) superior durability as physical damage to the surface doesn't compromise the formation of new DLibs. When DLibs depart the surface, the vacant spaces become more conducive to the collision of microscopic droplets, thereby establishing a continuous cyclic process of nucleation, growth, coalescence, and regeneration. These characteristics provide a robust foundation for developing next-generation fog collectors specifically engineered to address water scarcity in arid and economically disadvantaged regions.

Different from the static physical bumps on desert beetle backs, DLibs demonstrate several critical advantages over traditional static physical bumps. Physical bumps typically require complex artificial fabrication procedures to create precise bump structures. Furthermore, these physical bump structures frequently necessitate the application of asymmetric wettability patterns to function effectively. In stark contrast, liquid bumps leverage the inherent superhydrophilic nature of the liquid–air interface, which enables advanced efficient interception of tiny fog droplets. When these tiny droplets collide with liquid bumps, they directly merge and coalesce into the existing liquid bump structure without resistance. This inherent superhydrophilicity not only dramatically reduces production costs by eliminating complex fabrication procedures but also significantly enhances fog collection efficiency through superior droplet capture and incorporation mechanics.

Additionally, unlike physical bumps that require precise manufacturing, liquid bumps form spontaneously under the influence of surface tension during the fog collection process. The self-generating characteristic means that effective bump structures can be created simply by adjusting surface wettability and contact angle hysteresis of the base material, without requiring additional fabrication steps. The ability to form effective collection structures in situ dramatically reduces manufacturing costs while maintaining high performance. Furthermore, the dynamic nature of these liquid bumps ensures continuous renewal of the collection surface, providing consistent performance even after extended use under challenging environmental conditions. As long as the surface wettability remains sufficiently durable, liquid bumps can be continuously generated in the fog collecting process, ensuring the longevity of the bump structure and superhydrophilicity. The detailed comparison of DLibs and physical bumps is listed in Table 1.

Table 1 Comparison between physical bumps and DLibs

Aspect	Traditional physical bump fog collector	DLibs fog collector
Mechanism	Use static physical bumps to capture fog droplets.	Relies on liquid bumps formed on hydrophobic surfaces to collect fog water.
Surface type	Hydrophilic bumps and hydrophobic base surfaces.	Hydrophobic surfaces with low contact angle hysteresis.
Tiny droplet capture	Droplets captured on the physical bumps and detached with a certain volume.	Superhydrophilicity of liquid bumps maintaining higher capture efficiency.
Surface regeneration	Surface bumps remain static during the collection process.	Dynamic regeneration through droplet capture, growth and detachment.
Aerodynamic performance	Adjustable bump shape for enhancing airflow disturbance.	Aerodynamic performance is adjusted by controlling droplet size and spacing through surface contact angle and contact angle hystereses.
Fabrication complexity	Complex surface bump preparation presents challenges for large-scale production.	Rely on natural formation during the fog collection process on hydrophobic surfaces.
Material and cost	Increased costs due to preparation processes for bump surfaces and wettability asymmetries.	Only hydrophobic surfaces require preparation; the remaining features form naturally and reduce costs.
Sustainability and longevity	The hydrophilic durability of patterned surfaces may be limited.	By utilizing the collected liquid, the bumps can maintain a consistently elevated hydrophilic state.

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Therefore, the proposed concept and understanding of DLibs could offer a more efficient, cost-effective, and sustainable solution for water harvesting in arid regions. Inspired by nature, DLibs may provide both immediate practical benefits for addressing water scarcity in resource-constrained environments and valuable insights for designing next-generation high-performance fog collectors. The mechanisms explored in this study have the potential to be applied to various water harvesting applications, which could help transform our approach to sustainable water collection in water-stressed regions.

Static analysis: morphology of DLibs on surfaces with various wettabilities

To simulate the smooth back of desert beetles and investigate the influence of wettability on fog collection efficiency, various smooth surfaces with different wettability properties were employed. These surfaces were used to conduct an in-depth examination of how different types of wettability affect droplet states and fog collection performance on smooth surfaces during the fog harvesting process. For this purpose, a flat surface which is devoid of intricate structures is regarded as an ideal experimental platform. On an aluminum sheet with a surface area of 5 cm², various coatings are applied to create different wettabilities. Scanning electron microscopy (SEM) observations confirmed that all surfaces are smooth, ensuring no interference with airflow during fog collection (Fig. S1, ESI†).

