Droplet Self-Propulsion Based on Heterogeneous Surfaces

Luanluan Xue; Huizeng Li; An Li; Zhipeng Zhao; Yanlin Song

doi:10.7536/PC200675

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (1) : 78-86 DOI: 10.7536/PC200675

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Droplet Self-Propulsion Based on Heterogeneous Surfaces

Luanluan Xue ¹^,²
, Huizeng Li ¹^,^*
, An Li ¹^,²
, Zhipeng Zhao ¹^,²
, Yanlin Song ¹^,²^,^*

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Abstract

Actuating droplets for various dynamic behaviors has significant applications in the fields of biomedicine, microfluidics, and trace detection. The droplet movement depends on the adjustment of the forces on different positions of the droplets. Droplet self-propulsion based on heterogeneous surfaces shows advantages such as easy operation and energy conversation, which has been one of the research hotspots in droplet propulsion. In this article, recent research of droplet propulsion based on heterogeneous surfaces is reviewed. Firstly, the general principle of droplet self-propulsion based on surface heterogeneity is discussed. According to the different preparation methods, heterogeneous surfaces are divided into three categories: the heterogeneous wettability surface, the anisotropic structure surface and the synergistic surface. Their fabrications and applications are summarized, respectively. Finally, the limitations and the developments of heterogeneous surface are prospected and discussed.

Contents

1 Introduction

2 Droplet self-propulsion based on heterogeneous wettability

2.1 Gradient wettability

2.2 Patterned wettability

3 Droplet self-propulsion based on anisotropic structure

3.1 Static anisotropic structure

3.2 Dynamic anisotropic structure

4 Droplet self-propulsion based on synergetic effects

5 Conclusion and outlook

Graphical abstract

Keywords

functional interface / droplet self-propulsion / heterogeneous surface / wettability

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Luanluan Xue, Huizeng Li, An Li, Zhipeng Zhao, Yanlin Song. Droplet Self-Propulsion Based on Heterogeneous Surfaces. Progress in Chemistry, 2021, 33(1): 78-86 DOI:10.7536/PC200675

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1 Introduction

Droplet drive has important applications in daily life and industrial production. For example, in microfluidic technology, it is necessary to precisely control the movement of droplets for reaction detection^[1,2]. When collecting fresh water, water vapor needs to be quickly condensed into small water droplets and quickly transported and collected^[3]; In the case of anti-icing, it is necessary to reduce the adhesion force between micro-droplets and the surface to avoid droplet adhesion^[4].

At present, researchers have proposed a variety of methods to control the behavior of droplets, but most of these methods are based on the role of external fields to achieve droplet driving^[5,6]. For example, magnetic drive using a magnetic field to drive a magnetic fluid^[7]; Heating the substrate unevenly to generate the thermal drive of the Marangoni effect inside the liquid drop^[8]; Electric drive based on electrowetting phenomenon^[9]; And light actuation by a light responsive surface, etc^[10]. These methods have complex preparation processes, weak driving effects, and require continuous energy input. How to develop a simple, efficient and low energy droplet driving method is the main concern in the field of droplet driving research^[11,12].

In nature, many organisms use their anisotropic surface structure to realize droplet driving. For example, desert beetles use hydrophilic areas on their backs to condense droplets and then hydrophobic areas to drive them^[13]. Paederus uses surfactants to create a wettability gradient to drive liquid movement^[14]; The cactus drives condensed droplets from the top of the leaf to the root through the cone-like structure of its needle-like leaves^[15]. Anisotropic surfaces based on biomimetic preparation have the advantages of simple operation, sustainability and low energy consumption in driving droplets, which have attracted wide interest of researchers in recent years^{[16 ⇓~18]}.

In this paper, the main methods of droplet driving via anisotropic surfaces in recent years are reviewed. According to the different principles of droplet driving, the anisotropic surface can be divided into three types: wetting anisotropic surface, structural anisotropic surface and cooperative anisotropic surface. Their driving principles and preparation methods are introduced, and their respective characteristics are compared.Finally, the limitations and development direction of anisotropic surface driven droplets are summarized and prospected.

