# 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
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Classification**} & \multicolumn{1}{c}{**Mechanism**} & \multicolumn{1}{c}{**Transport Speed**} & \multicolumn{1}{c}{**Transport Distance**} & \multirow{2}{*}{**Fabrication**} & \multirow{2}{*}{**Robustness**} \\ \hline \multirow{4}{*}{Wettability} & Wettability & Different contact & 0.5 mm/s–6 mm/s [44] & & & \\  & gradient caused & angle, advancing & 0.7 mm/s & 7 mm [45] & & \\  & by surface & angle, and & [45] & 2.5 mm [46] & & Normal & Unstable \\  & composition & receding angle & 40 mm/s & & & & \\  & & & & [46] & & & \\ \cline{2-8}  & Wettability & Different contact & 5 mm/s–14.6 mm/s & & & & \\  & gradient caused & angle, advancing & [52] & 3.2 mm–5.2 mm & & \\  & by surface & angle, and & 46 mm/s–75 mm/s & [53] & Complex & Stable \\  & roughness & receding angle & [53] & & & & \\ \hline \multirow{4}{*}{Shape gradient} & \multirow{4}{*}{Cone-like shape} & \multirow{4}{*}{Unbalanced Laplace force} & 100 mm/s [64] & & & \\  & & & 400 mm/s & 20 mm [72] & & \\  & & LuBalanced & [72] & 50 mm [77] & Simple & Normal \\  & & Laplace force & 289 mm/s & & & \\  & & & [77] & & & \\ \cline{2-8}  & & Unbalanced & 78 \(\pm\) 12 mm/s & & & \\  & & Laplace force; & [26] & & & \\  & & Principle of & 0.1 mm/s–10 mm/s & 10–20 mm [26] & & \\  & structure & minimum surface & [82] & 20 mm [82] & & \\  & & energy & 1 mm/s–100 mm/s & & & \\  & & & [89] & & & \\ \hline \multirow{4}{*}{Cooperated gradient} & \multirow{4}{*}{Cooperated gradient} & \multirow{4}{*}{Cooperated gradient} & Unbalanced Laplace force; & & & & \\  & & Principle of & 116.7 mm/s [22] & & & \\  & minimum & 11.738 mm/s [99] & 5 mm–20 mm [99] & & \\  & surface energy; & 92 mm/s & 103 mm [76] & & & \\  & & angle, advancing & [76] & & & \\  & & angle, and & & & & \\  & & receding angle & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of different strategies of SDWT. superhydrophilic pore and developed a theory model of fog collection efficiency. The collection rate and efficiency of this super-fast fog collector were as high as 4.21 g\(\cdot\)cm\({}^{-2}\)\(\cdot\)h\({}^{-1}\) and 35.68%, respectively. Droplets of different compositions have different transportation motions on the same surface. So, researchers can use this theory to realize the spontaneous separation of mixed liquids. [PERSON] et al. [105] developed a superhydrophobic and superoleophilic coating mesh film. Because of the different wettability between the two sides of the pore, oil and water could be separated. [PERSON] et al. [106] fabricated an asymmetric repetitive array structure by imitating the peristome surface of nepenthes. These structures drove the oil droplet transport spontaneously and prevented water droplet transportation. This team designed a device that can separate small oil droplets mixed in water, as shown in Figure 7b. SDWT has gradually become a focus in the field of microfluidics due to its advantages of no external excitation, fast mixing speed [78; 79], and low cross-contamination. [PERSON] et al. [27] proposed an artificial nepenthes peristome by controlling the magnetic field to tilt the cone array. This surface allowed the mixture of various kinds of water, as shown in Figure 7c. The potassium permanganate and ascorbic acid from different paths mixed spontaneously, causing the potassium permanganate to gradually lose its distinctive dark-purple color because it was reduced by ascorbic acid. [PERSON] et al. [107] fabricated a hydrophilic track with a certain angle on a superhydrophobic aluminum surface. This pattern not only could mix the liquid but also could grab and release water droplets, providing a new idea for water droplet control in lab-on-a-chip devices. Figure 7: Applications of the SDWT. (**a**) Flog collection on a snowflake-like pattern. Reproduced with permission from [103], Copyright 2023, _Journal of Materials Chemistry A_. (**b**) Separation of oil and water. water on ratchet construction inspired by the nepthenes.