Droplets exhibit distinct bump heights and spreading widths on surfaces with various wettabilities (Fig. 2a). Liquid could only form a continuous water film on superhydrophilic surface. As the contact angle increased, the spreading radius of the droplets gradually decreased, while their bump height increased, resulting in the formation of nearly spherical droplets on superhydrophobic surfaces (Fig. S2, ESI†). To gain deeper insight into the morphological changes of droplets under varying contact angle conditions, physical modeling was conducted (Fig. 2b). The physical analysis explored the interrelationships among droplet volume, contact angle, curvature radius, width, and bump height, providing a comprehensive understanding of droplet behavior across different wettability profiles under ideal conditions.

where V represents the droplet volume, while R and θ denote the droplet's radius of curvature and surface contact angle, respectively. For hydrophilic surfaces, h = R − R cos θ and w = 2R sin θ. On hydrophobic surfaces, h = R + R cos θ, where h represents the droplet height and w corresponds to the width of the droplet. As the contact angle becomes larger, the droplets contract in width but extend further in height, indicating that hydrophobic surfaces can accommodate a greater number of dynamic liquid bumps on a fixed fog-collecting surface area. Moreover, liquid bumps of greater height can enhance the interception of tiny fog droplets and improve fog collection efficiency.

Fig. 2 Effect of surface wettability on DLibs’ morphology. (a) Droplet morphology on surfaces with different wettabilities, including superhydrophilic (SHL), aluminium oxide (Al₂O₃), waterborne polyurethane (WPU), acrylic resin (ACR), slippery surface, polydimethylsiloxane (PDMS), and superhydrophobic (SHB). (b) Contact angle comparison of various surfaces. (c) Relationships between the contact angle and the liquid size.

In addition to researching the effects of surface wettability, the contact angle hysteresis in the fog collection process was also investigated (Fig. 2c and d). A slippery coating and a PDMS coating were used as a comparison. Although their static contact angles are comparable, the slippery coating exhibits lower contact angle hysteresis, with reductions of 68% compared to the PDMS coating. Furthermore, the rolling angle of the slippery coating was reduced as well, which suggests that droplets on the slippery coating can refresh the surface with faster speed and with smaller sizes (Fig. S3, ESI†).

Dynamic analysis: behavior of droplet bumps during the fog collection process

In addition to the study of the static characteristics of droplets, understanding the dynamic behavior of DLibs during the fog collection process is of greater significance. By applying the fog to substrates with varying wettabilities and capturing the process using a high-speed camera, the dynamic behavior of DLibs was observed and analyzed to assess the impact of liquid bumps on fog collection efficiency (Fig. S4 and Movie S1, ESI†).

On superhydrophilic surfaces, no droplet formation was observed during fog collecting process, which indicates that superhydrophilic surfaces do not facilitate the formation of liquid bumps (Fig. 3a). Although superhydrophilic surfaces are not conducive to the formation of DLibs, they demonstrate significant advantages in water-harvesting applications where light transmittance is essential. For instance, in solar evaporation systems, a transparent superhydrophilic surface positioned at the top exhibits excellent anti-fogging performance, enabling efficient light transmission while maintaining high levels of solar transmittance and freshwater condensation efficiency.⁵⁰ As the contact angle increased, convex DLibs were observed on ACR surfaces (Fig. 3b). The DLibs were relatively low in height and the droplets spread over a large area, which caused droplets to take longer to accumulate and grow before detaching the surface. Once the droplet leaves, it will carry away a large area of the surrounding contactable droplet, resulting in a significant refresh area. Consequently, the resultant refresh area will lose DLibs during the following short period of fog collection until the droplets captured on the surface.

