Controlling Directional Liquid Transfer over a Ratchet-like Surface with Oriented Open-Wedges

Abstract

Directed transfer of liquids is widely used as an important mass transfer strategy in the nature biology, industry, agriculture, and medical fields. Based on the bionic concept, researchers have constructed various forms of surface structures to realize controlled transport of liquids. However, current methods for preparing surfaces with directional liquid transport often suffer from cumbersome preparation processes, complexity of the prepared structures, and high costs. In this paper, we prepared a topological structure surface capable of precisely controlling liquid transport by a facile method. By utilizing 3D printing technology, we designed and fabricated a wedge-shaped surface with a periodic tilting scale arrangement. We accomplished the precise regulation of liquid transport on the surface of the structure, which not only realized the bi-directional transport of liquid anisotropically to the uni-directional transport on the surface but also achieved the precise regulation of the distance and speed of liquid transport through adjusting the opening angle formed by the open-wedges. We expect that the as-prepared wedge-shaped surface will provide a facile and efficient strategy for directional liquid transport.

1. Introduction

Directional liquid transport as an important mass transfer strategy exists extensively in natural creatures to better adapt to their surroundings [1,2,3,4,5,6,7,8]. For example, the spider silk with periodic spindle-knots and joints can continuously collect tiny water droplets captured from the air and transport them into large ones directionally [7]; the peristome of Nepenthes alata enables the spontaneous directional flow of liquids to maintain a wet state imparted by the oriented microcavities with gradient wedge corners [5]. Inspired by the unique surface structures and novel fluid transport modes of living organisms in nature, researchers, utilizing advanced techniques such as standard microelectromechanical system processes [9] and 3D printing technique [10], have designed and prepared functionally biomimicking surface materials and devices that are widely used in microfluidics [11,12,13,14,15], water harvesting [16,17], biosensing [18], lubrication [19], heat transfer [20,21], and textiles [22]. However, despite the fact that the prepared structures realized pre-determined requirements for liquid-directed transport, there still remained objective problems, such as the complexity of the preparation process and complexity of the prepared structures, which often require different spatial layers (e.g., recessed cavities) [9,10]. At the same time, manipulating liquid to be transported in a precise and controllable (precise control of the direction and distance of the liquid transport) way is the current direction of research. Therefore, it is a great challenge and is desirable to design and construct concise surface structures to achieve directional liquid transport in a precisely controllable manner using a facile method.

Here, based on our previous work [23], we developed a concise plane surface with periodic-oriented open-wedges using 3D printing technology. Through adjusting the opening angle of the wedged scales, we achieve accurate regulation of the liquid on the surface, that is, controlling the direction of the liquid transfer—from bi-directional to uni-directional—and controlling the speed and distance of the liquid transfer. It is proposed that the controlled transport of liquid on the surface of the wedge structure is induced by the synergistic effect of the surface tension of water, the capillary force inside the wedge corner, the retention force of the inclined serrated structure, and the pinning effect of the scale sharp edges. We expect that the above results will provide a generalized strategy for the directional transport of liquids.

2. Materials and Methods

Fabrication of the periodic-oriented wedged-scales surface: The periodic-oriented wedged-scales surface with opening angles of 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, and 45° were prepared using a high-definition 3D printer (the printing accuracy is 2 μm, nanoArch S130, Shenzhen, China) with a light-cured resin material. The thickness and length of the scales were 100 μm and 500 μm. The plane length was 5 cm, the width was 0.5 cm, and the height was 0.2 cm. The as-prepared surface was treated with oxygen plasma (Plasma Cleaner, PDC-32-G-2, Chongqing, China) with superhydrophilic properties. There was no intersection between adjacent scales. The as-prepared surface was first washed with isopropyl alcohol for 10 min to remove the residual resin on the surface prior to performing the experiment, followed by soaking in deionized water for 12 h to remove the residual isopropyl alcohol on the surface, and finally blown-dry with nitrogen.

Characterization: The microstructures of the as-prepared surface were characterized using an optical microscope (BX51, Olympus, Hamburg, Germany) and scanning electron microscopy (JEOL, JSM-6700F, Tokyo, Japan) at an accelerating voltage of 3.0 kV. The contact angles of the as-prepared original and plasma-treated wedged-scales surfaces were measured using a contact angle system (LSA200, Lauda-Königshofen, Germany). The wetting behaviors over the as-prepared surface were characterized using a camera (Canon EOS40D, Tokyo, Japan).

