Fast prototype and rapid construction of three-dimensional and multi-scaled pitcher for controlled drainage by systematic biomimicry

Frequent rainfall leads to the loss of nitrogen, phosphorus, and other elements needed for biological growth in the land, which prompts Nepenthes to evolve a highly specialized conspicuous leaf—a 'pitcher', to supply the nitrogen it lacks by attracting, trapping, and digesting arthropods [37, 38]. Under the impact of raindrops, the liquid can easily wet the entire peristome of the pitcher, but dense raindrops may fall into the pitcher simultaneously so that the nutrients will be diluted and even overflow from the pitcher [39]. The pitcher lid is depressed when young to completely and firmly close the aperture, opens up at various angles at maturity, and has a double collar structure with an inward and an outward curve at the rim of the pitcher's mouth (figure 1(a)) [40, 41]. Taking the Nepenthes x ventrata as an example, the edge of the pitcher's mouth, termed peristome, has a broad collar-shaped structure and a narrow V-shaped gap at the lip neck in-between double collar structures (figure 1(b)). The cross-sectional shape of the collar is a T-shaped arch that connects the inside and outside of the pitcher with both arched arms sloping downward [42, 43]. The height of the peristome decreases from the lip neck to its wings side, showing an inclination angle of 30° (figure S2(d)). To compare the drainage ability of pitcher plants, 12 different species of pitcher plants with pitcher length (L) and pitcher width (W) were investigated [44]. We statistically investigated the distribution of the L/W of several representative genera of Nepenthes (figures 1(c) and S3) [45]. And we find that the Nepenthes x ventrata shows unexpected drainage ability through x-ray scanning (figure 1(d)). Nepenthes x ventrata forms a water film along the peristome to seal the pitcher to prevent subsequent droplets from entering (figure 1(e)). Besides this particular pitcher plant, we find that a smaller ratio of L/W means a lower coverage ratio of the lid, which leads to the filling of water in the pitcher, and the larger L/W can prevent raindrops by the lid.

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Next, we utilized x-ray imaging coupled with a drop-impact set-up to visualize the water film rupture and drainage on the pitcher's mouth (figure 1(d)). The real-time observations and an in-depth understanding of the mechanism can be achieved by in situ x-ray computer microtomography [46–48]. Our experiments involve releasing a water drop (radius R = 2.5 mm, velocity U = 1.0 m·s⁻¹) onto a pitcher's mouth and filming the film rupture and drainage dynamics inside the sample chamber of Micro-CT under 25 kV x-ray tube voltage (figure 1(d)). Water film formation and breakup on pitcher's mouth were captured by x-ray imaging (movie S1).

Water film pins along the margin of the inner rim of the peristome (figure 1(e-i)), which shows a concave shape by its own gravity (figure 1(e-ii)). Water shrinkages at the narrow V-shaped gap of the peristome on the lip-neck side, inducing the film rupture. Water then drains out of the pitcher with water overflowing from the inner peristome rim to the outer side (figure 1(e-iii)). Three-dimensional and multi-scaled structures of the pitcher lead to the breaking up of water film and then water being thrown out of the pitcher directionally at the lower side. The biomimetic design of this pitcher system would be favored for the revolution of existing apparatus for repeatable atmospheric monitoring and raindrop-repellent fume exhaustion.

Next, we proposed a novel method to mimic the pitcher structures in multi-scales and reconstruct the biomimetic system with the same superior drainage behavior. Micro-CT is used for observation and reconstruction [13]. With the help of computer-aided design (CAD) [8], we can transform the digital twin model into an artificial replicator by 3D printing. The design rationale for the systematic biomimicry is mapped in figure 2.

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A single pitcher was carefully cut off and then fixed to the sample frame in the sample chamber of a high-resolution 3D x-ray microscope (figure 2(a)). The rotating gantry construction with a short source-to-detector distance and high beam intensity can accelerate the rapid measurements of unstable samples to minimize the dehydrating deformation (figure S1). In detail, the pitcher mouth that combines the peristome and part of the pitcher sidewall was imaged. The gantry housing the x-ray source and detector remains stationary while the sample rotates by the sample stage in the horizontal direction [13]. Individual x-ray exposure slices were then converted into tomographic slices (figure 2(b)) and transformed into 3D models (figure 2(c)). The configuration minimizes the possibility of changes within the soft biological sample during the scan [49]. The digital model can be directly imported into the CAD for the extraction of key structures and the design of experimental variables. On this basis, a simplified model dominated by key structures was modeled with 3ds Max to further control the variables. With the help of CAD and stereolithography, we can adjust the 3D structural parameters and construct artificial replicators on demand (figure 2(d)).