2 Wetting anisotropic surface driven droplet

Wetting anisotropic surfaces drive droplets mainly through the difference in wettability between regions of the surface^[19,20]. As shown in fig. 1A, the contact angle of a droplet on a flat, smooth, chemically homogeneous, ideal surface can be calculated from Young's equation:^[21]

(1)

cosθ = σ sl - σ sg σ lg

Where the

σ sl 、 σ sg 、 σ lg

are the interfacial tensions between the gas-solid, solid-liquid, and liquid-gas interfaces, respectively. For the same droplet, the

σ sl 和 σ lg

is often difficult to change significantly, and the contact angle of the droplet is mostly determined by the

σ sg

^[22].

In real life, there are often uneven microstructures on the surface of materials^[23]. These microstructures can significantly affect the wetting behavior of the droplets. Wenzel suggested that when the rough surface is hydrophilic, water droplets can infiltrate the microstructure of the substrate (Fig. 1b)^[24]. At this point, the apparent contact angle of the droplet is:

(2)

cos θ * = r cosθ

Where R is the roughness of the substrate surface and θ is the intrinsic contact angle of the droplet on the surface; When the surface is hydrophobic, Cassie et al believe that the bottom of the droplet contacts the substrate to form a liquid-solid interface, while the other part directly contacts the air to form a gas-liquid interface (Fig. 1C)^[25]. At this point, the apparent contact angle of the droplet is

(3)

cos θ * = f (cosθ + 1) - 1

Where f is the area ratio of the liquid-solid interface and θ^* is the apparent contact angle of the droplet. It can be seen from formulas (2) and (3) that the apparent contact angle of the droplet is determined by both the intrinsic contact angle and the microstructure of the surface, and changing the intrinsic contact angle or the microstructure can significantly affect the apparent contact angle of the droplet^{[21,24 ⇓ ~26]}.

It is a common method to adjust the apparent contact angle of droplets by changing the chemical groups on the solid surface to change the

σ sg

and thus affect the intrinsic contact angle of droplets^[27]. In general, the

σ sg

is determined by the end groups of the chemical groups on the solid surface. The end groups with larger polarity, such as carboxyl, hydroxyl and aldehyde groups, can increase the interfacial tension of the gas-solid interface and reduce the intrinsic contact angle of the droplet. However, the less polar groups such as alkyl and fluoroalkyl can reduce the interfacial tension and increase the intrinsic contact angle of the droplet^[22]. When the intrinsic contact angle of the droplet on the surface is fixed, changing the microstructure of the substrate can also affect the apparent contact angle of the droplet^[28]. It is generally believed that when the intrinsic contact angle of the droplet is less than 90 °, the increase of the surface roughness tends to further reduce the apparent contact angle of the droplet. When the intrinsic contact angle is greater than 90 °, the apparent contact angle increases with the increase of surface roughness^[29]. The relationship between the apparent contact angle of the droplet and the intrinsic contact angle and the surface microstructure lays a theoretical foundation for the construction of wetting anisotropic surface driven droplets.

When a droplet contacts a solid surface with uniform chemical properties and microstructure, the wettability of all parts of the surface is the same, the apparent contact angle of all parts of the droplet is equal, and the internal pressure is balanced. When a droplet contacts an anisotropic surface with non-uniform chemical properties or/and microstructure, the wettability of different parts of the surface is different, and the contact angle formed by the droplet is also different, resulting in unbalanced Laplace pressure, which drives the droplet to move to the end with smaller contact angle (I^[30]. As shown in fig. 2, the driving force of the anisotropic surface to the droplet at this time is:^[31]

(4)

F = γ ∫ s (cos θ 1 - cos θ 2) ds

Where the θ₁ and the θ₂ are the contact angles in the advancing and retreating directions, respectively, and s is the three-phase contact line. According to the difference of wettability distribution, the wettability anisotropic surface can be divided into two types: gradient wettability surface and patterned wettability surface^{[32 ⇓~34]}^{[35 ⇓ ⇓ ⇓~39]}.

2.1 Gradient infiltration

A gradient wettability surface is an anisotropic surface with a continuous change in wettability. As shown in fig. 2, the more wetted side of the surface (right side) is more hydrophilic and the droplet exhibits a smaller contact angle, while the less wetted side (left side) has a larger contact angle. When a droplet contacts such a surface, the less wetted side produces a larger Laplace pressure, which drives the droplet toward the more wetted side^[30]. Because that wettability gradient on the substrate is continuously Chan, the asymmetric Laplace pressure exerted on the droplet is continuously present, so that the long-range directional drive of the droplet can be realized. Gradient wettability of solid surfaces can be achieved by constructing gradient physical microstructures and surface chemical properties.