Liquid transport over the as-prepared surface: Water droplets were continuously deposited on the as-prepared surface using a syringe pump (LSP Lab01, Shanghai, China) at a rate of 20 μL/min. The process of water spreading was recorded using a camera (Canon EOS40D, Tokyo, Japan). The spreading lengths of the water were measured, and the spreading velocity of water was calculated.

3. Results and Discussion

The as-prepared surface is shown in Figure 1, which consists of regular and periodic-oriented scales (Figure 1a,b), and the microstructures of the wedged scales were characterized using scanning electron microscopy (Supplementary Materials Figure S1). The inclined scales with sharp edges were arranged on the surface, forming a wedged corner with an opening angle, α, as depicted in Figure 1b₁. In order to clearly and explicitly characterize the orientation of the wedged surface, here, we denote the with-scale direction as WS and the against-scale direction as AS.

The controllable liquid transport with time was tested over the wedged surfaces with various opening angles. As shown in Figure 2a, on the surface of the wedge-shaped structure with an opening angle of 5° (Figure 2a₁,a₂), the water droplets spread along the directions of AS and WS within 0.6 s after the water droplets dropped on the surface, with spreading distances of 0.8 mm and 0.5 mm, respectively (Figure 2a₃). As time increased, the spreading distance of the water droplets in the AS and WS directions increased, with the spreading distance in the AS direction always greater than the distance in WS direction. When the time was gradually increased to 1.8 s and 3.0 s, the distance of the water droplets spreading in the AS direction was increased from 2.8 mm to 4.6 mm, and the spreading distance in the WS direction was increased from 1.8 mm to 3.0 mm (Figure 2a₄,a₅).

With the increase in the opening angle (Figure 2b₁,b₂,c₁,c₂), the distance that the water droplets spread on the surface of the wedged surface in both the AS and WS directions decreased gradually. As shown in Figure 2b,c, the lengths of water droplets transmitted in 0.6 s, 1.8 s, and 3.0 s in the AS direction were 0.5 mm (Figure 2b₃), 2.3 mm (Figure 2b₄), 4.1 mm (Figure 2b₅), 0.3 mm (Figure 2c₃), 0.8 mm (Figure 2c₄), and 1.6 mm (Figure 2c₅), respectively, when the opening angle of the wedge structure was increased to 20° and 40°; the lengths of water droplets transmitted in 0.6 s, 1.8 s and 3.0 s in the WS direction were 0.2 mm (Figure 2b₃), 1.0 mm (Figure 2b₄), 2.1 mm (Figure 2b₅), 0.1 mm (Figure 2c₃), 0.1 mm (Figure 2c₄), and 0.1 mm (Figure 2c₅), respectively. From the above results, it can be concluded that the opening angle of the wedged scale has an important role in regulating the behavior of the liquid transport on the surface (the liquid transport direction and distance). When the opening angle was small (5°), water droplets were able to spread over long distances in both directions (AS and WS) of the wedge structure. However, when the opening angle was larger (40°), water droplets could only be transported in the AS direction, while in the WS direction, the transport of water droplets was inhibited and pinned in the original position of the droplets.

In order to characterize in clearer detail the effect of the opening angle on the liquid transport direction and distance, a series of wedged surfaces with different opening angles (from 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, to 45°) were fabricated using the 3D printing technique (Supplementary Materials Figure S1). Time sequences of the liquid transport distances in the AS (Figure 3a) and WS (Figure 3b) directions (Supplementary Materials Figure S2) show the ability of the wedged scales controlling liquid transport. For a certain opening angle of the wedged scales, the distance of water droplet transport over it increases with time, so the transport distance can be regulated by controlling the time of water droplet transport; by increasing the opening angle of the wedged surface, it is possible to realize the bi-directional transport of water droplets from both in the AS and the WS directions to uni-directional transport only in the AS direction (for the wedged scales with the opening angles of 40° and 45°, the transport distance in the WS direction was 0 cm, the transport was suppressed). The opening angle of the wedged scales has a precise modulation on the water droplet transport (transport direction and transport distance).