Figure 2(d) shows three observation frames OFs (see the three boxes) of the biomimetic pitcher model. The pitcher peristome can be divided into different units at different scales. Using the digital twin model, we can reconstruct, re-design and refine the single unit as a micro-cavity with a width of 400 µm and the combining units as a pitcher with a height of 1 dm. Digital light processing (DLP) 3D printing can accelerate the construction of biomimetic pitcher models in one or arrays on demand within hours. Figure 2(e) shows the 3D printed model, which consists of periodically arranged micro-cavities tilting toward the outer rim of the peristome (figure 2(f)). The length (l), width (w), height (h), and radius (r) of single micro-cavities unit and the tilting angle (β) of the biomimetic pitcher are ∼0.6 mm, ∼0.6 mm, ∼2.0 mm, ∼0.5 mm, and ∼20°, respectively (table 1). Smooth (non-structure) and groove (grooved array structure) were also designed for control experiments (figure S4(a)). The water contact angles on the original, and superhydrophobic coating-modified, superhydrophililc coating-modified resin surfaces are 67.8°, 152.8°, and ∼0°, respectively (figure S4(b)).

Table 1. Parameter values of biomimetic structures.

Structure	Scheme	Length l/mm	Width w/mm	Hight h/mm	Radius r/mm	Tilting angle β/°
Micro-cavity unit		l1: 0.6	0.6	h1: 2.0	—	—
				h2: 0.5
		l2: 0.4		h3: 0.8
Micro-cavity array		l1: 0.3	3.1	1.5	1.5	—
		l2: 0.6
Biomimetic peristome		l1: 13.4	w1: 11.3	3.2	—	—
		l2: 6.2	w2: 5.2
Biomimetic pitcher		l1: 11.9	w1: 11.3	28.0	—	20
		l2: 5.3	w2: 5.2

In a short summary, the advantages of our biomimetic construction method include: (1) a single functional unit can be designed with optimized parameters based on the natural structures acquired from the 3D scanning; (2) 3D digital model can be reconstructed and refined by arraying, bending, or even curing the aligned arrays; (3) 3D printing can fast transform the digital design into real applications within hours.

High-speed videography (figure 3(a)) is performed to verify the film-assisted drainage behavior of the artificial pitcher to its natural prototype of N. x ventrata (figures 3(b)–(e)). The maximum rate of the water film rupture (${v_{\text{max} }}$) versus its diameter (w) and the inclined angle (β) of the natural pitchers was summarized in the heat map (figure S2(d)). The w and β of the peristome were concentrated around 4.5–5.5 mm and 30°–35°, respectively and the maximum rate of liquid film rupturing during directional drainage was concentrated at 200–250 mm·s⁻¹. Compared with the natural pitchers that drain water film within 16 ms, the biomimetic pitcher effectively controls and prolongs the sealed process to 70 ms (movies S2 and S3). Figures 3(f)–(i) shows three key factors that determine the drainage dynamics, including the pinning stabilizes the film (figure 3(g)), the dewetting triggers the directional breakup of the film (figure 3(h)), and the open siphoning controls the water thrown out of the pitcher (figure 3(i)). In detail, the water film forms along the ratchet teeth on the inner rim of the peristome after water droplets impact, sealing the pitcher. Liquid film remains stable because of the contact line pinning at the ratchet teeth [42], which is also the boundary between the superhydrophilic peristome and the superhydrophobic inner wall (figure 3(g)). Due to the directional transport of liquid by the arranged micro-cavities structure [21], the liquid film attached to the peristome is continuously thinning. At the same time, the meniscus, which provides the capillary force against the retraction of the liquid film, is gradually dewetting along the V-shaped gap (figure 3(h)). When the meniscus completely dewets, the liquid film eventually ruptures. Under the joint effect of the inertial force and the open siphon [42], the residual liquid is thrown out of the pitcher directionally (figure 3(i)). As a hypothesis, structural design should act on the water film formation and drainage dynamics and digital design could control these drainage dynamics.