Fig. 3 Dynamic behavior of DLibs during the fog collection process. (a) (i) The schematic figure of the fog collecting process. (ii) DLibs disappear on a superhydrophilic surface, and the fog condenses into a water film without forming distinct droplet bumps. (b) For the ACR surface, droplets adhere and expand their contact line before detaching, creating a large refreshed area but leaving no DLibs for subsequent fog interception. (c) Large numbers of DLibs grow and rapidly detach from the slippery surface, refreshing narrow pathways while retaining significant DLibs and maintaining high fog interception efficiency. (d) DLibs bounce on the superhydrophobic surface, picking up small droplets during each bounce. The scale bar is 3 mm.

In contrast, surfaces with higher contact angles exhibited the formation of distinct spherical droplet bumps, which enhanced tiny fog interception. On slippery surfaces, characterized by their superior sliding behavior, droplets rapidly detach from the surface in the form of individual droplets, refreshing narrow pathways as they slide. The rapid detachment reduced the droplets’ residence time on the surface and minimized the refreshed area. Even during the detaching process, a significant amount of DLibs will remain on the surface, ensuring the interception capability of DLibs (Fig. 3c).

Unlike the ordinary detachment process on a hydrophobic surface, the collected droplets leave the SHB surface while bouncing. The rapid detachment further reduces the residence time of droplets on the surface. However, as the fog collection proceeds, water will fill the micro/nanostructure of the SHB surface, transitioning the surface from the Cassie state to the Wenzel state. Therefore, with the increase of surface adhesion, the size of droplets that can be detached from the surface also increases. Correspondingly, the mode of droplet departure has gradually changed. At the initial stage, droplets detach rapidly and bounce during the coalescence process. Over prolonged fog collection, the size of the retained droplets gradually increases, while their bounce height upon leaving the surface decreases. The bounced droplet will collide and merge with other droplets on their path. As further surface energy is released during the coalescence, another bouncing is triggered. Through multiple bouncing events, a single droplet can gradually collect and carry away several smaller droplets dispersed across different locations, thereby achieving a high refresh rate while retaining numerous DLibs on the surface (Fig. 3d). As fog collection progresses, the adhesive force further increases, making it difficult for droplets to bounce after coalescence. Instead, the droplets leave the surface in a sliding motion, carrying other droplets along with them and completing the dynamic surface-refreshing process (Fig. S5, ESI†).

Furthermore, we investigated whether the observed Cassie-to-Wenzel state transition during prolonged fog collection is reversible. To this end, we conducted time-dependent fog collection experiments on the superhydrophobic surface. Initially, the fog collection process was recorded using a high-speed camera, capturing the droplet behavior in the pristine Cassie state. Subsequently, the same surface was continuously exposed to fog for 1 hour, and the departure size, retention time and spacing of DLibs were analyzed at 20-minute intervals. As fog collection progressed, the critical droplet size required for detachment and the spacing between droplets gradually increased, while the retention time of droplets remained constant. These observations suggest that part of the superhydrophobic surface transitioned to the Wenzel state, as evidenced by enhanced adhesion between the droplets and the SHB surface. To evaluate the reversibility of this wetting state, we subjected the fog-exposed SHB surface to drying at 50 °C for 30 minutes. After re-exposure to fog under identical conditions, the departure size, retention time, and DLibs spacing almost returned to values comparable to those observed in the initial Cassie state. This result indicates that the Cassie-to-Wenzel transition on the superhydrophobic surface is reversible through simple drying, allowing the surface to recover its initial fog collection behavior (Fig. S6, ESI†).

Effect of surface wettability on fog collection efficiency

A series of experiments were performed to investigate the influence of surface wettability on fog collection efficiency. The fog collection efficiency is tested under a constant fog flow velocity (∼2.2 m s⁻¹), the distance between the fog outlet and the surface was fixed at 10 cm, and the mass of collected water was tested every ten minutes. To enhance droplet detachment and minimize the interference of retained droplets on fog collection efficiency, aluminum sheets were tilted at 45° about their central axis, forming a rhombus-shaped orientation.

