Which properties of adsorbed droplets can describe heterogeneous nucleation on carbonaceous surfaces? Insights from molecular simulations and theoretical models.

Heterogeneous nucleation of cloud droplets or ice occurs on insoluble aerosol when the supersaturation of water vapor exceeds 100%. It is one of the least well understood processes in the climate system [Seinfeld et. al., PNAS, 2016]. The propensity of the different types of particles to nucleate cloud or ice droplets is affected by molecular scale chemical and topological properties of their surface [Kanji et. al., Meteor. Mon., 2017]. Heterogeneous nucleation is commonly represented in climate models by the classical nucleation theory (CNT) [Fletcher, J. Chem. Phys., 1958], whose only tunable input parameter is the contact angle, which does not allow for the inclusion such details. In this work we use the example of soot to demonstrate that a single contact angle, used as a thermodynamic parameter, is an ambiguous descriptor of the hydrophilicity - and therefore of the heterogeneous nucleation efficiency - of a surface. We also show that the adsorption nucleation theory (ANT) [Sorjaama, Atmos. Chem. Phys. 2007], in which the contact angle serves as geometric parameter that links the droplet shape to the amount water adsorbed at the surface, can account for molecular scale surface properties.

We perform molecular dynamics simulations of water nanodroplets on model graphene and soot surfaces whose hydrophilicity is tuned by A) uniformly scaling the interaction energy between the surface and droplet and B) by adding hydroxyl groups in different concentration and topology. We estimate the mean equilibrium contact angle of the droplets, and we present spatial distributions of local contact angles as novel and unusual approach to describe the real shape of nanodroplets, which strongly deviate from the idealized assumption of a spherical cap and fluctuate in time.

The average contact angle is a good descriptor of the hydrophilicity only in the case of type A systems, for which, in accordance with previous simulation results, we observe a linear relationship between the contact angle the surface hydrophilicity expressed as pairwise Ɛ_LJ parameter between the water and the surface. For the chemically and topologically heterogeneous type B systems we could not identify any significant correlation. Since the same mean contact angle can correspond to very different surfaces, CNT is not expected to differentiate between their heterogeneous nucleation activity.  The contact angle distributions on the other hand provide a unique description of the droplet shape for each of the systems. The distributions are bimodal for type A and trimodal for type B systems, with the marked differences in the weight and position of the hydrophobic peak. These distributions are however strictly geometrical properties of the droplet, and hence can only be used in the framework provided by ANT.