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Based on the distinct drainage dynamics, optimization of the core parameters affecting drainage was carried out to control rolling-over drainage dynamics (figures 4, S5, and S6). V-shaped gap is first drafted from the whole digital twin model for investigating the threshold capillary suction force. Figure 4(a) shows the real-time capillary force (Fγ_v-shaped) of biomimetic peristomes with varied opening angles α. The experimental setup was based on the dynamic contact angle measuring instrument and tensiometer (figures 4(b) and S6). When the opening angle of the V-shaped gap (α) is 5.5°, the meniscus can provide the minimum force (ΔFγ) to facilitate the fastest rupture of the liquid film (figure 4(c)). Meanwhile, the film rupture can be prolonged by choosing other suitable V-shaped angles.

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Next, the non-curved plates with different patterned microstructures were used to evaluate the effect of contact line pinning (figures 4(d) and S5). Compared with smooth surfaces, grooved or microcavities-arrayed microstructures can increase the maximum volume of liquid-holding by approximately 13.0% (figure 4(e)). Microstructures not only enhance the pinning effect but also affect the critical rate of overflow (figures 4(f) and S7). Subsequently, 2D plates were curved into 3D T-shaped arch structures. Water droplet spreads fastest on superhydrophilic surfaces with microcavities-arrayed microstructures (figure S7(b)). The minimum rate that can support the successful overflow of water along the T-shaped arch with the microcavities-arrayed surface is 213.9 mm·s⁻¹, which is lower than that on the smooth and grooved surface (figure 4(g)).

As an advantage of digital twin model assisted screening process, we can separate the structural designs into corresponding core parameters: (1) optimal digital twin model can be quickly extracted with the help of real-time acquisition of images and data; (2) complex model can be simplified to some of the key structures for discussion. For example, a double-curvature structure with the ring peristome and arched rim can be transformed into an arched single-curvature rim for priority discussion; (3) independent discussion on core parameters can speed up the extraction of the optimal digital twin model.

Refining the structure by digital twin model can assist the fabrication of the samples with different parameters in one pot by 3D printing, guaranteeing the uniformity and reproducibility of the artificial replicators. Figure 5(a) demonstrates that the superhydrophilic peristome and the superhydrophobic inner wall of the biomimetic pitcher are indispensable to achieving liquid sealing. Under this state, the peristome can be sealed by a smaller droplet, as long as the diameter of the droplet (d) is greater than 42.5% of the width of the biomimetic pitcher (D). Raindrops in the tropics are of comparable size with a diameter of 2–8 mm [50], which fully meets the conditions required for film formation. Figure 5(b) shows that the optimal inclination angle of the biomimetic peristome (β), which is about 20°, was obtained by rapid screening of the capacity of film forming and the maximum average rate of the rupture of the liquid film ($\bar v$). In addition, the stability of the liquid-sealed system is influenced by the impact of the sequence droplets. When the droplet diameter (d) is small or the impacting velocity (v) is low, the droplet will merge with the liquid film so that the liquid will eventually fall into the pitcher. Only when the droplet has a larger diameter or a faster impacting speed, the subsequent droplet will be drained out by sliding on the surface of the curved liquid film (figure 5(c)).

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Effective surface morphologies optimization and geometric deviation control have been achieved by the digital screening process. Biomimetic pitchers in a single unit and arrays both advance drainage properties with novel applications in detection, reaction, and smoke control (figure 6). First, an advanced gas detection device is developed. Ammonia gas and phenolphthalein solution are selected as the tested component and detection reagent (figure 6(a)). The phenolphthalein solution is released by a droplet dispenser to form the liquid film after impacting. Continuous absorption of ammonia gas changes the color of the phenolphthalein solution, and a gradual decrease of the surface tension of liquid film induces the film-rupture. The waste liquid after detection is drained spontaneously, and the biomimetic peristome and inner space will be free from pollution. Besides, the process of liquid drainage can also refresh the peristome for the next cycle of detection. This simple, real-time, and reusable biomimetic pitcher tank has great application potential in the fields of waste gas treatment, hazardous/toxic gas warning, and environmental quality real-time detection. Next, combining and arranging individual biomimetic pitchers into a certain structure can make this unique water-blocking and liquid-draining mechanism more widely and efficiently applied, such as the back-to-back (B–B) and face-to-face (F–F) models for the drop distribution (figure 6(b)).

