## 4 Discussion  
### Effects of Surface Wettability and Motion on Liquid Adhesion and Bouncing Behavior  
The traces of fluid particles [40] can reveal the intrinsic flow behavior of droplets. Traces can be obtained by numerical solution (computer simulation, see \"S3. Simulation Setup and Analysis\" in the Supplementary Material). The left and right sides of the center line in Figure 9 are the analysis results of models with hydrophilic and superhydrophobic surfaces, respectively. The tracking target is determined by a cube with a size of 10 mm \(\times\) 10 mm \(\times\) 0.3 mm, which contains the upper parts of nine droplets, with a total number of 199 particles (black dotted area in Figure 9 left). The motion range of droplet particles on hydrophilic surfaces is small, and, as can be seen from the analysis of the coordinates (Figure 9), most of the particles exhibit irregular slow outward-sliding behavior. In contrast, there is a regularity in the traces on the superhydrophobic surface, where the particles bounced in a direction and their traces were \"parabolic\". We use the z-axis and y-axis coordinates as examples to reveal the directional bounce behavior of droplets on superhydrophobic surfaces (Figure 10b). When the particle rebounds from the superhydrophobic surface for the first time, its z-coordinate-curve trend changes from falling to rising (the first inflection point). Due to the different projection angles of the droplet rebound, and to some droplets hitting the wall again, some particles show a rapid downward trend, while another portion of the particles rises again after the rapid drop due to the additional collision's kinetic energy (the second inflection point in Figure 10b). The state of the y-axis coordinate means that the particles on the hydrophilic surface fluctuate randomly (Figure 10 left) due to the pinning of the water adhering to the hydrophilic surface, which is difficult to detach. Motion induces multiple droplets to bounce backward (Figure 10 right, while differences in the surface curvature lead to diverse bounce directions. On the superhydrophobic surface, the bouncing direction is opposite to the motion direction, which accelerates the solid-liquid separation process; the trace is parabolic, and the y-axis coordinate increases monotonically with time. We have been able to confirm that, when the surface is made superhydrophobic, the motion of the model promotes the directionality of the droplet bounce-off, and the wetting phenomenon in the same area is minimized. These findings provide a confirmatory reference for the potential application of superhydrophobic surfaces on aircrafts and, in particular, in the development of aquatic UAVs. Figure 8: Comparison of the effects of surface curvature and wettability on lateral-pressure distribution. Water adhering to the hydrophilic surfaces of the wings quickly creates areas of high pressure on the wings, but asymmetrically, which induces nonequilibrium mechanical flight penalties. However, the coupling of superhydrophobicity to the surface curvature can disperse the impact force, no obvious high-pressure region is observed on the superhydrophobic surface, and the lateral interference is thus weakened (see details in “Supporting Movie S5” and “Supporting Movie S7” in the Supplementary Material). Figure 10: Coordinate changes of the simulated traces on the hydrophilic and superhydrophobic models. The staggered traces shown on the left (**a**,**c**,**e**) are on a hydrophilic substrate, implying that wetting occurs even while moving. The difference appears in the model after superhydrophobic modification, and the trend increases or decreases significantly. This suggests that droplets impacting a moving model are accelerated to bounce off (**b**,**d**,**f**). Figure 9: Comparative analysis of simulated-particle traces on hydrophilic- and superhydrophobic-model surfaces.