# Spontaneous Directional Transportation Surface of Water Droplet and Gas Bubble: A Review
## 3 Spontaneous Directional Transportation of Water Droplet (SDWT): Methods and Applications
### SDWT Based on Shape Gradient
#### 3.2.2 Repetitive Array Structure  
With the development of SDWT, researchers not only focused on the study of macroscopic wedge shapes but also noticed that water droplets can also transport spontaneously on the surfaces of some plants and animals that do not have apparent shape characteristics. To find the reason for this, they began to focus on the microscopic images of plant and animal surfaces and discovered that these surfaces with repetitive array structures were also capable of water transportation. In 1990, [PERSON] [80] found that the special skin of lizards living in desert areas could collect water in a rainy season and could transport water to the mouth in priority. [PERSON] [81] and his team studied the microstructure on the skins of three different lizards and found that there were honeycomb structures on their skin, which could be used to collect water. The water transportation on the scales of the desert horned lizard was most apparent. Thus, the team imitated this biological structure and fabricated repetitive array structures on polymethyl methacrylate (PMMA) using laser etching technology. As shown in Figure 5a, they made SDWT on this surface possible through a combination of mathematical modeling analysis and experimentation [82; 83; 84]. This structure used the asymmetric capillary structure to transport water and used interconnections to make the water droplet form a free liquid surface again so that the next cycle of transportation could be performed. Not only the skins of animals but also the surface of plants can transport droplets because of the repetitive array structure. The nepenthes plant has a specially evolved leaf for attracting, capturing, and digesting arthropods. [PERSON] [85] found that there were smooth, overlapping cells on the peristome surface of nepenthes, which allowed the peristome surface to be completely wetted by the rainwater and nectar it secreted. [PERSON] et al. [26] found that the special multilevel asymmetric structure on its peristome surface enhanced capillary action, allowing continuous SDWT and anti-gravity transportation. As shown in Figure 5b, during transportation, the lower layer of water would continue to fill the cavity of the structure, while the upper layer of water would keep spreading forward on a shallower upper track, and the water in the back would be difficult to diffuse backward because of the pinning force generated at the tip of the structure. Based on the above theory, the team fabricated a tilted arch array structure with sharp edges using photolithography to achieve the directional diffusion of water droplets [86; 87; 88]. Similarly, [PERSON] et al. [89; 90] designed a unique topological diode. They used the excess surface energy generated from a water droplet to drive it to break through the front contact line pinning and used the large pinning force to prevent the droplet from being transported backward. This method could achieve fast, long-distance directional transportation, and has good application prospects. However, such array structures are often damaged by external forces, affecting their lifetime. [PERSON] et al. [91] developed a two-dimensional asymmetric pattern to achieve unbalanced forces by controlling the length of the contact line between the water droplet and the hydrophobic pattern, as shown in Figure 5c, which finally resulted in SDWT. [PERSON] et al. [27] were also inspired by nepenthes and found that tilted cone arrays could also be used for SDWT. The use of tilted cone arrays for transportation was also found on araucaria leaf by [PERSON] et al. [92]. These leaves have a specific curvature and cross-arrangement, as shown in Figure 5d, which allows SDWT in three dimensions. Inspired by pine needles, [PERSON] et al. [93] designed asymmetric Janus pillars. As shown in Figure 5e, droplet merging occurs at the tip of the pillars, transforming excess surface energy into kinetic energy, and allowing droplet transportation along the pillars.