The experimental results demonstrated a strong correlation between surface wettability and fog collection efficiency (Fig. 4a). Among the tested surfaces, the superhydrophobic surface exhibited a fog collection efficiency that was 7.7 times higher than that of the superhydrophilic surface. Additionally, the slippery surface achieved a 16% improvement in fog collecting efficiency compared to the PDMS surface. Furthermore, fog collection experiments conducted on surfaces with typical wettabilities under varying temperature and humidity conditions consistently exhibited the same trend in collection efficiency (Fig. S7, ESI†). Even when the fog flow velocity was increased to 5 m s⁻¹ and 7 m s⁻¹, the relative ranking of surface performance remained consistent, with higher wettability surfaces achieving superior fog collecting efficiency. However, under high-speed airflow conditions, the superhydrophobic surface did not show further improvement in collection efficiency beyond 5 m s⁻¹. We attribute this to the detachment of small droplets before they could coalesce into larger DLibs, leading to a reduced number of interception sites on the surface and thus limiting performance enhancement at extreme wind speeds (Fig. S8, ESI†).

Fig. 4 Effect of surface wettability on fog collection efficiency. (a) Fog collection efficiency of various surfaces. (b) Departure size and retention time of DLibs. Superhydrophobic and slippery surfaces show smaller droplet departure diameters and shorter retention times compared to other tested surfaces. (c) Real-time analysis of water accumulation during fog collection. (i) Experimental setup for precision weight measurement using an air extraction device to eliminate interference. (ii) Growth of on-surface water accumulation can be divided into three stages. (iii) Comparison of growth rates across different surfaces, with the slippery surface exhibiting the fastest growth and stabilization. (iv) Schematic diagram of the fog collection process stages, illustrating the evolution from DLibs formation to stabilization.

Statistical analyses of droplet retention time and departure diameter further highlighted the differences (Fig. 4b). Superhydrophilic and aluminum oxide surfaces were excluded from these statistics because no distinct DLibs were formed (Fig. S4, ESI†). The superhydrophobic surface exhibited the shortest retention time and the smallest departure diameter. Besides, the slippery surface exhibited 25% shorter droplet retention times, and 23% smaller departure diameters compared to the PDMS surface, demonstrating that even with similar wettability, surfaces with higher slipperiness facilitate higher fog collecting efficiency. Besides, the bump spacing and droplet density are calculated as well. Surfaces with higher wettability tend to exhibit larger droplet spacing. However, droplet density showed no clear relationship with wettability, which we attribute to its strong dependence on both droplet size and detachment frequency during the fog collection process (Fig. S9, ESI†).

To further analyze the effect of DLibs on fog collection, we recorded and analyzed the amount of liquid attached to the surface during the fog collection process (Fig. 4c). When the fog-collecting plate is merely fixed on a shelf and its weight change is measured using a precision balance, a large proportion of the fog is trapped by the shelf, thereby increasing experimental error (Fig. S10, ESI†). To enhance precision, an air extraction device was employed to remove the fog that drifts away from the collecting plate, effectively eliminating the interference caused by the shelf. Droplet weight changes on the surface were then measured from the initial fog collection until stable collecting. The distance between the fog exit to the surface was maintained at 10 cm, and the weight change was recorded using a precision electronic balance. The mass change of ACR, slippery surface, and SHL surfaces were then analyzed by data fitting.

The fog collection process can be divided into three stages through data fitting. In the first rapid growth stage, many dispersed small droplets form on the surface, with relatively large spacing. At this stage, these small droplets are less affected by gravity and exhibit a significant bulge, which can be considered as small DLibs. Subsequently, the second stage mainly involves droplet convergence and growth. In the second stage, droplets gradually fill the surface, reducing inter-droplet spacing. The growth rate of the liquid volume decreases at this stage. In the third stage, as the collected water gradually begins to drip, the surface liquid volume stabilizes. As shown in Fig. 4c-ii, the surface liquid volume first increases rapidly, then increases slowly, and finally reaches a steady state. Linear fitting of these three stages reveals that the slippery surface shows the fastest growth rate in both stage 1 and stage 2, and it also reaches stability at the earliest. In contrast, for the superhydrophilic surface, there is no significant distinction between stage 1 and stage 2 as the fog water directly infiltrates the surface. Notably, the superhydrophobic surface demonstrates distinct behavior (Fig. S11, ESI†), as its surface liquid volume gradually increases over time, which is mainly caused by a gradual transition from the Cassie state to the Wenzel state.

