Kaiwen
Chen
ac,
Luyao
Chen
a,
Xianfu
Xiao
a,
Cheng
Hao
c,
Haonan
Zhang
bc,
Tongtong
Fu
c,
Wei
Shang
d,
Hui
Peng
a,
Tianyi
Zhan
*a,
Jianxiong
Lyu
*a and
Ning
Yan
*c
aCo-Innovation Center of Efficient Processing and Utilization of Forest Resources, College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China. E-mail: tyzhan@njfu.edu.cn; jianxiong@caf.ac.cn
bJiangsu Provincial Key Lab of Sustainable Pulp and Paper Technology and Biomass Materials, Nanjing Forestry University, Nanjing, Jiangsu Province 210037, China
cDepartment of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario M5S 3B3, Canada. E-mail: ning.yan@utoronto.ca
dCollege of Chemistry Engineering, Northeast Electric Power University, Jilin 132012, China
First published on 13th May 2025
Inspired by cactus spine and desert beetle back structures, we developed a wood-based wedge-shaped surface with gradient wettability for efficient and controlled spontaneous directional liquid transport. Utilizing the natural anisotropic and porous structure of wood, the wedge-shaped surface exhibited a continuous gradient wettability after chemical treatments combined with UV-induced modifications. The resulting surface enabled highly efficient directional liquid transport with transport rates reaching up to 8.9 mm s−1 on horizontal placement and 0.64 mm s−1 on vertical surfaces against gravity. By integrating geometric curvature and surface energy gradients, the innovative design achieved synergistic Laplace pressure-driven and wettability-driven liquid motions. To further demonstrate its potential for practical application, a fog-driven power device constructed using the gradient wettability wood with cactus spines not only enhanced water harvesting and energy conversion capabilities but also offered an environmentally friendly system. This study expanded the design toolbox for bioinspired liquid management surfaces, offering promising applications in water resource management, energy harvesting, and microfluidic devices.
New conceptsThis study introduces a new concept that synergistically utilizes the natural anisotropic structure of wood with biomimetic features derived from cactus spines and desert beetle backs. Unlike traditional synthetic materials or micro-nanofabricated surfaces, we leveraged the inherent properties of wood to create a wedge-shaped surface with a continuous gradient wettability. This innovative design integrates the geometric curvature and surface energy gradients, enabling spontaneous and highly efficient directional liquid transport both horizontally and vertically against gravity. This concept fundamentally differs from existing approaches by offering a scalable, environmentally friendly solution that eliminates the reliance on complex fabrication techniques. It bridges the gap between natural inspiration and practical application, expanding the toolbox for sustainable material design. Our findings not only advanced the understanding of liquid transport mechanisms but also opened new avenues for applications in water harvesting, microfluidics, and energy conversion technologies, demonstrating the transformative potential of bioinspired materials in addressing global resource challenges. |
Traditional directional liquid transport materials include metals12,13 (e.g., copper, aluminum), polymers14–16 (e.g., PDMS, PTFE), and silicon-based materials.17,18 Metals are widely used for their excellent mechanical strength and thermal conductivity, such as in the construction of gradient surfaces with copper nanopillar arrays for fog collection and liquid guidance.19 Polymers are favored for their ease of fabrication and customizable surface chemistry, as demonstrated by PDMS surfaces with microstructures that enable wettability gradient-driven droplet manipulation.20 Silicon-based materials, while popular due to their compatibility with micro–nanofabrication techniques, face limitations in broader application due to the high cost of processing equipment and chemical reagents.21 In terms of structural design, various geometries significantly influence the efficiency of liquid directional transport. Conical and wedge-shaped structures are among the most used, generating Laplace pressure gradients through curvature differences to drive droplets from high- to low-curvature regions.22–24 For example, Yu et al.25 reported a titanium dioxide-coated conical spine utilized for water collection and transport. This design demonstrated high efficiency in liquid guidance and revealed the mechanism of directional self-transport of water droplets beneath oil on its surface. Similarly, microgrooves or channels guide droplets along predefined paths, minimizing dispersion and enhancing control, which is particularly useful in microfluidic devices.26,27 Furthermore, hierarchical structures, combining microscale and nanoscale features, amplify wettability contrasts and enhance driving forces, resulting in markedly improved liquid transport performance.28,29 Despite these advancements, these designs often rely on precise and sophisticated micro–nanofabrication techniques, such as laser etching and chemical vapor deposition. Such processes are not only costly but also less environmentally friendly, posing significant challenges for large-scale implementation and sustainable applications.
