Evaporation of water nanodroplets: does size matter?

Understanding the wetting behavior and evaporation kinetics of droplets on solid substrates is central to many engineering and nanotechnological applications such as nanofabrication or spray cooling of electronics. While a broad range of continuum models exist to predict the evaporation of macroscale droplets, a similar level of understanding is lacking at the nanoscale, where many of the continuum assumptions break down and experimental systems become very hard to probe. Here, we use molecular dynamics to investigate the evaporation kinetics of water nanodroplets on heated substrates with different hydrophobicity. We find that all water nanodroplets follow a previously unidentified mode of evaporation characterized by large isovolumetric fluctuations. In contrast to diffusion-limited models, the evaporation rate of hydrophilic nanodroplets decays exponentially with the volume, and hydrophobic droplets follow stretched exponential kinetics. Our results show that the lifetime of the nanodroplets, which also disagrees with continuum predictions, strongly correlates with the time that takes the droplets to thermalize with the substrate, with hydrophobic droplets resulting in longer lifetimes. Interestingly, all droplets exhibit a non-uniform evaporative flux.