# 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  
The wettability gradient characteristic is the continuous change in the wettability on the surface. Since there are certain differences in the contact angle, advancing contact angle, and receding contact angle of the droplet on the surface with a wettability gradient, it will result in the spontaneous directional transportation of water droplets from the side that is not easily wetted to the side that is easily wetted. In 1978, [PERSON] [39] first predicted that a water droplet could directionally transport on a surface with the wettability gradient through theoretical and analytical modeling derivations. In 1992, [PERSON] et al. [40] prepared a surface with wettability gradients by exposing a polished silicon wafer to the diffusing front of a vapor of decyl trichlorosilane. On this surface, water droplets showed a contact angle ranging from 25\({}^{\circ}\) to 97\({}^{\circ}\). For the first time, water droplets could transport against gravity on a 15\({}^{\circ}\) tilted plane with a speed of 1 mm/s to 2 mm/s. In 2001, this team also similarly created a sample that had a contact angle of 100\({}^{\circ}\) in the surface center and a contact angle of 0\({}^{\circ}\) at the surface edge [41]. They performed condensation tests on this surface; as shown in Figure 2a, droplet 1 and droplet 3 spontaneously transported through the sample surface within 0.033 s, and droplet 2 did not move significantly at first due to its location. However, at 0.466 s, the position of droplet 2 had changed, which caused droplet 2 to be quickly transported across the surface within the next 0.033 s. [PERSON] et al. [42] developed a method by controlling the distance between the electrodes to achieve differences in chemical composition on the surface, as shown in Figure 2b. Water droplets could spread spontaneously up to 4 mm and transport against gravity on this surface. This method can quickly create a surface with a wettability gradient on different conductive metals. In 1997, [PERSON] et al. [43] produced TiO\({}_{2}\)-coated glasses with a wettability gradient using UV irradiation, which enabled the use of light to control the chemical composition. After that, [PERSON] et al. [44] developed a photodegradation technique to fabricate a wettability gradient by controlling the light intensity and duration. As shown in Figure 2c, a 2 \(\upmu\)L water droplet can transport on this surface at a maximum speed of 6 mm/s and a maximum distance of 3.5 mm. [PERSON] et al. [45] changed the oxidizer concentration by diluting it and controlled the oxidation time to oxidize vinyl-terminated self-assembled monolayers (SAMs) to carboxylic acid (COOH)-terminated SAMs. The behavior of droplet transportation on this surface is shown in Figure 2d. This is a simple preparation process and allows the generation of gradients at multiple points on the surface. [PERSON] [46] also produced a chemical composition gradient on the graphene surface using plasma processing in conjunction with a mask, which has the advantages of convenience, flexibility, controllable process, and environmental friendliness (Figure 2e). According to the Wenzel model, it can be seen that the contact angle of the surface with lower surface roughness is also smaller. Thus, a water droplet will spontaneously and directionally transport from the hydrophilic area with higher roughness to that with lower roughness. [PERSON] et al. [47] bonded a thin polydimethylsiloxane (PDMS) membrane on the top of a rough PDMS substrate and deflected it pneumatically, which achieved a wettability gradient using roughness difference for the first time. This method did not change the chemical composition of the surface and developed a new strategy for the fabrication of wettability gradients. [PERSON] et al. [48] fabricated square columns with different side lengths, heights, and spacing on the silicon surface by photolithography, which caused the roughness difference. Inspired by this, [PERSON] et al.