Surface wettability plays an important role in water recovery, distillation, dehumidification, and heat transfer. The rate of water collection depends on the rate of droplet nucleation, droplet growth, the initiation of droplet rolling, and droplet velocity. Nucleation and growth theory predicts a large advantage for hydrophilic surfaces, while geometric considerations stemming from high contact angles favor droplet shedding from hydrophobic surfaces. Some plants and animals overcome this tradeoff through biphilic surfaces with patterned wettability. For example the <i>Stenocara</i> beetle uses hydrophilic patches on a superhydrophobic background to collect fog from air. Cribellate spiders similarly collect fog on their webs through periodic spindle-knot structures on their silk fibers. In this study, we investigate the effects of wettability patterns on the rate of water collection from humid air. The rate of water collection per unit area is measured as a function of undercooling, and angle of inclination. It is then compared for biphilic, hydrophilic, hydrophobic, and surperhydrophobic surfaces. We then model each pattern by comparing the potential and kinetic energy of a droplet as it rolls downwards at a fixed angle. The results suggest that the rate-limiting step for condensate capture is the initiation of droplet rolling, and thus the fog-collecting strategies of the Stenocara beetle inhibit the rate of collection under these conditions.

Effects of <i>Stenocara</i> beetle-inspired biphilic patterns on the rate of water collection from humid air

253rd National Meeting (2017)

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Effects of Stenocara beetle-inspired biphilic patterns on the rate of water collection from humid air