Enhanced growth rates of nanodroplets in the free molecular regime: The role of long-range interactions

Recently, Pathak et al. (2013) conducted a series of non-isothermal D2O nanodroplet growth studies in the free molecular regime. They found that under highly non-equilibrium conditions, the condensation (q(c)) and evaporation coefficients (q(e)) can differ from each other and from the expected value of 1. Here, we confirm these observations by analyzing comparable experiments using n-propanol. We show that the best agreement with the non-isothermal Hertz-Knudsen growth law corresponds to setting (q(c), q(e)) = (1, 0.6) or (q(c), q(e)) = (1.3, 1). The approach of retarded evaporation yields values close to those observed by Pathak et al. for D2O, but is difficult to justify theoretically. Enhancing the condensation coefficient is consistent with long-range attractive interactions between the vapor molecules and droplets in the nanometer size range.