Shedding of a condensing droplet from beetle-inspired bumps

Abstract

Inspired by Namib desert beetle’s bumpy structures, condensing droplet shedding from various super-hydrophobic bumps with a hydrophilic patch present on their top was numerically studied using the lattice Boltzmann method (LBM). The droplet grows on the top patch due to condensation and then sheds from the bump due to gravity. A parametric study has been conducted. It was revealed that the bump shape, diameter, height, inclination and wettability all affect the shedding volume (volume at which the droplet sheds from the bump) of the droplet. The shedding volume decreases with the bump height till a threshold height, defined as the critical bump height, beyond which the shedding volume does not change any more. With the same patch area, the shedding volume is smaller on the bump with a hemispherical top compared to the bump with a circular cylindrical top. The shedding volume increases with bump diameter, but decreases with the patch contact angle and surface inclination angle. The same trends were also found for the critical bump height. Furthermore, compared to milli-scale bumps, droplet removal followed by shedding on micro-scale bumps was found to be inefficient. Based on the simulation results, a scaling law was obtained by data fitting to estimate the critical bump height for bumps with a cylindrical top, which can be used to guide the design of desert-beetle-inspired bumps for efficient water collection from fog.