Evaporation of Sessile Droplets: A Molecular Dynamics Study

The evaporation process of a sessile droplet plays a ubiquitous role in a wide range of natural phenomena and industrial procedures. Theories and models exist to predict the local evaporation flux as a function of the azimuthal angle. Although plenty of experimental and computational studies have been conducted on drop evaporation, data on the local evaporation flux with a good spatiotemporal resolution are still of great value and can be used to test the existing theories. Much of the challenges lies in the fact that evaporation is a transient process, making it hard to achieve good data statistics, and it is also difficult to measure the evaporation flux with a sufficiently fine spatial resolution. In this work we report molecular dynamics simulations of the evaporation process of a droplet adsorbed on a solid substrate with the contact line pinned by a chemical heterogeneity. We have designed a setup where the evaporating droplet is kept in a steady state by putting the escaping flux back to the center of the droplet. This setup allows us to average the local evaporation flux over a long time, thus obtaining data with a good accuracy and high spatial resolution. We have compared our simulation results to the predictions of various theories and models. In particular, our data show that for a wetted drop, the local evaporation flux increases from the apex of the drop to the contact line, in agreement with a model of drop evaporation based on vapor diffusion.