Droplet evaporation on superhydrophobic surfaces

Abstract

Over the last decades, a fabulous variety of synthetic superhydrophobic surfaces have been created, offering unique anti-wetting properties. A significant focus for these surfaces has been on their stay-dry and self-cleaning properties. However, unless in a saturated environment, water droplets lose mass through evaporation and this itself is a field of significant interest, which is illustrated by a flood of recent studies on surface contamination and potential surface transmission of infection by evaporating sessile droplets during the Covid-19 pandemic. Superhydrophobic surfaces alter a droplet's contact with a substrate and the surrounding environment, thus changing pinning and heat transfer properties. The droplet shape also alters the space into which vapor can diffuse. Despite the many excellent reviews on superhydrophobic surfaces, there does not appear to have been a focus on the overlap with evaporating sessile droplets. Here, we address this gap by outlining the diffusion-limited sessile droplet evaporation theory, applications on patterned superhydrophobic surfaces, effect of evaporative cooling on drop evaporation rates, and practical applications of drop evaporation on superhydrophobic surfaces, such as nanoparticle assembly, biomedical assay, analytical chemistry, and crystallization applications. Finally, we provide our personal views of possible future directions in these overlapping areas.