Bamboo-joint-like platforms for fast, long-distance, directional, and spontaneous transport of fluids

Abstract

Spontaneous transport of fluids without external force offers an enabling tool for a wide spectrum of fields. However, the development of a universal spontaneous transport platform for liquids remains a challenge. In this work, a novel bamboo-joint-like platform with tapered micro-tubes as transport units is presented, which not only enables the spontaneous transport and extrusion of liquids but also enables customized and optional assembly of transport devices. Spontaneous transport characterized with long-distance, anti-gravity transport, directional transport, and liquid extrusion characteristics was found to show excellent transport capacity. The results indicated that both transport distance and speed varied periodically with time, which was mainly due to the difference in curvature caused by asymmetric structure and capillary force. The desired spontaneous transportation was successfully obtained even when the supply rate speed was up to 632.5 μl/min and length of platform reached a scale of hundreds of millimeters. Transport units were easily fabricated via a commercially available 3D printing technique, so that the customized and directional spontaneous directional transport can be realized for liquid distribution, serpentine loop transportation, and speed control. With the comprehensive use of transport units and connectors, it is very easy to implement self-service construction of a universal complex multi-functional transportation platform.

I. INTRODUCTION

Spontaneous transport of fluids is a ubiquitous phenomenon that helps organisms acquire special survival skills.^1–6 Unlike existing industry systems in which liquid transport consumes a lot of energy, spontaneous transport has attracted more and more research attention due to low energy consumption promising application value in microfluidic technologies and design of advanced devices.^5,7–10 Inspired by nature, the mechanism of spontaneous transport is gradually revealed.^7,11–16 Under the action of capillary force and Laplace pressure, the special microstructure and wettability gradient were generated on the surface of organisms, which realizes the energy transformation from surface energy released by the transported liquid into kinetic energy and gravitational potential energy.^17,18 In the past few decades, bio-inspired spontaneous transport systems have been developed rapidly, which realize functions of improving the economics of fabrication technology, breaking the transportation distance limit, and increasing the versatility of the transportation platform.^7,19–23

Currently, functional surface with the capacity of spontaneous transport is highly desired in areas such as water harvesting, heat transfer, and droplet manipulation.^24–27 Numerous research studies of spontaneous transport phenomena have revealed the secret underlying biological examples. Structure gradient, wettability gradient, and their combinations are identified as the key factors for obtaining spontaneous liquid transport. According to further investigation on the structural features of organisms, such as shorebird beak,²⁸ cactus spine,²⁹ and pitcher's peristome,³⁰ it can be known that the spontaneous transport ability is mainly attributed to the multi-level surface structures. Another typical case found that the wettability gradient on the back of the desert beetle enhanced the ability to capture water in extreme environments.¹⁵ By tapping into multi-level structure and wettability, directional water collection was achieved on spider silk.² Although various methods offer an enabling tool to design spontaneous transport systems, it is still a challenge to obtain a spontaneous transport system meeting actual application scenes by mimicking the complex structure of living organisms.

Under the commendable progress of fabrication technology,^24–27 bio-inspired surfaces with various topographical patterns are becoming increasingly popular for spontaneous transport systems.^27,31 Ghosh et al. reported a facile wettability patterning method to fabricate a wedge-patterned surface on open platform for pumpless fluid transport and achieved complex droplet handling tasks.³² To combat the gravity, a novel superhydrophobic pump was developed to obtain spontaneous antigravity water transport with a centimeter-scale height.^33,34 Various adaptive strategies, e.g., asymmetrical wettability barriers,³⁵ microgrooves with a pit array,^20,21 and hierarchical microchannel,³⁶ have been devoted to realizing directional and long-distance spontaneous transport. However, most reported methods require specialized design and fabrication based on the transport path and desired function. Therefore, the development of a facile and universal platform for spontaneous liquid transport is not only a salient requirement but also may be the last obstacle to make the technique applicable for a variety of application scenarios.

