Nanoscale Coatings for the Enhancement of Atmospheric Water Capture

Abstract

Atmospheric water capture has the potential to combat water scarcity and improve the well-being of millions of people. Drops of dew condense on surfaces that are cooled below the dew point, and surface wettability plays a critical role in atmospheric water capture. In this Thesis, the role of surface chemistry in water capture performance was investigated, focussing on: 1) quantifying the relationship between surface wettability and nucleation density Ns, coalescence and early stage droplet growth, 2) enhancing droplet shedding behaviour for optimal heat transfer, using low-friction, slippery covalently attached liquid-like (SCAL) surface coatings, and 3) the development of novel methods for the preparation of biphilic SCAL coatings, including patterning using soft lithography for spatial precision. The key findings were that for conventional surfaces, the more wettable the surface, the higher the Ns, leading to faster droplet growth through coalescence. For smooth surfaces, Ns reduced exponentially as the surface became less wettable, and the results were fit to an exponential

decay function allowing for the prediction of Ns, based on surface wettability.

Low-friction SCAL surfaces of both hydrophilic (polyethylene oxide, PEO) and hydrophobic nature (polydimethylsiloxane, PDMS) were studied to increase the surface’s droplet shedding ability. The volume at which condensed droplets detached from the surface was found to be 30 times smaller on SCALS with low contact angle hysteresis (6°) than on conventional non-slippery surfaces . Droplet growth rates for hydrophilic SCALS were seven times larger hydrophobic SCALS, but despite this, both SCALS collected similar volumes of water. Droplet

growth cycles were three times faster on hydrophilic SCALS than on hydrophobic SCALS, making the hydrophilic SCALS better suited to real-world dew collection applications.