2.1.1 Gradient wettability due to physical microstructure

Microstructure has a significant effect on surface wettability. Laser etching, plasma etching and focused ion beam etching are commonly used in the fabrication of microstructures^[40,41]^[42,43]^[44]. The wettability gradient of the surface can be achieved by changing the processing size, density and structural orientation of the microstructure^[29]. The gradient wettability obtained by this method is more stable, but the preparation method is more complex and the wettability gradient is smaller.

Wang et al. Constructed physical microstructures with different degrees of density on the substrate surface by photolithography, and prepared a wetting gradient surface^[32]. As shown in Figure 3A, they first coated part of the silicon substrate with photoresist, and then photolithographed the rest of the substrate. After washing away the photoresist, they obtained a rough surface with a local nano-columnar structure. Changing the area ratio of the photoresist can adjust the area ratio of the rough surface and affect the wettability of the droplet. With this method, the wettability gradient surface with contact angle of 92 ° ~ 164.8 ° can be obtained.

2.1.2 Gradient wettability due to surface chemistry

The basic idea of preparing gradient wetting surface by changing the surface chemical properties is to change the density of hydrophobic or hydrophilic functional groups on the substrate surface by photolysis or grafting methods, so as to prepare gradient wetting surface. The commonly used preparation methods include electrochemical modification of ^[33][45], microprinting, photodegradation and plasma diffusion^[46]^[47]^[48]. This kind of method has simple process and large surface wettability gradient, but its stability is poor and it is easily affected by the surrounding environment.

Zheng et al. Prepared the wettability gradient caused by the difference of chemical properties by the anodic oxidation method^[33]. They used a graphene sheet as the cathode and a copper sheet as the anode, as shown in Figure 3 B. After electrification, the distance between different positions of graphene sheet and copper anode is different, and the resistance is also different, thus forming different current densities in different areas, which affects the oxidation degree of graphene sheet. In addition, the solution slowly discharged from the container can form an oxidation time gradient, which further increases the difference in the oxidation degree of the substrate. It is found that the gradient surface prepared by this method has an increased wettability gradient. Near the bottom of the A3 region, the contact angle is 35 °, while the top of the A1 region has a contact angle of 115 °.

2.1.3 Gradient wettability due to a combination of microstructure and chemical properties

By combining the physical microstructure and the surface chemical property, the gradient wettability surface with large wettability gradient and good stability can be prepared by combining the advantages of the two. Compared with the single treatment method, the surface obtained by this treatment method has the advantages of large wettability gradient, fast driving speed and long driving distance.

Using micromachining technology and chemical vapor deposition method, Wang et al. Constructed a wettability gradient with synergistic physical structure and chemical properties on the surface of silicon wafers^[34]. They first treated the silicon substrate at high temperature to form a 2 μm thick SiO₂ film. The surface is processed by a photolithography method to obtain a patterned photoresist coating. Then, through reactive ion beam etching and C₄F₈ gas deposition, the wetted surface was obtained, in which the area covered by the photoresist coating was hydrophilic and the exposed area was super-hydrophobic. As shown in fig. 4A, they gradually reduced the proportion of photoresist coverage from left to right, thus producing a gradient wetting surface with hydrophobic left and hydrophilic right. The contact angle of the surface can vary widely from 15.5 ° to 166 °.

2.2 Patterned wettability

A patterned wetting surface refers to an anisotropic surface that has a different wettability in certain specific areas of the surface than surrounding areas. There are two common types of such surfaces, patterned wetted surfaces in which a hydrophilic pattern is built on a hydrophobic surface and patterned wetted surfaces in which a hydrophobic pattern is built on a hydrophilic surface^{[3,37 ⇓ ~39,49 ⇓ ⇓ ⇓ ⇓ ~54]}^[55,56]. Unlike a gradient wetting surface, the wettability on a patterned wetting surface does not change continuously. The driving force applied to the droplet is not continuous, and the asymmetric force is applied only at the boundary of the pattern^[57]. However, the forces acting on the droplet in different regions of the patterned wetting surface are quite different, and the forces acting on each part of the droplet can be more accurately controlled by reasonably designing the wetting/non-wetting region patterns, so that various driving behaviors of the droplet can be realized^[39]. Imilar to that gradient wettability surface, the patter wettability surface can also be produced by physical or chemical method.