Wood, with its unique microstructure and sustainability, exhibits immense potential and advantages in directional liquid transport. As a natural material, its porous network and anisotropic structure provide an ideal foundation for constructing liquid transport channels.30–34 The internal vessels and tracheids create vertically aligned pathways, enabling rapid liquid flow with minimal resistance.35–39 Moreover, the distinct characteristics of earlywood and latewood, such as differences in density and porosity, enhance the diversity and controllability of liquid transport. Earlywood, with its thin cell walls, facilitates efficient fluid conduction, while latewood, with thicker cell walls and a denser structure, effectively regulates flow.40–42 This natural hierarchical arrangement requires no complex processing, making wood inherently suited for designing surfaces for directional liquid transport. For example, Wang et al.43 leveraged the structural differences between earlywood and latewood, combined with selective chemical modifications, to create wood surfaces with specific gradient wettability for liquid directional transport. Overall, wood's inherent structural features, ease of processing, and functionalization position it as a versatile and sustainable material for advanced liquid transport systems. Recent studies have explored wood's potential for directional liquid transport, but few have utilized bioinspired principles to enhance its performance.
Nature offers an abundance of efficient designs for directional liquid transport. For instance, a cactus spine exhibits a conical curvature that generates Laplace pressure gradients, driving liquid motion, while the backs of desert beetles demonstrate gradient wettability that facilitate directional droplet movement. These systems combine the geometric structure and surface energy to achieve exceptional liquid transport efficiency under challenging conditions. Inspired by these mechanisms, bioinspired materials have increasingly been used to mimic nature's designs for advanced applications. However, these efforts often rely on synthetic materials or micro–nanofabrication techniques, which are costly, environmentally unfriendly, and difficult to scale up.
This study bridges the gap between natural materials and bioinspired principles by integrating the anisotropic structure of wood with the bioinspired mechanisms of cacti and desert beetles. By leveraging wood's hierarchical architecture and natural transport channels, combined with surface modifications that mimic gradient wettability and curvature-driven pressure differentials, this work introduces a sustainable and scalable solution for directional liquid transport. The conical geometry of cacti spines and a wedge-shaped wood surface was combined to obtain a controlled gradient wettability, enabling synergistic liquid motion driven by both Laplace pressure and surface energy gradients. Compared with previously reported wood-based liquid transport systems that mainly focus on either structural anisotropy or chemical modification alone, our design integrated geometric shape, inherent anisotropic structure, and a precisely tunable gradient wettability. This unique combination significantly improved directional liquid transport efficiency, avoided the complexity and high cost associated with traditional micro–nanofabrication methods, and enhanced scalability and environmental friendliness. Experimental results revealed that the gradient wettability wedge-shaped wood enabled unidirectional liquid transport under both horizontal and inclined conditions. In horizontal placement, the liquid transport rate reached up to 8.9 mm s−1, while under inclined placement (e.g., at a 90° angle), the liquid (4 μL) still achieved upward transport against gravity at a rate of 0.64 mm s−1. These findings systematically elucidated the mechanism of the synergistic interaction between geometric driving forces and gradient wettability. Furthermore, utilizing this gradient wettability wedge-shaped wood, a highly efficient fog-driven energy device was developed, demonstrating performance significantly superior to traditional fog collection systems. This innovative design not only expanded the methodology for creating bioinspired liquid management surfaces but also provided critical insights for advancing developments in the field of water collection and energy conversion.