In this work, we developed a bamboo-joint-like platform by assembly of tapered micro-tubes, which has advantages of facile fabrication and excellent universality. Being commonly used as a transport unit, a tapered micro-tube without complicated multi-level surface structures is easily fabricated by a commercial 3D printer and is convenient for self-service construction of a complex multi-functional transportation platform. This work will offer more options for the modular design of an advanced microfluidic device.

II. EXPERIMENTAL

A. Material preparation

The fluid transport platform was made by assembly of translucent photosensitive resin tubes (a tube with a length of 30 mm and an inner diameter of 0.5 mm at the inlet). These transport units are commercially produced by Hangzhou Xianlin 3D Technology Co., Ltd., China, using a low cost 3D printing technology. Before the experiment, the transport platform was cleaned by immersion in alcohol solution with an ultrasonic cleaning machine for 10 min and then washed with de-ionized water, followed by drying with hot air. The transport platform was immersed in an aqueous solution of sodium dodecyl benzene sulfonate (SDBS) with a mass fraction of 5% for 10 min. After treatment, hot air was applied to dry the inner cavity to ensure that there was no trapped liquid. If the tubes are continue to be used, sodium dodecyl benzene sulfonate attached on the tubes will be removed and hydrophilicity will gradually decrease but will recover after retreatment.

To observe the water flow in the inner channel of the conical tube, water droplets were dyed with red or blue inks. In the experimental process, a micro-pump was used for liquid supply at a supply rate of 63.25 μl/min. The water was not injected straight into the tube but dropwise added to contact the end face of the tube, and the droplets absorbed into the tube by capillary force. The entire experimental procedure was recorded with a camera (SONY RX100V, Japan) for post-test analysis and processing.

B. Material characterization

Material surface morphology was characterized using a scanning electron microscope (SEM, SUPRA 55 SAPPHIRE, Germany). Wettability of the inner wall of the transport unit was evaluated by measuring surface water contact angle, using an optical contact angle meter (JC2000D4F, POWEREACH, China).

II. RESULTS AND DISCUSSION

A. Characterization of the transport unit

Optical image and SEM images of the transport unit are as shown in Fig. 1(a). The transport units are designed to be transparent to enhance the visualization of liquid transportation. As shown in the SEM images, the surfaces are highly porous with micro- and nano-structures. The transport unit is hydrophobic. When a water droplet contacted the inner wall, it did not spread, and the wetting process was not observed within 0.18 s. When water droplets contacted the inlet of transport unit, the liquid was difficult to be absorbed, and only a small amount of liquid entered the tube after 77 s [Fig. 1(b)]. The inner wall of the transport units became hydrophilic after being treated with SDBS, and the liquid spread rapidly when contacted with the surface as shown in Fig. 1(c). Then, liquids could be easily absorbed in the transport units and transported until the unit was filled when liquid droplets were positioned near the inlet.

FIG. 1.

(a) Optical and SEM images of the transport units. Wetting process of a liquid on the inner wall of the transport unit before (b) and after (c) treatment by an aqueous solution of SDBS. Length of the transport unit is 30 mm.

B. Spontaneous and directional transport unit of fluid

It is an intriguing phenomenon that fluid flows spontaneously without external force in micro-tubes. We functionalized the interior of a tube (inner diameter of 0.5 mm) to become hydrophilic. When a droplet approached the inlet of the tube, the droplet was absorbed into the tube rapidly under capillary forces. However, when the whole tube was filled with water, the liquid did not escape from the other side of the tube due to its surface tension. Instead, the liquid accumulated at the inlet of the tube, resulting in a failure of the liquid's spontaneous transport [Fig. 2(a)]. This experiment shows that such a tube can only continuously transfer liquid by capillary force, and it is extremely difficult to realize the expected spontaneous transportation and output of fluid to the target area.

FIG. 2.

