# Spontaneous Directional Transportation Surface of Water Droplet and Gas Bubble: A Review
## 3 Spontaneous Directional Transportation of Water Droplet (SDWT): Methods and Applications
### Applications of SDWT  
Through the above sections, we summarized various methods of the SDWT and their principles. Table 1 shows an overview of the characteristics of different transportation strategies. These spontaneous water transportation technologies have a wide range of applications, such as fog collection in dry areas, droplet control in lab-on-a-chip devices, and heat transfer. Figure 6: SDWT based on cooperated gradient. **(a)** SDWT due to shape gradients and wettability gradients on spider silk. Reproduced with permission from [65], Copyright 2010, _Nature_. **(b)** SDWT induced by wettability gradient, microgrooves, and cone-shaped structures on cactus surfaces. Reproduced with permission from [21], Copyright 2012, _Nature Communications_. **(c)** Ultra-fast SDWT on the trichome of sarracenia trichome due to the unique hierarchical microchannels and cone-shaped structures. Reproduced with permission from [99], Copyright 2018, _Nature Materials_. **(d)** Continuous, directional, and long-distance transportation on the surface of serial comics inspired by spider silk and butterfly wings. Reproduced with permission from [67], Copyright 2020, _Advanced Materials Interfaces_. Freshwater scarcity was a worldwide problem restricting human development. As early as 1992, fog collection was realized on a surface with a wettability gradient by [PERSON] [41]. However, due to the complexity of its fabrication and low collection efficiency, researchers started to focus on other methods. They successively found fog collection phenomena on the backs of beetles [19], spider silk [65], and cactus spines [21; 97] and were inspired by them. [PERSON] et al. [100] proposed a mixed-surface film by adding hydrophobic Ag and hydrophilic nano TiO\({}_{2}\) to a bio-based film. Its fog collection rate is as high as 1043 mg\(\cdot\)cm\({}^{-2}\cdot\)h\({}^{-1}\). Inspired by cactus, [PERSON] et al. [101] also achieved efficient fog collection using cone-like copper wires with a wettability gradient. To solve the limitation of low droplet shedding efficiency, [PERSON] [23] developed a proposal that combined the structure of the cactus spine and Janus membrane. This proposal used a cone-like shape structure with microstructure to collect the fog and transport the water droplets. Then, the Janus membrane was placed vertically to the cone to collect water droplets. This study realized continuous water collection. Due to the small size of the cone surface, researchers designed a star-shaped pattern on a flat surface to achieve more efficient fog collection [102]. The fog collection reached 2780 mg/cm\({}^{-2}\cdot\)h\({}^{-1}\) when the surface was tilted 45\({}^{\circ}\). But it had a limitation in that it could not collect the droplets created by fog collection if the surface was parallel to the ground. To solve this limitation, [PERSON] et al. [103] designed a snowflake-like pattern with a small hole, as shown in Figure 7a. The droplet could spontaneously transport to the hole by Laplace pressure and then be collected by the hole with a wettability gradient. In addition, [PERSON] et al. [104] designed a super-fast fog collector based on the Laplace pressure difference generated by a miniature fog droplet on a superhydrophobic surface with a wettability gradient.