A plasticizing solution was prepared by dissolving 2 g of PVDF–HFP in 100 mL of DMF at 60 °C in a water bath for 1 hour until fully dissolved. The DWW was immersed in the plasticizing solution under a vacuum pressure of 0.095 MPa for 12 hours, resulting in PVDF–HFP-impregnated wedge-shaped wood (PWW). The PWW was washed several times with deionized water and stored in anhydrous ethanol for further use (Fig. 1). The wedge angles (5°, 9°, and 15°) were specifically selected to systematically evaluate the influence of geometric gradients on directional liquid transport performance. A smaller wedge angle (5°) generates higher Laplace pressure, facilitating rapid liquid transport ideal for microfluidics or precision liquid handling, while a larger angle (15°) provides greater structural stability, which is more suitable for scaling up to practical applications such as large-area fog harvesting. The 9° wedge angle was deliberately chosen as a representative middle ground, offering a balance between transport efficiency (as seen in 5°) and structural robustness (as in 15°), making it ideal for evaluating scalability under practical conditions.
The HWW was then exposed to UV light with a wavelength of 365 nm, positioned 5 cm away from the lamp. A light-blocking film was used to cover the entire surface of the HWW and was gradually moved along the wedge's length from the base to the tip (Fig. 1). The film's movement rates were set to 10 mm h−1, 7 mm h−1, and 5 mm h−1, respectively, to produce three gradient wettability wedge-shaped woods (GWW): G1-GWW, G2-GWW, and G3-GWW.
In the manual water-dropping experiment, a micropipette was used to add 5 μL of water at the tip of the device. A high-resolution camera recorded the device's response, including the rotation of the wedge and the detachment of the droplet. In the simulated fog environment, a commercial ultrasonic humidifier (Model: JSQ107, Guangdong Zhigao Air Conditioner Co., China) was used in a sealed chamber to simulate fog conditions with a fogging rate of 200 mL h−1 and a fog density of 10 g m−3. The device was positioned 50 mm directly below the nozzle of the humidifier, with real-time monitoring of droplet capture, transport, and detachment processes.
To evaluate the stability and repeatability of a single fog-harvesting unit, an additional test was conducted. In this setup, the fog stream was aligned horizontally with the cactus spine of the individual fog-harvesting unit. The fog flow rate was controlled using a wind tunnel setup with an adjustable speed range of 0.5–2 m s−1, monitored using an anemometer. The humidifier generated fog with a consistent density of 10 g m−3, and the device was tested at a distance of 30 mm from the nozzle. The detachment frequency and weight fluctuation of droplets collected by the single unit were recorded over 10 continuous cycles, with each droplet's weight monitored using a precision balance (accuracy: 0.1 mg). This setup ensured consistent fog exposure to the cactus spine, allowing detailed assessment of the stability and repeatability of the device under controlled conditions.
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Fig. 2 Morphology and structure of the samples. (a) Digital photographs and 3D profiles of the sample surfaces; (b) SEM images of the microstructures in cross-section and longitudinal section. |
To evaluate the environmental stability of the gradient wettability on wedge-shaped wood surfaces, particularly under prolonged sunlight exposure, a 7-day outdoor test under natural solar radiation (average intensity ∼50 mW cm−2) was conducted. Three representative samples (G1-GWW, G2-GWW, and G3-GWW) were selected, and their gradient wettability were measured before and after exposure. As shown in Fig. 3c, all samples retained their characteristic gradient wettability, with less than 8% reduction in contact angle across the measured positions. Among them, G1-GWW exhibited the highest stability, with only minor changes in contact angles, while G3-GWW showed slightly larger variations due to its steeper initial gradient. This stability was attributed to the chemical structure of PVDF–HFP, whose strong C–F and C–H bonds provided inherent resistance to photodegradation and environmental oxidation.49,50 These results indicated that the PVDF–HFP-based gradient surfaces possessed reliable short-term UV resistance and maintained functional surface performance under realistic outdoor conditions. Future work will focus on integrating UV-resistant coatings to further enhance long-term durability for extended applications.