(a) The straight micro-tube can transport liquid by capillary force but cannot achieve liquid extrusion. (b) Conical micro-tube for liquid transport and output. (c) The liquid transport speed of the conical micro-tube with different cone angles. (d) Bamboo-joint-like platform for liquid transport. (e) Symmetrically uniform flow of liquid in the straight micro-tube without directional transport properties. (f) Asymmetric flow of liquid in the conical micro-tube. (g) The directional transport of liquid in the bamboo-joint-like platform and the shape of the liquid column within the conical micro-tube. (h) Asymmetric flow and directed transport models of liquids in the conical micro-tube. All bars denote 10 mm.

In our previous study, we developed asymmetric structures to realize the spontaneous and long-distance transport of a liquid.^11,37 However, it was difficult for those structures to be processed inside a micro-tube. A 3D printing method was used to process the internal asymmetric conical structure, so that the spontaneous transport of liquid can be realized under a curvature effect. The experimental results show that when the droplets approached the inlet where the inner diameter of the conical tube was smaller, they were readily absorbed into the tube and then transported along the tube under the effect of an asymmetric structure and capillary force. Finally, the fluid overcomes the surface tension at the outlet and flows out from the outlet, where the inner diameter of the conical tube was larger [Fig. 2(b)]. This shows that the inner conical micro-tube can realize not only the spontaneous transport of liquids but also the liquid extrusion, which is difficult to achieve using existing techniques.

To study the influence of a conical micro-tube structure on the performance of the spontaneous transport of a liquid, the liquid transport speed was investigated when the cone angle α = 2°–10° [Fig. 2(c)]. The experimental results showed that the liquid transport speed decreased with time at different cone angles. When the cone angle was smaller (α = 2°–6°), the capillary force was stronger due to a smaller inner diameter of the cone tube in the initial stage of liquid transport. In this case, the liquid transport speed was fast under a combined action of capillary force and curvature effect. However, as the diameter of the conical tube increased, the capillary action weakened and the liquid transport speed decreased dramatically. When the cone angle α = 8°–10°, the inner diameter of the cone tube increased rapidly, and the capillary force was not obvious, so the initial liquid transport speed was slow. Since curvature effect played a leading role in this process, the fluctuation of liquid transport speed was slight. In general, the smaller the cone angle, the faster the liquid transport speed. As the cone angle increased, the liquid transport process became smooth. Considering the liquid transport speed, transport stability, and processing difficulty of the conical micro-tube, the optimal cone angle α of the conical micro-tube was set to α = 4° in the subsequent research.

The above research proved that the conical micro-tube could realize the spontaneous transport and extrusion of liquids. However, as the transport distance increased, the transport speed would drop sharply. At the same time, as the diameter of the asymmetric channel of the conical micro-tube gradually increased with the distance, more and more liquids would be filled in the tube, which is not an ideal situation for microfluidic transportation, especially for liquids that are expensive or difficult to obtain. To solve this problem, we proposed to extract a 30 mm long conical micro-tube as a transport unit to enhance the capillary effect and the curvature effect and then combine the transport units together to form a bamboo-joint-like platform [Fig. 2(d)]. When the liquid drops were positioned close to the inlet of the bamboo-joint-like platform, the liquid was spontaneously transported in the first transport unit. In conical micro-tubes, the liquid would be transported from the smaller diameter end to the larger diameter end, under the action of the capillary effect and the curvature effect. At the joint of the bamboo-joint-like platform, the inner diameter of the conical micro-tube decreased dramatically and the curvature effect became resistant to liquid transport. Therefore, overcoming the resistance at the joints is the key factor determining the feasibility of the proposed method. According to the experiment, when the first transport unit was filled with liquid, the liquid overcame the resistance at the joint and was smoothly transported to the next unit. It is feasible to adopt a bamboo-joint-like platform to realize spontaneous and long-distance transport of the liquid.

