Fog collection on wire arrays

The upright, wire-like leaves of Stipagrostis sabulicola, a grass species in the Namib Desert, shows excellent fog collection performance. Inspired by such wire geometry, we aim to understand the mechanism of fog collection on individual vertical wires and multiple-wire arrays. We visualized how fog droplets are collected on a wire and measured the resulting fog-collection rate while varying wire diameter, surface wettability, and wind speed. Results show that fog-collection rates on a single wire are determined by deposition efficiency, an aerodynamics-related parameter. Surface wettability has a negligible effect on the fog collection rate at low wind speeds (e.g., 0.5 m/s). By contrast, there appears to be a strong effect of wettability at high wind speeds (e.g., 3 m/s), where surfaces with the lowest droplet adhesion performs worst. Such phenomenon is explained by the competition between the air drag force and the droplet retention force on a surface, both of which are determined by surface wettability and wire diameter. Building on the knowledge gained from individual wires, we optimize the design of multiple-wire arrays, which shows a four-fold enhancement in fog-collection rate compared to a single wire of the same surface area.