# 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 Wettability Gradient
[49] proposed a surface with different roughness by changing the laser processing parameters. On this surface, the contact angle of the water droplet ranges from 126\({}^{\circ}\) to 145\({}^{\circ}\) and the surface can achieve SDWT (Figure 3a). Figure 2: Fabrication and droplet motion behavior on the surface with wettability gradient caused by different surface compositions. (**a**) Condensation and droplet transportation on surface fabricated by vapor phase diffusion. Reproduced with permission from [41], Copyright 2001, _Science_. (**b**) Schematic illustration of the formation of wettability gradients on a graphite plate and water contact angles along the gradient surface. Reproduced with permission from [42], Copyright 2014, _Angeeu. Chem. Int. Ed._ (**c**) Droplet motion behavior on wettability gradient surfaces fabricated by photolithography. Reproduced with permission from [44], Copyright 2007, _Langmuir_. (**d**) Fabrication and droplet motion behavior on the surface generated by controlled surface reactions. Reproduced with permission from [45], Copyright 2010, _Langmuir_. (**e**) A well-controlled chemical gradient on the surface fabricated by plasma processing. Reproduced with permission from [46], Copyright 2013, _ACS Nano_. [PERSON] et al. [50] fabricated nano-pillars using photolithography and plasma etching, and then tilted these nanopillars by the stresses generated by unilateral gold film on nano-pillars. Its surface nanostructure and water droplet transportation are shown in Figure 3b. [PERSON] et al. [51] created a V-shaped array on the surface of an elastic material. By changing the curvature of the elastic film, the parameters of the V-shaped arrays can also be changed, which might lead to a difference in the wettability gradient. When the curvature changes, the water can be transported in different directions. In addition to the above methods, [PERSON] [52] presented a way to change the ratio of hydrophilic and hydrophobic regions of a surface to achieve a wetting gradient. The schematic illustration of the structure and the droplet motion behavior are shown in Figure 3c. Similarly, [PERSON] et al. [53] fabricated a wettability gradient (CA = 15.5\({}^{\circ}\)\(\sim\)166\({}^{\circ}\)) by etching silicon nanopillars and adjusting the area of hydrophilic silica planes. As shown in Figure 3d, the speed of the droplet can be up to 75 mm/s, and the droplet can transport in curves along a designed track. In addition, [PERSON] et al. [54] used photolithography combined with low surface energy modification to fabricate circular arrays with different diameters and spacings on the copper surface. On this surface, a water droplet could easily transport past the surface within 2.5 s.  
Figure 3: Wettability gradient caused by different surface roughness. (**a**) Water transportation on a laser-etched surface. Reproduced with permission from [49], Copyright 2008, _Thin Solid Films_. (**b**) Nanostructure and water droplet motion behavior of surfaces with different nanopillars. Reproduced with permission from [50], Copyright 2010, _Nature Materials_. (**c**) Structure of a surface that has different ratios of hydrophilic and hydrophobic regions and water droplet motion behavior on it. Reproduced with permission from [52], Copyright 2011, _Langmuir_. (**d**) Wetting behavior and water droplet motion behavior of a surface consisting of hydrophobic silica nanopillars and hydrophilic SiO\({}_{2}\). Reproduced with permission from [53], Copyright 2017, _Scientific Reports_. The surface with a wettability gradient for water droplet transportation was one of the first areas that researchers began to study. Many researchers focused on the fabrication of wettability gradient surfaces using more convenient processing methods. However, the commonly used processing methods, such as vapor phase diffusion [39; 40; 41; 55], SAMs remold [44; 45], photolithography [43; 48; 50; 54], and so on, are still complex and unsuitable for large-area processing.