Separation and collision dynamics in biomimetic wind-driven fog harvesting

In regions where foggy days are recurrent and other water sources scarce, harvesting fog is a practical solution. Physical and behavioral techniques developed over eons by plants and animals in these regions can provide useful insight to do it better. O.unguicularis, a Namib desert beetle, goes to the top of the sand dunes on foggy days and uses its forewings to intercept the inertial microdroplets carried by the wind. Adaptation of this beetle has been extensively studied through the lens of surface wettability and its role in transporting accumulated water. Yet, how the interaction between flow dynamics and surface topography of beetle elytra can induce collision of droplets and increase accumulation of water has not been considered. Here, we show in careful experiments accompanied by numerical flow simulations that surface morphology, independent of wettability, plays a dominant role in deposition efficiency of beetle-like geometries. Furthermore, we investigate the potential role of hydrodynamic forces in near collision dynamics where surface mechanics and texture can limit the possibility of collision. The results suggest that manipulation of the collision mechanism via geometry and mechanics can be an alternative driver of the physical adaptation of the fog basking beetles.