To fully characterize the effect of the wedged surface on the transport behavior of water droplets, we simultaneously calculated the velocity of water droplet transport over the surface with different opening angles. Here, we define the velocity of water droplet transport, v, as the ratio of the distance traveled by a water droplet to the spreading time 3 s. As shown in Figure 4, the velocity of water droplet transport on the wedged surface in both the AS direction and WS direction tends to decrease with the increase in the opening angle. The decrease in water droplet transport velocity on the wedged surface with increasing opening angle corroborates the decrease in transport distance on the wedged surface with larger opening angle in the same time. The speed of water droplet transport in the WS direction is always greater than that in the AS direction for wedged scales with the same opening angle, which is also consistent with the fact that the distance of water droplet transport in the WS direction is always greater than that in the AS direction. In the WS direction, when the opening angle was increased to 40°, the velocity of droplet transport decreased to 0 m/s, indicating that the droplet transport was suppressed, i.e., the distance of droplet transport did not change but was pinned at the origin with the increase in time. The change in water transport velocity characterizes the dynamic process of water transport in detail.

It is well known that on an isotropic hydrophilic surface, the affinity of water for the solid substrate drives the water to spread uniformly in all directions. In contrast, on an inclined sawtooth-structured surface through a plasma treatment (Supplementary Materials: Figure S3), according to our experiment results, the water droplets can be precisely regulated in the AS and WS directions with different transport speeds and transport distances. We inferred that the controlled transport of liquid on the surface of the wedge structure is caused by the synergistic effect of the surface tension of water, the capillary force inside the wedge corner, the retention force of the inclined serrated structure, and the pinning effect of the scale sharp edges.

When a water droplet is drop-added to the surface of the prepared structure, it is transported both along the AS direction and the WS direction under the effect of surface tension simultaneously (Figure 5a₁–a₃). Inside the microcavity formed by the wedge-shaped structure in the AS direction, the capillary force generated in the wedged corner tended to promote water moving to the tapered end to fully fill the wedged scales in the AS direction (Figure 5a₂,b):

$$F_c=2w\gamma \cos \left(\alpha /2\right)$$

(1)

where α is the opening angle of scales, and w is the width of the wedged surface [24,25,26]. Meanwhile, on the oriented scales with the ratchet-like features, the moving water is exposed to the retention force:

$$f_{\mathrm{i}}=4\pi \gamma w\sin \left[\left({\theta }_{r,0}+{\theta }_{a,0}\right)/2\right]\times \left[\sin \left(a_{\mathrm{i}}+∆{\theta }_0/2\right)\right](∆{\theta }_0={\theta }_{\alpha ,0}-{\theta }_{r,0})$$

(2)

which impedes the motion of the spreading water, where w is the width of the wedged surface, α is the opening angle of the scale, and θ_α,0 and θ_r,0 are the intrinsic advancing and receding contact angles [27]. As the oriented scales have different opening angles in the AS and WS directions (α_AS ˂ α_WS), the retention force that impedes the movement of water in the AS direction is smaller than the retention force that impedes the movement of water in the WS direction (f_AS ˂ f_WS, Figure 5a₃,c), according to Equation (2). Meanwhile, because of the pinning effect and the retention force, the water was pinned on the scales’ sharp edges and could not spread further in the WS direction [28,29]. This means the unbalanced driving force imparted by the oriented scales combined with the capillary force inside the microcavity in the AS direction led to the anisotropic water transport behaviors in the AS and WS directions. The overall result was the directional liquid transport.

The opening angle α is a key factor that influences the net driving force F. When the opening angle α increases, the capillary force F_c in wedged corners decreases, and the retention forces f in both the AS and WS directions increase, resulting in the decrease in F, which in turn influences the water transport distance and direction.

4. Conclusions

In summary, by using a facile strategy, we developed a ratchet-like surface with periodic-oriented scales, realizing controllable liquid transport. The wedged scales with various opening angles endowed the surface with the ability of transferring liquids with a steerable direction and distance. The water droplet deposited on the as-prepared surface spread from bi-directional to uni-directional with the increase in the opening angles, and the spreading distance decreased both in the AS and WS direction until the water droplets were pinned on the sharp edges of the scales in the AS direction where in situ transmission was suppressed. Not only limited to resin materials for 3D printing, we also prepared wedge-shaped Polydimethylsiloxane (PDMS) surfaces by two steps of replication based on the 3D printing wedge-shaped structure, which realizes the precise control of the fluid (Supplementary Materials: Figure S4) and greatly expands the scope of the application of wedge-shaped material surfaces in the industrial field, as well as the possibility of large-scale application. We anticipate that the wedged surface with concise structure may provide a generalized strategy for the liquid transport in a controllable manner.