According to the thorough analysis of surface wettability and the fog collection process, the facilitating effect of DLibs on fog collection efficiency was demonstrated. On hydrophilic surfaces, the refresh frequency is relatively slow, and the refresh area is extensive, which leads to the accumulation of droplets on the surface before detaching. The convex DLibs lack an effective interception area, and their tight spacing further obstructs collisions between DLibs and tiny airborne droplets. When droplet departure creates a large refreshing surface, this extensive flat area loses its ability to intercept fog. In contrast, a more hydrophobic surface with smaller contact angle hysteresis generates smaller droplets that depart more quickly, facilitating the formation of DLibs at appropriate intervals and thus maintaining efficient air interception throughout fog collection. Even when the droplet leaves the surface, it only produces a small refresh area but improves aerodynamics, which will be discussed in the following parts.

The wind field of DLibs on various conditions during the fog collection process is further simulated by COMSOL Multiphysics. To demonstrate the influence of wettability, three models were built, including SHB, hydrophilic, and SHL surfaces. The DLibs are all with the same model spacing but different bulge heights. In the simulation results, the wind vector can be decomposed orthogonally into two directions: the collision direction (perpendicular to the droplet) and the departure direction (parallel to the droplet). For superhydrophobic and hydrophilic surfaces, the collision direction is essentially parallel to the surface, whereas for superhydrophilic surfaces, it is perpendicular due to the liquid spreading on the surface. The collision direction markedly increases the probability of air-carried tiny droplets colliding with the DLibs.

The collision velocity can directly affect the efficiency of fog collection. Generally speaking, fog collection efficiency is the combined effect of three kinds of efficiency.⁵¹

η_c = η_Aη_Dη_Dr (2)

where η_A is aerodynamic efficiency, η_D is deposition efficiency and η_Dr is drainage efficiency. Among these parameters, deposition efficiency is a key indicator for assessing droplet trapping ability, which represents the proportion of air-carried tiny droplets which flow towards the liquid bumps and collide with DLibs. This work mainly focuses on investigating how surface wettability influences droplet collision and capture by the surface. Therefore, deposition efficiency is the most important variable, which can be quantified by the Stokes number St.^52–54 When St is greater than 1.1,where ρ_water and ρ_air denote the densities of water and air, respectively, d and D are the diameters of the droplet and the target, and Re is the Reynolds number.where u represents the velocity of air flow, μ_air is the viscosity of air. When u (the wind speed) is 2 m s⁻¹, D = 1 mm, and d_droplet = 10 μm, the Stokes number (St) is approximately 1.23. The equation demonstrates that increased wind speeds and larger droplet sizes contribute to higher fog collection efficiency.⁵⁵

Based on the equation of fog collection efficiency, our simulation primarily analyzes the flow velocity in the collision direction towards droplets and fog collision area to assess the influence of surface wettability on fog collection efficiency (Fig. 5a). For hydrophobic surfaces, the higher droplet bump leads to an expanded fog collision area, thereby enhancing the probability of droplet trapping. Moreover, the wind speed near the droplets on hydrophobic surfaces is higher, with the maximum speed reaching 1.9 times that of hydrophilic surfaces and 33 times that of superhydrophilic surfaces. Combined with the actual fog efficiency, the COMSOL simulation can effectively explain DLibs on hydrophobic surfaces increase the probability of fog-carried tiny droplets collision and improve fog collection efficiency.

