Bio-inspired hierarchical topography for texture driven fog harvesting

Fog harvesting is recognized as one of the most sustainable means of freshwater collection. Synthetic fog harvesting surfaces have been predominantly inspired from the desert beetle's exoskeleton which exhibits a bumpy topography. This topography underlies an alternating hydrophilic-hydrophobic pattern which has been the basis of several bio-inspired fog harvesting surfaces. However, replication of such hydrophilic-hydrophobic patterns involves multiple processing steps and tedious incorporation of functional/chemical groups at precise locations. On the other hand, surface topography or texture has proven to be insufficient in realizing an efficient fog harvesting surface. This is because micro- or nano-scale textures alone fail to simultaneously maximize the rate of droplet condensation and disposal, which are the two key aspects of efficient fog harvesting. Herein, we report that a hierarchically-textured surface, consisting of micro-lenses arrays covered with high aspect-ratio nanoscale fibrils, can fulfil these two key requirements for maximizing fog harvesting efficiency. While the micro-lenses enable faster droplet condensation, the cluster of nanoscale fibrils impart superhydrophobicity that aids in intermittent droplet disposal. Together, the topography achieves a fog collection efficiency ∼5-6 times higher than that of the planar counterpart. Moreover, this hierarchical texture is fabricated by a simple one-step nanoimprinting approach which is scalable to arbitrarily large-area flexible substrates.