# 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
#### 3.2.1 Cone-like Shape  
Cone shapes formed between non-parallel plates are the most common three-dimensional asymmetric structures. The shape gradient characteristics caused by cones can be divided into concave curvature gradients on the inner surface of the cone (Figure 4a) and convex curvature gradients on the outer surface (Figure 4b). The concave curvature gradient on the inner surface of the cone causes the water droplets inside the cone to have an asymmetrically curved surface. This may result in the water droplets being affected by the unbalanced Laplace pressure and thus spontaneously transporting from a position of bigger curvature to a position of smaller curvature. In 1712, [PERSON] [56] first discovered that when oil of orange is placed between two glasses that are not parallel to each other, it will spontaneously transport towards the direction where the gap thickness is small. [PERSON] et al. [57] also discovered a way of SDWT inside a cone by studying the movement of the shorebird's peak during feeding, as shown in Figure 4c. The team designed a tweezer that can be controlled, and from theory, they analyzed SWDT between asymmetric plates that are repeatedly opened and closed [57; 58]. Convex curvature gradients on the outer surface of the cone similarly result in an unbalanced Laplace pressure, driving the droplets at the outer surface of the cone from positions of smaller curvature to positions of larger curvature. In 1986, [PERSON] [59] discussed the balance of a water droplet on an elongated cylinder. [PERSON] [60; 61] calculated the surface energy of the water droplet by finite element analysis, explaining why small droplets on an elongated cylinder looked like a clamshell, while large droplets tended to form a barrel shape. Inspired by the above studies, [PERSON] et al. [62] showed a strategy for SDWT on a conical wire for the first time and pointed out the driving force was the Laplace pressure difference. Researchers focused on SDWT on a conical outer surface for a long time. They found that the conical structure of the cactus thorn helps it to easily collect water in deserts [21]. [PERSON] et al. [63] found that the shape of the green bristleggrass bristle was a cone, with microgrooves on the surface, and had barbs so that the water droplets could spontaneously transport from the top of the bristle to the bottom (Figure 4d). [PERSON] et al. [64] also pointed out that the water droplet on the setae of desert scorptions can achieve a fast speed. They found that the shape of the scorpion setae was also a cone, and it can accelerate the speed of water droplets to 100 mm/s by the water film caused by fog collection. Cone-like shapes can also be found on spider silk [65] and many other bionic, artificial fog-collecting surfaces [21; 23; 66; 67]. Inspired by the cone-like shape of cactus thorns, the researchers proposed a wedge-shaped pattern on a flat surface. However, initially, the researchers did not realize that the wedge pattern could lead to SDWT, and they only created uniformly shaped trackson the super-wetted surface to control the route of the water droplet [68; 69; 70]. Until 2013, [PERSON] et al. [71] produced a wedge-shaped pattern by depositing hydrophobic copper on hydrophilic aluminum. When 2-80 uL of water droplets fell on the narrow side, they were able to transport from the narrow side of the wedge to the wide side of the wedge angle. They pointed out that the droplet motion behavior and the velocity of the water droplets depend on the wedge angle and the contact angle of the droplets on aluminum and copper. This study provides ideas and a basis for the SDWT on patterned super-wetted surfaces. As shown in Figure 4e, SDWT on a single wedge-shaped pattern (SWP) was noticed first. [PERSON] et al. [72] used UV light in combination with a photomask to produce SWP and analyzed the motion behavior and force of the droplet on the pattern. The maximum water transportation flow rate of this pattern can reach 350 uL/s, and the maximum instantaneous speed of water droplets can reach 400 mm/s.