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To prove the practicability of artificial biomimetic fume pipes, we constructed a set of homemade apparatuses to simulate rainfall and soot emissions. The actual test results are shown in figure 6(c). When the pipes are unobstructed, the smoke can be exhausted smoothly on sunny days (ii); while the pipes are liquid-sealed to prevent droplets from falling into the gas duct on rainy days (iii). When the rain stops, the sealing films rupture and drain to re-open the pipes to exhaust the smoke (iv).

Mature pitcher plants, N. x ventrata, growing in a greenhouse (in Beijing, China) were purchased commercially from XCCT Corporation (Zhejiang province, China) and carefully maintained, covered with plastic film, in a greenhouse under controlled conditions at a temperature of 20 °C–25 °C and relative humidity of 75% to 90%. The plants received 12 h photoperiod per day (8:00 am–8:00 pm). Commercial prototyping resins were used to print biomimetic pitcher structures with a large-area projection micro-stereolithography printer. Form 3L-Model Series White resin, RS-F2-GPWH-04, was printed with a commercial printer (Form 3L, Formlabs, USA). The water was dyed with brilliant blue G to enhance visualization. Deionized water was acquired from Milli-Q with a resistance of 18.2 MΩ. Isopropyl alcohol and ethanol were purchased from Sigma-Aldrich, Inc., United States.

Different from measuring by a caliper previously [51], as an advantage, the peristome width (a) and the pitcher width in minor axis (D) and major axis (b) were recorded by x-ray snapshots and measured by ImageJ (figure S2(c)). Three independent trials were performed for each measurement.

DLP 3D printer (X-maker M-One Pro, MakeX, China) was used at the X–Y axes resolution of 33 μm over large areas (approximately 63.4 mm × 35.7 mm) by scanning a tightly focused image across the build plate to polymerize photosensitive resins layer by layer. All parts with fine features were printed with 20 μm layers. Stand density was controlled by the filling rate of the 3D model (X-maker, MakeX, China). All 3D printed parts were rinsed and sonicated in isopropyl alcohol for approximately 5 min. Parts printed using the commercial printer were post-cured for 5 min at 60 °C while exposed to 405 nm light (Form Cure, Formlabs). Before wettability treatment, the printed artificial pitcher was cleaned with ethanol and deionized water. The treatment process of the superhydrophilic peristome of the artificial pitcher as well as the superhydrophobic inner wall is as follows: first, the whole printed artificial pitchers were immersed in the superhydrophobic solution to add a rough structure on its surface. Second, to obtained a superhydrophilic peristome, the artificial pitcher was treated with O₂ plasma (PT-5ST, SANHOPTT, China) at 200 W five times for 2 min each time. Third, to make the inner sidewall superhydrophobic, the artificial pitcher was dip-coated three times by controlling a motorized vertical mobile device (Mark-10, ESM 301, USA) to immerse samples in a superhydrophobic solution. To prepare the superhydrophobic solution, 0.5 g Capstone ST-200 (Dupont, USA), 1.0 g hydrophobic fumed silica nanoparticles (Evonik Degussa, Germany), and 30 ml ethanol (Aldrich, China) were mixed, and then stirred in a sealed bottle for 1 h. Other experimental materials were printed by a SLA 3D printer (Form 3L, Formlabs, USA) at X–Y axes resolution of 25 μm and a Z-axis resolution of 50 μm.

Pitchers were cut into different parts with a razor blade, and then placed on a sample stage that can be moisturized for a certain period time for observation (a watch glass inserted with a wet layer of dustless wiper). The images of the high depth of field and large format were obtained using optical digital microscope (Olympus DSX1000, Japan), through the depth of field superposition with a resolution of 137 µm in the vertical direction and the combination of the image with 20%–30% overlap rate in the horizontal direction. Other optical images were recorded by the digital camera (Nikon D750, Japan).