2.2.1 Patterned wettability due to physical structures

Patterned wetting surfaces can be made by adjusting the distribution of physical microstructures on the substrate. The commonly used methods are 3D printing, reactive ion beam etching, laser etching and so on^[58,59]^[36,52]^{[60 ⇓ ⇓~63]}.

Ding et al. fabricated conical microstructures on substrates using 3D printing technology^[35]. The smaller the spacing of the conical structures is, the worse the wettability of the substrate is. By reasonably designing the widths of the different strip wettability patterns, it is ensured that the droplet contacts at least two different wettability regions at the same time after being placed on the substrate (Figure 5), thereby realizing the continuous driving of the droplet.

The microstructure wettability pattern may also enable impact deflection of that droplet^[36]. The silicon substrate used by Wang et al. Is composed of patterns with different wettability. At low temperatures, the droplet impinges on the surface and deflects toward the more wettable side. When the temperature is higher, the Leidenfrost temperature is lower in the region with higher wettability, while the Leidenfrost temperature is higher in the region with lower wettability. When the droplet impinges and rebounds, the region with higher wettability presents the Leidenfrost state, while the region with lower wettability presents the contact boiling state, and the droplet deflects toward the region with lower wettability^[36].

2.2.2 Chemically-induced pattern wettability

The wettability pattern can also be obtained by chemical modification. There are two common methods: selective grafting and selective de-grafting^[37,64]^[38,39]. The selective grafting refers to a method of treating an untreated surface selectively shielded by a sacrificial layer by using a method such as vapor deposition or liquid deposition to obtain a patterned wettability. This method has low cost and high speed, but it needs to prepare a uniform and high-precision sacrificial layer, so the preparation process is more complex^[28].

Song et al. Used inkjet printing technology to prepare the sacrificial layer. By accurately controlling the concentration and viscosity of the ink, the spreading of ink droplets and coffee ring effect were effectively avoided, and the sacrificial layer with uniform thickness and high precision was obtained^[37]. Chemical vapor deposition is used to graft fluorosilane on the surface not covered with the sacrificial layer, and then the sacrificial layer is washed away to obtain the wettable surface with a triangular super-hydrophilic pattern. This surface can drive the droplet towards the base of the triangle (fig. 6a).

The general steps of selective de-grafting are as follows: first, the substrate is modified by vapor deposition or liquid deposition, then the substrate is selectively shielded by masks or photoresists, and finally, the surface groups are selectively removed by UV etching, plasma treatment and other technologies^[38,39]. The method has the advantages of simple process and high precision of the prepared wettability pattern.

Song et al. produce a variety of patter wetting surface by selective degrafting^[38]. They first grafted perfluorosilane on the surface of the substrate by chemical vapor deposition. Then the surface covered by the mask was irradiated by ultraviolet light to selectively remove the perfluorosilane on the substrate surface, and a series of patterned wetting surfaces were prepared. They found that when a droplet impinges on the wetting surface of the strip pattern, the hydrophilic area adheres to the droplet more than the hydrophobic substrate, and the droplet bounces to the side of the hydrophilic pattern. Also, the lateral bounce of the droplet is proportional to the area of overlap of the droplet with the hydrophilic pattern at maximum spread. When the hydrophilic region of the substrate has a spiral pattern, the translational kinetic energy of the droplet is converted into rotational kinetic energy after impact (Fig. 6B)^[39].

3 Tructure anisotropic surface driven liquid drop

In addition to the anisotropic wettability, the anisotropic structure is also capable of driving droplets simply and efficiently. Many animals and plants in nature use their anisotropic structure to drive droplets^{[15,65 ⇓ ⇓ ⇓ ⇓ ⇓ ⇓ ⇓ ⇓ ~74]}. For example, the cone-shaped spine structure of cactus can directionally transport small water droplets formed by fog enrichment to the stem^[15,68,69]. Waterbirds can directionally transport water droplets into their mouths through the periodic opening and closing of their beaks^[70]. Its basic principle is that after the anisotropic structure makes the droplets contact, different parts of the curved liquid surface with different curvatures are formed, and the unbalanced Laplacian force is generated in the droplets, so as to realize the droplet drive^[75]. According to the different driving principles, the anisotropic structure can be divided into static anisotropic structure and dynamic anisotropic structure^{[67,69,76,77]}^[70,78,79].

3.1 Static anisotropic structure

Static anisotropic structure refers to its own asymmetric structure. When a droplet wets the surface of the structure, different curvatures of the liquid surface will be formed spontaneously at different parts of the droplet, thus driving the droplet to move spontaneously^[75]. The common structures are cone structure, spindle structure and pitcher plant structure.