Further analysis was conducted on the bottom regions of the gradient wettability wedge-shaped wood samples (G1-GWW, G2-GWW, and G3-GWW) to investigate their microstructure and compositional changes. This choice was made because the formation of gradient wettability was closely tied to UV exposure time. The bottom, as the initial position of the movable light-shielding mask, underwent the longest UV irradiation, providing a clearer representation of the photocatalytic reaction's effects on the microstructure and chemical composition. Systematic characterization of the bottom region was therefore critical for uncovering the intrinsic mechanisms underlying the formation of gradient wettability. SEM analysis revealed that TiO2 nanoparticles were uniformly distributed across the microstructure of the sample surfaces, ensuring consistency in the photocatalytic modification process (Fig. 4a). Notably, in G3-GWW-bottom, prolonged UV exposure caused visible surface cracking, likely due to changes in surface tension induced by the abundant hydroxyl groups generated during the photocatalytic reaction, which led to the formation of microcracks. FTIR analysis highlighted significant chemical changes on the sample surfaces. Key features include the O–H stretching vibration peak at 3300 cm−1 (hydroxyl groups), the C–H stretching vibration peak around 2900 cm−1, and the Ti–O vibration peak within 500–700 cm−151 (Fig. 4b). With increasing UV radiation exposure, the O–H peak intensity in G3-GWW-bottom showed a marked increase, indicating a substantial rise in hydroxyl content and supporting the transformation of the surface from hydrophobic to hydrophilic. Changes in the C–H stretching vibration peak further confirmed the impact of UV photocatalysis. Additionally, the relative weakening of the Ti–O peak intensity suggested that the photocatalytic reaction generates more active sites on the TiO2 surface, thereby enhancing the gradient-driven wettability effect. The XPS full spectrum analysis revealed that the bottom surfaces of all three samples contain C, O, Ti, and F elements, with the F peak intensity following the trend G1-GWW-bottom > G2-GWW-bottom > G3-GWW-bottom (Fig. 4c). This decrease in F element content with increased UV exposure time likely resulted from PVDF–HFP decomposition and F element desorption under high UV energy. The Ti characteristic peak (458.5 eV) shifted to higher binding energies and became sharper, indicating changes in the chemical environment of Ti, such as Ti–O bond rearrangement or enhanced binding with hydroxyl groups46 (Fig. 4d). Moreover, the O–Ti peak at 530.0 eV gradually weakened, suggesting intensified redox reactions on the TiO2 surface during the prolonged UV exposure, which might have consumed surface O–Ti bonds (Fig. 4e). These findings demonstrated that UV radiation significantly influenced the surface element distribution and chemical bonding, thereby enabling continuous surface transition from the hydrophobic to hydrophilic state. This understanding provided critical insights into the construction mechanisms of wood surface with a gradient wettability and laid a theoretical foundation for its application in directional liquid transport and functional surface design.
Fhorizontal = FL + FW | (1) |
![]() | (2) |
Additionally, the gradient wettability driving force (FW) arises from differences in surface free energy, expressed as:
FW = γ(cos![]() ![]() | (3) |
In summary, when the surface is placed horizontally, the combined effect of the geometric driving force (FL) and the gradient wettability driving force (FW) propels the droplet along the surface in a directional manner.
Further analysis of the influence of wedge angle on droplet transport distance and velocity is shown in Fig. 6c and d. The results revealed that the wedge angle not only significantly impacted droplet transport velocity but also played a critical role in determining the maximum transport distance and load-carrying capacity. Specifically, the 5° wedge angle exhibited a steep increase in both transport distance and velocity as the droplet volume increased from 1 to 4 μL, with the transport distance rising from 7.8 mm to 19.5 mm and velocity from 3.47 mm s−1 to 8.9 mm s−1. However, droplets larger than 4 μL could not be stably transported, likely due to the combination of high capillary force and insufficient lateral confinement, resulting in detachment from the surface. This indicated that while the 5° wedge angle provided a strong geometric driving force (resulting from a pronounced curvature difference, R1 < R2), its structural limitations reduced the droplet stability and load-carrying capacity, making it suitable for efficient transport of small droplets. In contrast, the 9° wedge angle demonstrated superior transport performance across a broader range of droplet volumes. Both transport distance and velocity increased steadily with volume from 1 to 7 μL, with transport distance saturating at 20 mm from 7 μL onward, and velocity stabilizing at approximately 7.5 mm s−1 for droplets between 7 and 9 μL. These results suggested that the 9° wedge angle achieved an optimal balance between geometric and gradient wettability driving forces, making it ideal for medium-sized droplet transport with high stability and load-carrying capacity. For the 15° wedge angle, both transport velocity and distance exhibited slower, more linear growth. The droplet transport distance increased gradually from 2.01 mm (1 μL) to 20 mm (15 μL), while velocity rose from 1.68 mm s−1 to a peak of 3.85 mm s−1 at 14 μL before slightly declining. This behavior was attributed to the relatively weaker geometric gradient inherent to the wider wedge angle, with transport relying primarily on the wettability gradient. However, the larger wedge angle ensured superior droplet stability and load-carrying capacity, making it suitable for the steady transport of large droplets, despite the reduced transport efficiency compared to narrower wedge geometries. As shown in Table S3 (ESI†), GWW demonstrated superior performance in terms of droplet transport velocity and directional liquid spreading compared to other materials with gradient wettability. This enhancement is primarily attributed to the synergistic effect of the optimized wedge-shape structures and the precisely engineered gradient wettability, which facilitated rapid and sustained droplet motion.
