# 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 Cooperated Gradient  
In nature, many plants and animals do not just use a single transportation method but combine both to achieve more efficient water droplet transportation. SDWT based on  
Figure 5: SDWT based on a repetitive array structure. (**a**) The microstructure of the lizard skin and SDWT on a surface inspired by it. Reproduced with permission from [82], Copyright 2015, _Journal of The Royal Society Interface._ (**b**) Continuous SDWT and anti-gravity transportation on the peristome surface of nepenthes. Reproduced with permission from [26], Copyright 2016, _Nature._ (**c**) SDWT on a two-dimensional asymmetric pattern. Reproduced with permission from [91], Copyright 2018, _Materials Horizons._ (**d**) Three-dimensional capillary ratchet-induced SDWT on an araucaria leaf. Reproduced with permission from [92], Copyright 2021, _Science._ (**e**) Droplet-bouncing transportation inspired by pine needles. Reproduced with permission from [93], Copyright 2020, _Sci Adv._  
the cooperated gradient is a combination of both wettability gradient and shape gradient, bringing out the strengths of both methods. In 2010, [PERSON] et al. [65] found that the spindle knots of spider silk could lead to unbalanced Laplace pressure after studying the water collection processes on Uloborus wakkenaerius, as shown in Figure 6a. At the same time, they discovered that spider silk had a wettability gradient due to the difference in the microstructure and shape gradient of the silk. These two factors eventually led to the efficient water collection of spider silk. Meanwhile, polymer-based membranes have been developed in the fields of promoting osmotic energy conversion and droplet-triboelectric nanogenerators due to their excellent mechanical properties, controllable dimensions, and other advantages [94; 95; 96]. Inspired by the above, this team fabricated bioinspired bead-on-string silkworm silk using a polymer combined with silkworm silk. They dipped natural silkworm silk (NSS) into a solution of PVDF (poly(vinylidene uoride) in DMF (N,N-time-thyformamide). The polymer solution film broke up along the NSS to form periodic polymer droplets and formed a periodic spindle structure after phase separation. Due to the good biocompatibility of PVDF, it formed a regular geometrical structure on BSS and had better water collection performance and mechanical properties [97]. Analysis of rapid fog collection on the surface of cactus spines was carried out by [PERSON] et al. [21] This study found that cactus stems were covered by lots of cone-like spines and trichomes and there were multistage microgrooves on the surface of cactus spine. The cone-like shape produced the difference in Laplace pressure, and the microgrooves were sparser near the roots, leading to differences in roughness and then forming a wettability gradient. As shown in Figure 6b, the above two characteristics led to cooperative water transportation. [PERSON] et al. [98] combined a wettability gradient with a single wedge-shaped wettability pattern, which allowed for water droplet transportation on an upside-down surface. [PERSON] et al. [76] also solved the problem of the block caused by the pinning force on SSWP in this way. [PERSON] et al. [99] found in their observations of sarracenia that there are unique hierarchical microchannels on the trichome, as shown in Figure 6c. Two neighboring high ribs formed a large channel that contained 1-5 low ribs, which led to a water film on it. This structure allowed ultrafast SDWT at a speed that was three times faster than that on spider silk and cactus horn. Combining multiple structures from nature allowed for more efficient SDWT. [PERSON] et al. [66] proposed a slippery asymmetric bump by combining the wettability gradient on the back of the beetle, a wedge-shaped pattern of cactus thorn, and the microstructure of the nepenthes. This bump increased the rate of droplet formation during fog collection by six times and greatly improved fog collection efficiency.