Surface morphology enhances deposition efficiency in biomimetic, wind-driven fog collection

Passive interception of fog from the wind is an effective solution for accessing water in regions where fog is frequent and other sources scarce. A Namib Desert beetle is often cited as bioinspiration for further advancement, in a narrative which focuses on patterned wettability of its bumpy elytra as a means of transporting accumulated water from its back to its mouth. However, surface transport in fog collection is secondary to the role of the fluid dynamics of droplet deposition, in which inertial droplets migrate across diverging streamlines approaching an obstruction. 3D geometry of biological surface features inevitably affect this process, but its specific role in flow physics of fog collection has not previously been explored. Here, we report experimental measurements of deposition efficiency of targets with identical surface chemistry but varying surface morphology. We find a nearly threefold increase in collection upon addition of millimetric bumps to a spherical target, and provide insight into the micromechanics underlying the performance. Modifying surface morphology can be easier than overall geometry for both manufactured structures and evolved organisms and should therefore be both considered in the design of separation devices and expected in other biological systems for which extraction of particles from flow is important.