Molecular Simulations Supporting Adsorptive Theory of Heterogeneous Droplet Nucleation

Αerosol-cloud interactions constitute the largest source of uncertainty in assessments of regional and global anthropogenic climate change. Better understanding of aerosol-cloud interactions can help narrow down the uncertainty range and improve model predictions. A key issue is the predictive understanding of heterogeneous nucleation of liquid water and ice on insoluble particles. Variety of aerosol types, wide temperature ranges, and challenges related to direct observation of the fundamental steps of nucleation hinder the improvement of atmospheric models, and invoke large uncertainties in aerosol-cloud-precipitation and aerosol-cloud-climate assessments. The current conceptual framework of heterogeneous nucleation is based on the classical nucleation theory which assumes the formation of critical sized droplets on insoluble surfaces to be a single step process. This view is challenged by the fact that adsorption isotherms on partially wettable surfaces extend to the supersaturated regime before condensation, which suggests that adsorption preceeds heterogeneous nucleation. Adsorption nucleation theory (ANT), which introduces the effect of multilayer adsorption before droplet condensation, is formulated in terms of macrodroplet contact angles used as purely geometric descriptors of the thickness of the adsorbed layer. It proved successful at modeling droplet nucleation on graphene and is able to account for surface heterogeneities. ANT the may provide the long needed generalised framework for heterogeneous nucleation of droplets in case in case its only assumption, namely the equality macroscopic and microscopic contact angles is valid across a range of temperatures and surfaces which are of interest for atmospheric science. A systematic set of measurements combined with molecular simulations is performed to elaborate this assumption. We present contact angles of water, estimated from long molecular dynamics simulations in comparison with measured contact angles, at temperatures ranging -70 to 25 °C, at pure and partially oxided graphene, silica and select metal oxide surfaces. Droplet binding energies and the effect of surface oxidation on binding thermodynamics is also characterized. Our results are used to prove the validity of ANT and refine the theory of droplet nucleation.