A single pitcher was carefully cut off, cleaned with deionized water, dried with N₂ gas in sequence and then fixed to the sample frame in the sample chamber of a high-resolution 3D x-ray microscope (SKYSCAN 1272, Bruker, Belgium), shown in figure 2(a). The pitcher mouth that combines the peristome and part of the pitcher sidewall was imaged at 6.57 µm resolution (4032 × 2688 pixels) with no filtering, no averaging, and a rotation step of 0.1° (rotation angle from 0° to 180°, acquired 1800 frames of raw images) under 30 kV x-ray tube voltage by SKYSCAN 1272 (Bruker, MA, USA), shown in figure 2(a), inset. Raw images were analyzed with NRecon (Bruker, MA, USA), shown in figure 2(b), with GPU acceleration to convert them into exposure slices at a layer thickness of 6.57 µm (without lid structure). Individual x-ray exposure slices were then transformed by CTvox (Bruker, MA, USA) to construct 3D copies of the samples, shown in figure 2(c). 3ds Max (Autodesk) was used to design the artificial pitcher.

Fresh pitcher samples from the greenhouse were taken six preparing steps before the SEM observation. (1) Samples were cleaned with water and dried with N₂ before the following steps; (2) fixed geometry at 4 °C for 24 h by 90 ml 50% alcohol, 5 ml glacial acetic acid, and 5 ml formalin; (3) dehydration by immersed in a series of alcohol with concentrations from 50%, 70%, 85%, 95%, to 100%. Each lasted for a minimum of 1 h and ended with 100% ethanol overnight; (4) shoot apices of peristome were subjected to vacuum infiltration in a fixative solution (5% formaldehyde, 5% acetic acid, and 50% ethanol) for 30 min and then kept at room temperature overnight; (5) frozen in liquid nitrogen and dried overnight in a lyophilizer at −40 °C. (6) Peristome was mounted on aluminum stubs, dissected under a stereomicroscope, and sputtering a thin layer of platinum (EM ACE, Leica) for 5 min at 25 mA to make them electroconductive for SEM imaging. We observed the microstructures of the peristome surface and artificially fabricated samples using the Environmental SEM operating at an acceleration voltage between 10 kV and 15 kV and a working distance between 9.0 mm and 13.5 mm (QUANTA FEG 250).

Cooperating with reasonable lighting modes and intelligent software for controlling the size of droplets, unique drainage processes were filmed at a suitable frame rate by high-speed cameras that matched the rate at which the droplets impacted, and the liquid film broke. Then the behavior of the liquid film was analyzed by Photron FASTCAM Viewer and the rate of the movement was traced by Photron FASTCAM Analysis. Both natural and artificial pitchers were stuck to an angle-adjustable platform to ensure that the experimental position of these pitchers was the same as the natural position (figure 3(a)). Drop impact was captured in oblique and side views simultaneously with two high-speed cameras, each filming at a minimum of 1000 frames per second (FPS). A combination of high-speed cameras, including FASTCAM AX200 (figures 6(b) and (c), 1024 × 1024 pixels at 1000 FPS), and Freefly Wave 4 K ((figures 3(c), (e), 4(g), 5 and 6), 4096 × 896 pixels at 1000 FPS) attached to a TOKINA atx-i 100 mm F2.8 FF and LAOWA FF 100 mm F2.8 CA-Dreamer macro-lens respectively, were used in the experiments throughout the research. The drops were released from a splash water drop kit (MIOPS Splash, DE, USA) at a fixed height to control the impact speed above the artificial pitcher tank. The size of the drop and the impact frequency can be controlled by the remote-control app, Miops mobile 4.23. And water film formation time, water film duration, and water film breakup time were calculated directly from the high-speed images for each trial.