Fig. 5 COMSOL Multiphysics simulations of the wind field on surfaces with varying wettability and droplet densities. (a) Effect of surface wettability on fog collection efficiency. (i) Hydrophobic surfaces with high droplet bumps exhibit larger fog collision areas and higher colliding speeds. (ii) Hydrophilic surfaces show smaller collision areas and lower colliding speeds. (iii) As a contrast, superhydrophilic surfaces display negligible velocity deflection due to the absence of droplet bumps, reducing fog interception capacity. (b) Effect of DLibs spacing on fog collection efficiency. (i) Large-spacing droplet bumps produce significant fog disturbance with high collision velocities and effective fog collision areas. (ii) Medium-spacing DLibs reduce wind disturbance, limiting effective collision areas and lowering collision velocity. (iii) Small-spacing droplets substantially restrict fog flow to the droplet tops and minimize collision efficiency.

The influence of DLibs on airflow dynamics can be further analyzed by the thickness of the boundary layer. Under our experimental conditions, the Reynolds number (Re = ρud/μ) was approximately 1.49 × 10⁵, indicating laminar flow over the surface. In laminar flow, the boundary layer thickness (δ) evolves as δ = 5x/Re^0.5, where x is the length of the fog collector along the direction of fog flow. A thinner boundary layer generally facilitates faster deceleration and enhances the deposition probability of tiny droplets.

We hypothesize that tiny droplets carried by fog initially exhibit a velocity close to the fog velocity. Upon entering the boundary layer, droplets experience progressive deceleration due to the velocity gradient within the layer. The longer the droplet resides in the boundary layer without contacting the surface, the more likely it is to be carried away by fog flow and escape collection. Therefore, efficient fog collection requires a thin boundary layer to increase collision rates.

The presence of DLibs creates bumps to disturb the fog flow, compressing the boundary layer around the bumps and enhancing the velocity gradient near the surface. COMSOL Multiphysics simulations show that DLibs induce significant thinning of the boundary layer and generate accelerated airflows directed toward the liquid bumps, which improves the probability of interception (Fig. S12, ESI†).

Similarly, the effects of different droplet detach frequencies on the wind field were simulated and analyzed (Fig. 5b). To simplify the model, we assume that droplet size remains uniform, while droplet spacing on the surface varies. The easier droplets detach from the surface, the wider their spacing becomes. The simulation results show that the fog disturbance caused by DLibs decreases as droplet spacing decreases. As illustrated in Fig. 5b-i, the large-spacing droplet array produces the most significant fog disturbance. Fog flow can almost fully collide with each droplet and maintain a high collision velocity. In contrast, the medium- spacing droplet array substantially reduces wind field disturbance, with only about half of the droplet surface area colliding with the fog. The effective collision area decreases by approximately 50%. Meanwhile, the collision velocity also drops to 24% of that in the small-spacing array (Fig. 5b-ii). More critically, the small-spacing droplet array nearly eliminates wind field disturbance, restricting the fog flow to the outermost region at the top of each droplet and further reducing the effective fog-collision area (Fig. 5b-iii). In this scenario, the wind speed is reduced to 65% of that in the large-spacing array. Through the different collision velocities and collision areas simulated in COMSOL, it can be concluded that DLibs with different spacings directly influence fog collection efficiency. Therefore, the detaching of droplets plays a pivotal role: the more easily droplets detach the surface, the more effectively the air-carried tiny droplets collide with DLibs. In addition, the accuracy of the COMSOL simulation analysis is also proved by the filming of the fog collecting process in the top view (Fig. S13 and Movie S2, ESI†). To better reflect real fog collection conditions, we conducted additional simulations incorporating droplets of varying sizes and densities. The results consistently showed that smaller droplet spacing leads to weaker airflow deceleration, thereby reducing the probability of fog droplet collisions and lowering collection efficiency (Fig. S14 and Movie S3, ESI†).

Impact of surface wettability on the efficiency of harp-like fog collectors

To further verify the impact of DLibs on tiny fog droplet interception, the performance of the harp-shaped structure in fog collection was investigated (Fig. 6a). Aluminum foil strips with a width of 1 mm and spacings of 0.5 mm, 1 mm, and 1.5 mm were used to construct harp-like arrays with varying geometrical configurations. Across all spacing configurations, four typical wettability surfaces were applied on harp-like collectors, including SHL, ACR, slippery surface, and SHB. Under experimental conditions, the distance between the fog outlet and the harp-like fog collector was maintained at 10 cm, and fog collection efficiency was measured every 10 minutes. The results demonstrate that in all three spacing conditions, increasing the surface contact angle consistently led to a significant improvement in fog collection efficiency. Specifically, under 1 mm spacing, the SHB surface showed a 57% higher fog collection efficiency than the SHL surface (Fig. 6b), and similar enhancement trends were observed in the other spacing configurations.