In addition to spontaneous transport, directional transport is also highly desirable in microfluidic applications.¹⁸ The fluid was injected from the middle into a straight tube with an internal diameter of 0.5 mm and no tapered structure, and the droplets were found to flow uniformly in the symmetrical direction from the injection point, which indicates that the ordinary micro-tube does not have the capability to realize directional transport [Fig. 2(e)]. The experiment was carried out with a tapered micro-tube, and the liquid was first transported to the section where the inner diameter was smaller, which is in line with the curvature effects of shore bird beat revealed by scholars²⁸ [Fig. 2(f)]. When the liquid filled the cone tube with a smaller inner diameter, the liquid did not output from the nozzle, but flowed to the cone tube with a larger inner diameter. This indicates that the liquid in the conical micro-tube has a preferential movement direction and does not show symmetrical flow. The bamboo-joint-like platform consisting of three transport units was used to carry out the experiment [Fig. 2(g)]. In the transport unit for liquid injection, the liquid flow mode still had the preferential movement direction, i.e., first flowed to the end with smaller inner diameter and then flowed to the end with larger inner diameter. When the transport unit was fully filled with liquids, the liquids overcame the resistance at the node of the bamboo-joint-like platform and flowed to the next transport unit rather than flowing back to the previous transport unit. When observing the state of the liquid in transport unit, it was found that both end faces of the liquid column were concave in shape under the action of surface tension [locally enlarged image of Fig. 2(g)].

The mechanical analysis of the above phenomena is as shown in Fig. 2(h). When the droplet was inhaled from the middle of the conical tube [Fig. 2(h1)], the radii of the concave surfaces at both ends of the liquid drop are r₁= −R₁/cos(α/2 − θ) and r₂= −R₂/cos(α/2 + θ), respectively, where R₁ and R₂ are the radii of the conical micro-tube at the end face of the liquid drop, α is the cone angle of the conical micro-tube, and θ is the contact angle between the liquid and the conical micro-tube. Therefore, the Laplace pressure difference ΔP = ΔP₁ − ΔP₂= 2γ(1/r₁ − 1/r₂) = 2γ[cos(α/2 + θ)/R₂ − cos(α/2 − θ)/R₁], where γ is the interfacial tension between the droplet and the tapered micro-tube. cos(α/2 + θ) ≈ cos(α/2 − θ): this is because the inner wall of the tapered micro-tube had been treated to be highly hydrophilic, leading to a small contact angle θ. Also, since R₂> R₁, then ΔP < 0, the droplets were driven to the direction where the radius of the tapered micro-tube was smaller. When the small end of the conical tube was filled with liquid [Fig. 2(h2)], the radii at the nodes at both ends of the liquid column were the same, i.e., ΔP ≈ 0. However, the inclined angle between the two ends of the liquid column and the tapered micro-tube would cause pinning force hindering the liquid flow. The pinning force was closely related to the contact angle between the liquid and the surface. When the average contact angle was less than Pi/2, the magnitude of pinning force was positively correlated with the angle.⁷ The angle between the S2 surface of the next transport unit and the liquid column was obviously larger than the angle between the S1 surface of the previous transport unit and the liquid column, so the liquid can be transported smoothly from the small end to the large end and eventually into the next transport unit.

According to the above analysis, the conical micro-tube structure can realize the spontaneous transport and extrusion of liquid, while neither will be achieved by ordinary tubes. The bamboo-joint-like platform, which consists of these conical micro-tubes as transport units, is able to achieve a directional and continuous liquid transport.

C. Characteristics of bamboo-joint-like transport of fluid

To further understand the spontaneous liquid transport based on the bamboo-joint-like platform, we carried out further research on the spontaneous transport, anti-gravity transport, and directional transport.