In order to analyze the effect of the angle of V-shaped gap (α) on the liquid film rupture and the effect of surfaces with different structures on storage capacity of liquid, we measured and discussed the data obtained by means of adhesion force measurement (figures S5(a) and S6(a)). By means of the software 3ds Max, half-peristome models with the same parameters except for different angles of V-shaped gap (α ε (3.7°, 5.5°, 7.1°, 8.7°, 10.5°)) were designed. The structure of samples in the top view and 3D view is shown in figure S6(b), and the structural parameters were listed in table 1. After wettability treatment, the peristome was superhydrophilic and the inner wall was superhydrophobic. We gripped the sample on the mechanical measurement probe and lifted it out of the water surface from the submerged state. The mechanical changes in real-time during the process of simulating liquid film rupture from dewetting to liquid bridge rupture were measured. The smooth, grooved, and microcavities-arrayed microstructures were converted to straight plates of which the wide was 10 mm and the length was 30 mm (figure S5(a)). The structured side of the plates was treated to be superhydrophilic, and the rest was treated to be superhydrophobic. After prewetting, the plates were loaded on the adhesion force measurement equipped with a liquid injection device to measure the liquid-holding capacity of different samples at the same flow rate of 2 ml·min⁻¹ (figure 4(d)). The mechanical changes of the stable attachment and unstable fall of droplets under different structures were recorded by the instrument itself.

In order to explore the minimum rate at which the residual liquid completes the rolling-over drainage in the way of overflowing around the peristome when the water channel is formed, we designed the enlarged model of the peristome fabricated by DLP 3D printing and verified the role played by the arched biomimetic peristome with arranged micro-cavities structure (figure S7(a)). A needle that can vary the injection speed was placed close to one side of the biomimetic arched sample and vertically upwards, and a high-speed camera was placed in the front view to record the behavior of the liquid flow (figure 4(f)). The image of the experimental setup and the optical microscope images of the sample with biomimetic peristome microstructures are shown in figure S7(a). Except for the upper surface of the curved sample being treated as superhydrophilic, the rest of the sample is superhydrophobic.

Two different wettability, superhydrophobic and superhydrophilic, were modified on the peristome or the inner wall of the biomimetic pitcher by dip-coating. The process of impacting biomimetic pitchers with different wettability by releasing droplets of different sizes at the same height was filmed by a color high-speed camera. The different diameters of droplets (d (2.90 mm, 3.68 mm, 4.98 mm, 6.10 mm, 7.29 mm)) were released from the same height of 7.5 cm by a droplet generator.

Based on the microcavity unit (table 1), samples with different angles were designed by means of the software 3ds Max. The behaviors of droplets with the same diameter (d = 4.98 mm) and velocity (v = 1.23 m·s⁻¹) impacting on the peristome at different angles of inclination were filmed by a color high-speed camera.

Droplets of different diameters (d ε (2.90 mm, 3.68 mm, 4.98 mm, 6.10 mm, 7.29 mm)) and velocities (v (0 m·s⁻¹, 1.23 m·s⁻¹, 1.73 m·s⁻¹)) impacting the liquid film were also released by a droplet generator from different height and recorded by a color high-speed camera.

B–B and F–F units are developed for multifunctional applications (figure 6(b)). For B–B units, three biomimetic pitchers can be sealed simultaneously when a single droplet impacts on the overlap. If three kinds of water-soluble solutes for detecting different gas components are placed on biomimetic peristomes respectively, the detection reagents can be quickly prepared under the impact of the single droplet (figure 6(b-i)). Subsequently, liquid films will rupture and be drained when the detected gas components reach the critical value (figure 6(b-ii)). This array application, which realizes simultaneous liquid sealing and separate detection, can significantly increase the efficiency of gas detection. Furthermore, the sequential drainage behavior for F–F units can also be applied to the mixing of different liquid components (figure 6(b-iii)). Combining B–B units and F–F units into an array structure similar to a 'Chinese knot' can greatly improve the efficiency of droplet distribution and gas detection, and further expand the application range of these artificial biomimetic pitchers.

A set of homemade apparatuses was constructed to simulate rainfall and smoke emission. The arrayed structure was designed with 3ds Max and directly printed by a large-area DLP 3D printer. By pressurizing the designed hermetic water tank (the pressure is about 0.2 MPa), droplets quickly dropped from the array of needles equipped at the bottom of the tank. With the heating of candles at the bottom, the smoke rises on thermals and is exhausted through the fume pipe at the top.