Fig. 6 Influence of the surface wettability of harp-like fog collectors. (a) Schematic of the harp-like fog collector with aluminum foil strips coated with surfaces of varying wettabilities. (b) Fog collection efficiency of harp-like fog collectors with various wettabilities and different spacings. (c) The dynamic behavior of DLibs on harp-like fog collectors with various wettabilities. Fog forms a continuous water film on the SHL surface. (d) For the ACR surface, small DLibs converge into larger droplets, eventually forming thick liquid columns. (e) DLibs always exist on slippery surfaces during the growing and detaching cycle. (f) Tiny droplets rapidly bounce off the SHB surface, maintaining larger droplet spacing.

In the fog collection process, surfaces with different wettabilities exhibit distinct dynamic behaviors (Movie S4, ESI†). DLibs do not form on SHL surfaces. Instead, a continuous thin liquid film covers the entire array surface (Fig. 6c). In contrast, ACR surfaces initially produce small DLibs that gradually converge into larger ones. Once large droplets slip away, a thick liquid column covers the surface (Fig. 6d). As a result, the fog collection efficiency of ACR surfaces is only about 3% higher than that of SHL surfaces.

By comparison, slippery surfaces exhibit significantly higher fog collection efficiency (Fig. 6e). On slippery surfaces, DLibs can continuously form after larger droplets slip off, creating a dynamic cycle of droplet bump formation, convergence, and slippage. This process greatly enhances the ability to capture fog droplets, and the collection efficiency is 17% higher than the ACR surface. In addition, the SHB surface behaves differently, as tiny droplets rapidly bounce away after convergence (Fig. 6f). The continual droplet departure maintains a larger spacing between DLibs, increasing the droplet-bump area exposed to fog flow and preserving higher wind speeds at the surface. Therefore, the SHB surface results in a substantial improvement in fog collection efficiency, being 57% higher than the SHL surface.

Conclusions

Efficient fog collection is critically important for providing clean water in arid and resource-limited regions. In this work, we reconsider the structure of beetles’ backs, discovering that a smooth hydrophobic surface can also achieve highly efficient water harvesting as observed in rough patterned surfaces. To answer this question, an aerodynamic mechanism was proposed to explain how hydrophobic surfaces can significantly improve fog-harvesting performance. Dynamic droplet bumps (DLibs) were found on hydrophobic surfaces, which promotes frequent collisions between fog-carried tiny droplets and the collector interface. Moreover, lower contact angle hysteresis helped maintain appropriately spaced DLibs, sustaining airflow disturbances and enhancing droplet capture. This mechanism was further demonstrated on a harp-like fog collector, where hydrophobic surfaces exhibited improved fog collecting efficiency. We anticipate that these findings will provide valuable design insights for advanced fog-harvesting systems and stimulate innovative approaches in liquid manipulation, water reuse, energy conservation, etc.

Author contributions

The study was conceptualized by M. C. and H. B. The methodology was developed by H. B., H. S., Z. L., T. Z., and X. W. The investigation was carried out by H. B., X. W., M. C. (Mingren Cheng), and Z. W. Visualization was done by H. B. and M. C. (Moyuan Cao). M. C. (Moyuan Cao) supervised the project. All authors contributed to the discussion. The original draft was written by H. B. and M. C. (Moyuan Cao), with review and editing contributions from H. B., Z. W., S. H., and M. C. (Moyuan Cao).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Haoyu Bai and He Sun contributed equally to this work. This work was supported by the National Key R&D Program of China (2022YFA1504002), the National Natural Science Foundation of China (52373247 and 22075202), and the Young Elite Scientists Sponsorship Program by Tianjin (TJSQNTJ-2018-17).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00553a

‡ Equal contributions.

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