The platform consisting of four transport units is as shown in Fig. 3(a). When the liquid approached the inlet of the bamboo-joint-like platform, it was absorbed into the interior rapidly. At 62.0 s, the first transport unit was fully filled. At 100.0 s, the liquid smoothly entered the third transport unit. At 272.0 s, the existing liquid escaped from the outlet of the fourth transport unit. Since the cavity was tapered, the diameter of each cross section was different. For a single transport unit [Fig. 2(h)], local curvature R(L) for a transport distance of L is

$$R(L)=L\tan \left({\displaystyle \frac{\alpha }{2}}\right)+{\displaystyle \frac{d_1}{2}.}$$ (1)

FIG. 3.

(a) Long-distance transport of the bamboo-joint-like platform. (b) Theoretical analysis of kinematics characteristics of long-distance transport in bamboo-joint-like platforms. (c) Experimental results of kinematic characteristics of the bamboo-joint-like platform during long-distance transport. (d) The transportation process of blue-dyed water after red-dyed water filled the first transport unit. (e) The red-dyed water was transported after the two transport units were filled. (f) Experimental results of kinematic characteristics of the blue-dyed water in Fig. 2(e). (g) Anti-gravity transport process of bamboo-joint-like platform. (h) Directional transport characteristics of the bamboo-joint-like platform during gravity transport. (i) Bamboo-joint-like platform for long-distance transport with high speed rate of 632.5 μl/min. All bars denote 10 mm.

Since the cavity was filled up during transportation, the volume of the liquid in the tube was equal to the volume of the tube cavity. So, the volume V₀ of liquid filled in transport unit is

$$V_0=\pi {\int }_0^L{R}^2(L)dL.$$ (2)

The bamboo-joint-like platform consists of many units. For the bamboo-joint-like platform, when the transportation distance is L, n = [L/L₀] represents the number of transport units, where L₀ represents the length of transport units, and [L/L₀] represents the integer part of L/L₀. Then, the liquid volume V is

$$V=n\pi {\int }_0^{L_0}{R}^2(L)dL+\pi {\int }_0^{L-n}{R}^2(L)dL.$$ (3)

From the point of view of conservation of mass, we know that the mass of liquid in the bamboo-joint-like platform must be equal to the mass of liquid we supplied. Since the density of the liquid remained unchanged during the experiment, the conservation of mass intuitively means that the volume of the input liquid is going to be equal to the volume of the bamboo-joint-like platform. In the experiment, the input rate of liquid from the inlet of the bamboo-joint-like platform is v, and the volume V of liquid transported is

$$V=vt,$$ (4)

where t is the input time. By solving Eqs. (3) and (4), we can obtain the relation between liquid transportation distance and speed and time as shown in Fig. 3(a) [Fig. 3(b)]. The theoretical model shows that in the process of spontaneous transport of liquid, the transport distance and transport speed both changed periodically with time. The transport distance and speed curves have three inflection points, which correspond to the three nodes of the bamboo-joint-like platform, respectively. The liquid transport speed at the joint was the largest, of which the theoretical maximum was ∼40 mm/s. Analysis of experimental data shows that the results are consistent with the theoretical model [Fig. 3(c)]. However, during the experiment, there was friction resistance between the liquid and the inner wall of the bamboo-joint-like platform, making the maximum transportation speed smaller than its corresponding theoretical value. Comparing the liquid transportation speed curves of the bamboo-joint-like platform and transport unit, there were no obvious changes in the maximum transportation speed, and the change trend does not produce obvious changes [Figs. 2(c) and 3(b)]. This shows that the bamboo-joint-like platform does not have a significant impact on the liquid transport process.