Taking the conical structure as an example, as shown in Fig. 7A, after the droplet wets the outer surface of the conical structure, the droplet is subjected to a static pressure directed to the bottom end of the conical structure, and the magnitude is:^[76]

(5)

Δ p = - ∫ x B x A 2 γ (R + R 0) 2 αdx

Where, the

x A 、 x B

are the positions of the front and rear ends of the droplet, γ is the surface tension of the liquid, α is the half-angle of the cone, the

R 0

is the radius of the droplet, and R is the radius of curvature of the conical structure.

Zheng et al. fabricated a tapered glass tube by microelectrode drawing (Fig. 8A)^[80]. When a droplet wets the glass tube, the Laplace pressure near the bottom of the cone is small and the Laplace pressure near the tip is large. Driven by the Laplace pressure difference, the droplet spontaneously moves towards the root of the cone.

When the droplet is located inside the conical structure, the driving force generated by the anisotropic structure is:^[31]

(6)

Δ p = 4 γ cosθ x B α - 4 γ cosθ x A α

Where θ is the contact angle of the droplet on the solid surface, as shown in Fig. 7B. In this case, the direction of the driving force of the droplet is related to the contact angle. When θ < 90 °, the liquid surface is concave and the droplet moves to the top of the cone; When θ > 90 °, the liquid surface is convex and the droplet moves to the bottom of the cone.

Wang et al. imitated the primary structure of the surface of pitcher plants and prepared hydrophobic semi-conical channels (Fig. 8B)^[66]. They found that when a droplet was placed in a pipe, the contact angle of the droplet on the pipe wall was greater than 90 degrees. The droplet is therefore subjected to a static pressure directed towards the wider part of the duct, driving the droplet from the tip of the duct towards the wider part.

In addition, Jiang et al also found that the bow-shaped cavity on the surface of pitcher plants can pin one side of the droplet, resulting in asymmetric spreading of the droplet^[67]. Wang et al. Prepared a cavity structure imitating the pitcher plant on the silicon surface, and realized the "droplet diode" effect^[81]. However, Zhao et al. Used magnetic field to control the orientation of magnetic fluid as a dynamic template to prepare similar cavity structures, which further simplified the preparation method and could control the morphology of the cavity^[82]. Compared with the gradient wetting surface, the method of driving droplets by static anisotropic structure has the advantages of simple preparation and fast driving speed^[80].

Aquatic plants can spread their seeds on the water surface by capillary force. Inspired by this, Yao et al. And Zhao et al. Created a convex structure on the substrate and injected a lubricant to form a lubricated meniscus (Fig. 8C)^[71,83]. When the droplet is within the capillary distance, the liquid surface near the bulge is raised, and the two sides of the droplet show different contact angles, α > β, and the radius of curvature of the liquid surface near the bulge is larger, so the droplet is driven to move close to the bulge. Because the lubricating surface reduces the viscous resistance, it also has a good driving effect on small droplets.

3.2 Dynamic anisotropic structure

Dynamic anisotropic structure refers to the structure that can produce or intensify asymmetry under the action of external fields such as light, heat and magnetism^[84,85]. Droplets driven by dynamic anisotropic structure have the advantages of fast response, adjustable driving speed and flexible control.

Bush et al. Found that water birds can directionally drive water droplets into their mouths through the opening and closing of their beaks^[70]. As shown in Fig. 9a, when the beaks are closed, the angle between the beaks becomes smaller, and the three-phase lines on both sides of the droplet tend to expand, but the contact angle on the side near the mouth is larger, so the three-phase lines here move first; When the beak is open, the angle between the beaks becomes larger, and the three-phase lines on both sides tend to shrink, but the contact angle on the side far from the mouth is smaller, and the droplet shrinks to the side facing the mouth. Through the periodic opening and closing of the beak, the three-phase lines on both sides of the droplet move alternately, thus realizing the directional movement of the droplet.

Inspired by arterial blood transport, Yu et al. Made a dynamic tubular structure with photoresponse properties to drive droplets^[78]. They used linear liquid crystal polymers containing azobenzene photosensitive groups as raw materials, deposited a layer of liquid crystal polymers on the inner wall of the glass tube by template method, and dissolved the glass tube with hydrofluoric acid to obtain a self-supporting liquid crystal polymer tubular structure. When light is applied, the liquid crystal molecules are aligned along the polarization direction of the light, resulting in an increase in the diameter of the tube. When the light intensity changes in a gradient, the original tubular structure becomes a conical structure, and the liquid is driven by the Laplace pressure difference to transport to the side with a smaller diameter.