In summary, the wedge angle had a dual effect on droplet transport performance.53 Smaller wedge angles (e.g., 5°) provided stronger geometric driving forces, enabling efficient transport of small droplets but had a limited load-carrying capacity. Medium wedge angles (e.g., 9°) achieved an optimal balance among the transport velocity, distance, and stability, making them versatile for a wide range of droplet sizes. Larger wedge angles (e.g., 15°) were better suited for stable transport of large droplets but at the cost of reduced velocity and efficiency. These findings provided clear guidance for the design and optimization of gradient wettability wedge wood, enabling the selection of appropriate wedge angles based on specific application requirements.
To further investigate the impact of tilt angle on the droplet transport behavior on the gradient wettability wedge-shaped wood (G3-GWW), the migration distance and transport speed of droplets with different volumes were systematically examined under different tilt angles (0°, 30°, 45°, 60°, 90°) (Fig. 7b and Fig. S11, ESI†). At the horizontal state (0°), the droplets exhibited the best transport performance, with the migration distance significantly increasing with the droplet volume. For example, a 3 μL droplet could travel up to 15.2 mm, and droplets ≥6 μL achieved a maximum distance of 20 mm. Corresponding transport speeds increased from 2.56 mm s−1 to 5.56 mm s−1, demonstrating a clear volume responsiveness. As the tilt angle increased, both the migration distance and speed showed a decreasing trend. At 30°, although the 6 μL droplet maintained a 20 mm travel distance, the speed of larger droplets decreased markedly. For instance, the speed of an 8 μL droplet dropped to 1.51 mm s−1, indicating that gravity was gradually becoming a limiting factor. At 45° and 60°, the droplet transport performance further decreased, and the transport speed of larger droplets significantly slowed. At 60°, for example, the speed of a 5 μL droplet was only 0.54 mm s−1, reflecting a strong counteracting effect of the larger gravitational component on the driving mechanism. Under vertical conditions (90°), the droplet motion was the most constrained, with the maximum migration distance of 20 mm and speed of only 0.39 mm s−1 (for a 4 μL droplet), indicating strong suppression of directional motion by gravitational forces at steep inclinations. To better understand the impact of inclination on droplet transport in gradient wettability wedge wood, it was essential to analyze the underlying mechanism under the combined effects of gravity and surface driving forces (Fig. 7c). The directional transport of droplets on inclined gradient wettability wedge wood was governed by the geometric driving force (FL), gradient wettability driving force (FW), and the gravitational component (Fgsin
α). The total driving force can be expressed as:
Finclined = FL + FW − Fg![]() ![]() | (4) |
FL > Fg![]() ![]() | (5) |
The study of inclined gradient wettability wedge wood highlighted the synergy between geometric design and surface functionalization to achieve controllable directional droplet transport, overcoming the challenges posed by gravity and other external factors. This research not only validated the cooperative mechanism of gradient wettability and geometric driving forces but also provided theoretical insights and technical guidance for designing liquid management and transport systems in complex environments. Furthermore, it demonstrated broad application prospects in fields such as energy harvesting, microfluidics, and sensing technologies.