The above research focuses on the kinematic characteristics of liquid-filled bamboo-joint-like platform. When the liquid starts to output, the subsequent liquid transport process is a liquid–liquid–solid interaction process, which is different from the liquid–gas–solid interaction at the initial stage. To further study the liquid transport behavior of the bamboo-joint-like platform, we used red ink and blue ink to dye water, respectively. When the first transport unit was filled with red-dyed water, we injected blue-dyed water from the inlet and found that red-dyed water entered the second transport unit quickly [Fig. 3(d)]. The red-dyed water and blue-dyed water were mixed in the second transport unit before the liquid was extruded. When both transport units were filled with red-dyed water, we injected with blue-dyed water, and it could be found that blue-dyed water was mixed with red-dyed water in the second transport unit and then output from the outlet before all red-dyed water in the first transport unit was produced [Fig. 3(e)]. The transport distance and speed of the front end of blue-dyed water were analyzed, as shown in Fig. 3(e); it was found that the liquid movements in the liquid–liquid–solid mode were similar to those in the liquid–gas–solid mode as shown in Fig. 3(f). Therefore, when the bamboo-joint-like platform was fully filled with liquids, the transport characteristics of the liquids did not significantly change, and the proposed new method can be applied to long-distance stable liquid spontaneous transport and making it output to a target area.

In practical applications, micro-fluidic manipulation and analysis are not always carried out on a horizontal plane. Therefore, the anti-gravity transport of fluids will be helpful to many microfluidics applications.³⁸ The bamboo-joint-like platform was placed at an inclination of 10°, and anti-gravity liquid transport between transport units was realized [Fig. 3(g)]. When the bamboo-joint-like platform was tilted, and the liquid was injected from the middle of a transport unit, the liquid initially flowed to the section with a smaller inner diameter and then flowed back to the section with a larger inner diameter [Fig. 3(h)]. When the transport unit was fully filled with liquids, the liquids flowed to the next transport unit. This shows that the bamboo-joint-like platform retains the function of directional liquid transport even against gravity.

Within a small supply rate, the liquid transport velocity is positively correlated with the liquid supply rate. However, when the supply rate exceeds a threshold, the increment of the transmission speed cannot keep up with it, making droplets accumulate on small ends and then fall to the ground by gravity. Even if the supply rate of the liquid is increased at this point, the transport speed of the micro-tubes will not increase. To investigate the high-speed transport performance of the bamboo-joint-like platform during long-distance spontaneous transport, we increased the liquid supply velocity from 63.25 μl/min to 632.5 μl/min. For a 240 mm long bamboo-joint-like platform consisting of eight transport units, the liquid can be quickly transported to the outlet within 58 s and output smoothly [Fig. 3(i) and Movie S1 in the supplementary material].

The bamboo-joint-like platform has excellent transport capacity in terms of spontaneous transport, anti-gravity transport, and directional transport. The liquid motion characteristics of each component transport unit in the bamboo-joint-like platform are similar to that of a single transport unit. This shows that it is feasible to select the optimal conical microtubule as the transport unit and then optimize the assembly into a platform for multifunctional application in liquid transport.

D. Optionally assembled platform for multifunctional applications

After the fundamental research of the bamboo-joint-like platform, we then explored various liquid delivery applications using the spontaneous transport devices through 3D printing and optional assembly. Since high-speed transport performance has been confirmed, the liquid supply rate in the applied research should be higher than that in the basic research experiment. Using the transport unit proposed in this study as the basic design element, Y-type liquid distribution device was manufactured through 3D printing. After the liquid entered the device from the inlet, it could be divided into two branches, and the spontaneous and directional transport of liquid were realized [Fig. 4(a)]. A serpentine loop transportation device is often used in the field of microfluidic analysis. The serpentine loop transportation device with 368.7 mm of flow channel, which was invented and manufactured in this research, can successfully realize spontaneous and directional transport of the liquid [Fig. 4(b)]. The anti-gravity transportation characteristics of the bamboo-joint-like platform in Fig. 3(g) provide reference for special liquid transportation devices. In the designed bridge-shaped anti-gravity transportation device, the angle between the tangent line of the starting point of the middle arc and the horizontal direction is 10° [Fig. 4(c)]. After the liquid entered the device, the red-dyed water was transported to the outlet of the device. When the inner channels of the three branches were designed with different wedge angles, it was very easy to obtain three different flow rates of the liquid [Fig. 4(d)]. The smaller the wedge angle, the faster the flow rate of liquid.

