# Spontaneous Directional Transportation Surface of Water Droplet and Gas Bubble: A Review
## 4 Spontaneous Directional Transportation of Gas Bubble (SDBT): Methods and Applications
### Applications of SDBT  
Researchers have proposed various patterns such as wedge-like and cone-like shapes to drive gas bubbles underwater based on Laplace pressure. Table 2 shows an overview of the characteristics of different transportation strategies for SDBT. Due to the advantages of spontaneous transportation and simple structure, SDBT driven by surface gradient force has important application prospects in various aspects of daily life and industrial production, such as gas collection, hydrogen precipitation reaction, and underwater sensors. Environmental protection is an eternal theme for humankind. The release of hazardous gases due to accidents and industrial development exacerbates the greenhouse effect. Therefore, underwater directional gas transportation and collection are especially important for environmental protection. [PERSON] et al. [128] used the designed 2D superaerophilic surface to transport hydrogen and superaerophilic sponge and tubes to collect gas bubbles, as shown in Figure 11a. [PERSON] et al. [129] also designed an underwater gas bubble collection device that consisted of a single layer of superaerophilic mesh and quartz tubes. This device could be sealed and was able to collect underwater gas bubbles. Gas bubbles also have an important impact on catalytic production, especially in the hydrogen precipitation reaction. If gas bubbles cannot be removed in time, they will attach to the electrode, resulting in insufficient reactions between the electrode and the electrolyte, which affect the production efficiency in the end. [PERSON] et al. [130] fabricated a multifunctional electrode for bubble generation, transportation, and collection by combining a superaerophilic sponge with an aerophilic cone-shaped electrode. The cone-shaped electrode functioned as a reduction electrode to generate hydrogen. The generated hydrogen would transport along the aerophilic surface toward the root where there was a superaerophilic sponge. Once the hydrogen gas touched the superaerophilic sponge, it would be absorbed immediately, which allowed the electrode to have enough area for continued hydrogen precipitation reactions, as shown in Figure 11b. Figure 11: Applications of underwater SDBT. **(a)** Gas collection on a flat electrode. Reproduced with permission from [128], Copyright 2021, _Advanced Functional Materials_. **(b)** Gas transportation and collection on cone-shaped electrodes. Reproduced with permission from [130], Copyright 2016, _Advanced Functional Materials_. **(c)** Monitoring with bubble sensors. Reproduced with permission from [131], Copyright 2018, _ACS Applied Materials & Interfaces_. **(d)** Microbubble reactor. Reproduced with permission from [132], Copyright 2021, _Advanced Functional Materials_. The control of underwater gas bubbles is flexible and versatile, which not only plays a role in the study of gas collection but also has good application prospects in the field of sensor monitoring. [PERSON] et al. (2013) fabricated a superhydrophobic cone by electrochemical etching and found that the equilibrium position of a gas bubble whose volume is a constant value was fixed on the cone. This meant that the equilibrium point could be changed by increasing or decreasing the volume of gas bubbles, as shown in Figure 11c. Thus, this team designed an underwater sensor based on such a theory. The spontaneous and directional motion property of underwater gas bubbles also provides a new method to conduct chemical reactions. [PERSON] et al. (2014) injected hydrogen and oxygen bubbles into two different narrow sides of the pattern. The two gas bubbles transported towards each other along the track and merged at the middle position of the pattern due to the driving of the Laplace pressure. The mixture bubble undergoes a violent combustion reaction upon ignition. [PERSON] et al. (2013) fabricated a gas bubble microreactor based on a magnetically controlled V-slip surface.