The role of surface morphology in aerodynamics of biomimetic fog harvesting

Harvesting water by intercepting atmospheric fog is a proven solution in certain regions where surface water is scarce. The strategy of fog harvesting in Stenocara, the Namib desert beetle with bumpy elytra, is repeatedly explained in a narrative involving hydrophilic bumps and hydrophobic valleys that modify the transport of the accumulated water. While transport can be crucial in some cases, the potentially dominant role of the bumps as aerodynamic features that change the boundary layer and subsequent impaction rate of droplets has not yet been studied. In this study, we investigate the influence of surface morphology on the dynamics of droplet impaction and explore non-trivial geometries for potential enhanced performance. For this purpose, a custom, low speed table-top wind tunnel with fog inlet was implemented to measure the initial accumulation rate on targets with various surface morphologies, before coalescence and surface transport. Results show an approximately threefold increase in impaction rate of microdroplets with the addition of bumps in a beetle-like geometry. We hypothesize that the bumpy adaptation, where present in fog basking beetles, was driven by their aerodynamic role in accumulation, rather than their role in wetting properties and surface transport.