FIG. 4.

Spontaneous transport devices fabricated by assembly of liquid transport units. (a) Y-type liquid distribution device. (b) Serpentine loop transportation device for long-distance transport. (c) Bridge-shaped anti-gravity transportation device. (d) Speed control device, the wedge angles of the inner channel of three branches from left to right are 6°, 4°, and 2°, respectively. All bars denote 10 mm.

Although the 3D printed device can realize customization of spontaneous and directional transport devices, in the actual production application, it is difficult to meet all the special design and processing requirements of the device, so the device mentioned above cannot meet all the requirements flexibly. In order to expand the universality of this study, we have designed four types of tube joints that were used in conjunction with transport units, namely, straight-through joint, right-angle joint, Y-type joint, and T-type joint, as shown in Fig. 5(a). A linear transport device was formed by assembling a straight-through joint and a transport unit, which had the same transportation function as that of the bamboo-joint-like platform manufactured by 3D printing [Fig. 5(b)]. By assembling the Y-type joint and a transport unit, a liquid diversion device was formed, which could divide the liquid into two branches and then output them from the outlet, respectively [Fig. 5(c)]. A Y-type three-way joint was adopted to adjust the direction of the inlet and outlet of the transport unit. After assembly, a liquid flow merging device was formed, which could merge the two branches of liquids (i.e., red-dyed water and blue-dyed water, respectively) into one flow channel and then output it from the outlet [Fig. 5(d)]. A T-type transportation device was formed by assembling a T-type joint and a transport unit, which could divert the liquid and change the transport direction and then output it from the outlet [Fig. 5(e)]. A U-shaped transport device was formed by assembling a U-shaped joint and a transport unit, which could make the liquid flow along the designated flow channel [Fig. 5(f)]. With the use of our designed transport unit and connectors, we can build a liquid transport platform with customization functions [Fig. 5(g)]. When oil and water were input from two inlets, respectively, the liquid could be mixed in proportion and then output by adjusting the flow rate and time of the input liquid [Fig. 5(h)].

FIG. 5.

Optionally assembled platform for liquid spontaneous transport applications. (a) Straight-through joint, right-angle joint, Y-type joint, and T-type joint, which are fabricated by 3D printing. (b) Linear transport, (c) liquid diversion, (d) liquid confluence, (e) T-type transport, (f) U-type transport, (g) complex flow channel assembled using transport units, and (h) liquid proportional mixer. All bars denote 10 mm.

IV. CONCLUSION

A bamboo-joint-like platform with a tapered micro-tube as the transport unit was designed, which can achieve spontaneous transport and extrusion of a liquid to a target area without external force. With this facile method and novel design concept, a complex multi-functional liquid transport platform was developed, which can realize spontaneous transport, long-distance transport, anti-gravity transport, and directional transport under the effect of asymmetric structure and capillary force. During spontaneous liquid transport based on the bamboo-joint-like platform, the transported liquid not only overcame the resistance at the joint between transport units but also successfully realized the liquid output function, which can be hardly achieved by most existing methods due to the constraint of surface tension. The bamboo-joint-like platform has excellent transport capacity even at a liquid supply rate up to 632.5 μl/min, and the liquid transport characteristics of each component transport unit are similar to that of a single transport unit. The 3D printing device can realize the customization of a spontaneous transport platform by using the transport unit as the basic design element. Four types of connectors were designed to optionally assemble the spontaneous transport platform of the liquid according to actual functional requirements, which greatly expand the universality. This universal spontaneous liquid transport platform can be modularly designed, which has a great application value in the design of advanced devices.

SUPPLEMENTARY MATERIAL

See supplementary material for the bamboo-joint-like platforms for fast, long-distance, directional, and spontaneous transport of fluids.

DATA AVAILABILITY

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