# 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
This pattern also enabled droplet merging and anti-gravity transportation. In order to improve the transportation performance of SDWT, [PERSON] et al. [73] fabricated SWP on a superhydrophobic aluminum by laser processing and studied the effect on transportation caused by different parameters of the laser processing. They pointed out that the microgrooves fabricated by laser processing led to faster water transportation speeds. SWP has shown excellent transportation performance in the field of SDWT. But because of its single wedge shape, it has a generous size at the end of the pattern, which may cause fluid loss. If the water droplet becomes smaller during transportation, the driving force coming from Laplace pressure will also be smaller, which results in a short transportation distance. To solve this problem, [PERSON] et al. [74] proposed a superhydrophobic serial wedge-shaped pattern (SSWP) in which the wedges are connected head to tail (Figure 4f) to decrease fluid loss and increase the water transportation distance. However, this brings another problem: when the water droplets are at the junction of the pattern, they will be impeded by pinning force, which leads to a loss of speed. To solve the problem, researchers began to study the optimization of the junction. [PERSON] et al. [74] proposed a method of arc junction transition optimization which increased the speed by 5 percent. Inspired by nepenthes, as shown in Figure 4g, [PERSON] et al. [75] proposed a method of wedge junction transition optimization, resulting in the normalized transportation distance (transportation distance/droplet diameter) of 23.4 for a 3 uL droplet. [PERSON] et al. [76] proposed a method of streamlined junction transition optimization, which allows large water droplets to pass the junction smoothly. However, a faster speed is always needed because of the significant use of SDWT. [PERSON] et al. [77] were inspired by nepenthes and proposed a superhydrophilic serial cycloid-shaped pattern (SSCP). They replaced the wedge-shaped pattern with the cycloid-shaped pattern, which made the wedge angle continuously change during the transportation process and reduced the number of junctions (Figure 4h). The average water transportation speed within 50 mm on SSCP was 40% higher than that of the SSWP. These methods all have a cone-like shape and use the different Laplace pressures caused by the shape to achieve SDWT (from narrow to wide). This water transportation type has potential applications in fog collection, microfluidic devices, microchemical reactors, and liquid mixing [78; 79]. But it has the disadvantages of fixed transportation routes and fluid loss. Figure 4: SDWT based on a cone-like shape. (**a**) Schematic illustration of SDWT on the inner surface of the cone. (**b**) Schematic illustration of SDWT on the outer surface of the cone. (**c**) SWDT between asymmetric plates of a shorebird’s beak. Reproduced with permission from [57], Copyright 2008, _Science_. (**d**) SDWT on the surface of green bristegrass bristle. Reproduced with permission from [63], Copyright 2014, _RSC Adv._ (**e**) Force conditions and water droplet motion of SWP. Reproduced with permission from [72], Copyright 2014, _Lab Chip._ (**f**) SDWT on SSWP. Reproduced with permission from [74], Copyright 2022, _Chemical Engineering Journal_. (**g**) Schematic illustration of wedge junction transition optimization. Reproduced with permission from [75], Copyright 2022, _Small Methods_. (**h**) Schematic illustration of SSCP and SDWT on it. Reproduced with permission from [77], Copyright 2023, _Nanoscale_.