To evaluate the stability and sustainability of the device during the fog collection process, we monitored the detachment frequency and weight fluctuation of individual fog-harvesting units (Fig. S12, ESI†). Experimental results revealed that during 10 collection cycles, the weight of each collected droplet remained consistently around 6.2 mg, with droplets detaching approximately every 100 seconds. This consistent performance underscores the device's excellent stability in dynamic fog-harvesting conditions. During manual droplet experiments (Fig. 8c), the device responded sensitively to the weight of water droplets, demonstrating excellent mechanical flexibility and stability. In a simulated fog environment (Fig. 8d and Video S6, ESI†), the device efficiently captured and transported water droplets, showcasing its practicality and high efficiency. By integrating the kinetic energy of droplets with the mechanical response of the wedge structure, this fog-driven device not only efficiently collected water but also revealed significant potential for converting ambient environmental energy into useful mechanical motion. To further improve the fog harvesting efficiency and validate scalability for practical applications, a multi-spine array integrated with GWW to create an array-type fog harvesting collector was fabricated (Fig. S13, ESI†). The array-type collector effectively expanded the fog-capturing area and increased droplet collection density, achieving an impressive fog harvesting rate of approximately 7.2 kg m−2 h−1. The array structure facilitated uniform droplet distribution across multiple wedge-shaped wood surfaces, which accelerated water transport and improved the mechanical responsiveness of the fog-driven power device. These results clearly indicated that the array-type cactus spine–GWW collector not only significantly enhanced fog harvesting efficiency but also scaled easily for real-world implementations. Moreover, the comparative analysis summarized in Table S4 (ESI†) highlights that the integration of cactus spine-inspired structures with gradient wettability wedge wood in a modular design achieved superior fog-collection efficiency compared to previously reported bioinspired fog-harvesting surfaces. This remarkable enhancement originates primarily from the combination of droplet nucleation, directional transport, and rapid removal facilitated by the combined structure and wettability features. Such comparative evaluations validated the effectiveness of our structural design and demonstrated its advantages over existing droplet transport and fog collection systems.
This device offered multiple critical advantages. Firstly, its bioinspired architecture synergistically integrated the fog collection capability of cactus spines with the liquid transport efficiency of GWW, achieving a unique cooperative effect. The implementation of the multi-spine array structure further improved the collection yield and operational stability, providing a scalable and modular strategy for practical deployment. Secondly, the successful implementation of a scalable multi-unit array structure enhances both collection yield and operational stability, providing a practical modular strategy suitable for real-world applications. Additionally, the device design is structurally simple, inherently self-powered, reusable, and highly cost-effective, further highlighting its practical utility. Moreover, this innovative device has substantial potential for addressing critical applications such as water harvesting in arid environments, industrial condensate management, and small-scale energy harvesting. Further optimization can enhance its adaptability to complex environments. For example, improving the gradient wettability and geometric structure of the GWW can enable more efficient liquid transport and energy conversion. Incorporating additional bioinspired materials and multifunctional coatings can further boost its collection efficiency and durability. This fog-driven power device provided a novel approach to liquid management and energy utilization technologies, offering critical scientific and technological support for achieving sustainable development goals.
Notably, this work not only offered a scalable approach for developing bioinspired liquid management surfaces but also provided a sustainable alternative to synthetic materials by capitalizing on wood. As a renewable and biodegradable natural resource, wood confers considerable environmental benefits, such as reduced reliance on petroleum-based synthetic materials, diminished environmental footprint, and enhanced sustainability. Additionally, the simplicity and cost-effectiveness of the processing methods render this wood-based design readily scalable for real-world applications, including outdoor fog harvesting facilities and sustainable liquid management systems. Furthermore, the integration of geometric design and surface functionality unlocks new opportunities for tailored applications in microfluidics, environmental engineering, and biomedical devices. In this context, future research could focus on further optimizing gradient control for diverse liquids and environmental conditions, as well as enhancing the durability, responsiveness, and multifunctionality of the wood-based surfaces.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00440c |
This journal is © The Royal Society of Chemistry 2025 |