# Spontaneous Directional Transportation Surface of Water Droplet and Gas Bubble: A Review
## 1 Introduction  
In recent years, fluid transportation has gradually gained attention because of the development of precision medicine, micro-devices, and green energy collection. Researchers found that surfaces for water and gas transportation had numerous applications such as fog collection, heat transfer, drug transportation, and so on [1; 2; 3; 4; 5; 6; 7]. However, when a water droplet touches a solid surface, it will perform some indeterminate behaviors, such as impacting, jumping, trapping, and spreading. An underwater gas bubble demonstrates similar behaviors (adhesion, merging, etc.) when it contacts a solid surface. Researchers have undertaken many studies to make spontaneous directional transportation (SDT) possible, meaning that fluid can be transported on an expected trail. Currently, the means of controlling fluid for spontaneous and directional transportation can be divided into two methods: fluid transportation using external energy and spontaneous directional fluid transportation without external energy input. Directional transportation using external energy is often achieved by magnetic field [8; 9; 10], electric field [11; 12; 13], thermodynamics [14; 15], optics [16; 17; 18], and so on. Despite the fact that these methods can achieve directional fluid transportation, their application areas are limited due to the need for external energy input. Researchers have found that the spontaneous directional transportation of fluids without external energy input can also be achieved by using gravity or buoyancy. However, these methods can only transport fluid vertically upwards or downwards, and the accuracy of the fluid transportation is difficult to guarantee. When researchers studied organisms in nature, they found that many organisms hadevolved unique microstructures on their surfaces that enabled spontaneous directional water transportation (SDWT) and spontaneous directional gas bubble transportation (SDBT). Inspired by organisms in nature, researchers developed a variety of functional surfaces to enable spontaneous directional fluid transportation. The entire transportation process of the water droplet or gas bubble on functional surfaces does not require external energy input. Therefore, the application field of spontaneous directional fluid transportation will be more diversified in the future and is one of the key areas for the future development of fluid transportation systems. In 2001, [PERSON] (1966) from Oxford University found that numerous hydrophobic bulges and hydrophilic groove structures on the back of the desert beetle allowed the captured water droplet to be transported from its back to its mouth. The Janus membrane is also referenced, which is used for oil-water separation ([PERSON] and [PERSON], 2012). In 2012, [PERSON] et al. (2012) from Beihang University found that cacit evolved cone-shaped spines to adapt to desert environments. The fog droplets would spontaneously transport from the tip to the root due to the unbalanced Laplace pressure. Based on this, researchers designed a variety of structures inspired by the cactus spine for fog collection ([PERSON] and [PERSON], 2012; [PERSON], 2012; [PERSON], 2012) and underwater gas supply ([PERSON] and [PERSON], 2012). In 2016, [PERSON] et al. (2016) found that water could transport continually on the peristome of nepenthes against gravity because of its multiscale structure, which optimized and enhanced the capillary force. Inspired by this, researchers designed an artificial bionic multiscale surface, which could control the water droplet transportation in a lab-on-a-chip device ([PERSON] and [PERSON], 2012) and capture the bubbles underwater ([PERSON] and [PERSON], 2012). In this review, we first look back at the important basic theories related to fluid transportation. Then, we summarize the theories and methods of SDWT and SDBT and discuss their limitations. In addition, we review the applications of SDWT and SDBT in the fields of fog collection, fluid control, heat transfer, gas collection, and underwater sensors.