Jiang et al. Utilize the dynamic structure of the magnetic response to drive the droplet^[79]. They first prepared flexible tubes of PDMS by template method, and then covered the tubes with a layer of PMDS containing ferromagnetic particles. When the magnet is close to the pipe wall, the magnetic PDMS layer shrinks, the pipe diameter decreases, and the liquid moves in the direction of smaller cross-sectional area. As the magnet moves along the tube wall, the area of constriction moves with it, and the liquid in the tube moves in the same direction. This method moves droplets at speeds of up to tens of centimeters per second, which is the fastest speed that similar structures have been able to achieve in previous studies (Figure 9B).

4 Synergetic effect driving droplet

In fact, when the anisotropic wettability or anisotropic structure is used alone to drive the droplet, the driving speed and distance of the droplet are limited. Combining the advantages of anisotropic wettability and anisotropic structure, using synergistic effect to improve droplet driving speed and distance is one of the current research hotspots (Figure 10).

As shown in fig. 11A, Ju et al. prepared a tapered structure by using a gradient electrochemical etching method, and then introduced a wettability gradient on the tapered structure by gradient chemical modification to obtain an anisotropic surface with both structural difference and wettability difference. Such a surface greatly increases the Laplace pressure differential experienced by the droplet and increases the collection rate and transport rate of the droplet^[68].

Jiang et al. Revealed the droplet driving principle of spider silk. They found that the surface of spider silk has periodic spindle-shaped knots. Moreover, the roughness of the spindle-shaped junction is greater than that of the junction, and it has greater wettability. When the droplet is placed on the spider silk, it will be transported spontaneously and rapidly to the spindle-shaped knot region under the action of asymmetric structure and wettability gradient^[69]. As shown in Fig. 11b, Jiang et al. subsequently prepared a hydrophilic PMMA spindle-shaped knot with synergistic effect by imitating the spider silk structure, which realized the condensation and rapid directional driving of tiny droplets^[86].

In addition, the polymer with light response or temperature response is modified at the spindle-shaped junction to change its wettability under external field stimulation, which affects the synergistic effect of the anisotropic surface, thus changing the driving direction of the droplet^[87]. As shown in fig. 11c, the PMMA-b-PNIPAAm block polymer has temperature responsiveness. At room temperature, the polymer is hydrophilic, and the droplets can move towards the spindle-shaped knot under the synergistic effect of the wetting gradient and the asymmetric structure. When the temperature rises, the polymer shows hydrophobicity and the droplets are driven in the opposite direction.

5 Conclusions and outlook

Droplet drive is of great significance in the fields of agricultural production, energy, environment and biomedicine. Inspired by nature, researchers have developed a variety of droplet driving methods using wettability gradients and asymmetric structures to achieve rapid and efficient droplet driving, which play an important role in many fields, such as directional collection and transportation of micro-droplets, biological detection, anti-icing, power generation, efficient heat dissipation and so on.

However, there are still some shortcomings in the current research. In terms of preparation, anisotropic surfaces often need to introduce micro-nano structures, which are easily destroyed by changes in temperature, pressure and chemical environment. How to improve the stability of anisotropic surface is one of the difficulties in current research^[31]. Recently, Deng's team at the University of Electronic Science and Technology of China proposed a strategy for the preparation of stable superhydrophobic structures, which provides a new idea for the design and preparation of anisotropic surfaces^[88]. In terms of application, the size of droplets driven by anisotropic surfaces is more than millimeters. How to drive smaller droplets, develop the application of anisotropic surface in micro-and nano-scale fluid driving, and improve the theory of droplet driving still need further research^[75]. Precise control of droplet driving speed and distance to achieve fine and intelligent droplet driving is also a hot and difficult point in current droplet research^[31,75]. In addition, the development of droplet directional transport on asymmetric surfaces for applications in microfluidic chips, biomedicine, and efficient energy harvesting is an important development direction in this field in the future^[89,90]. In a word, the droplet driven by anisotropic surface has the advantages of low energy consumption, high efficiency and simple method, and is an important method for droplet manipulation. The solution of the above problems will improve the droplet driving technology, promote the development of droplet research field, and broaden the application prospects of droplet research.

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