Nanodroplets of immiscible fluid pairs adopt nonspherical shapes

To help understand recent experimental results for nonane/water condensation [Pathak, et al. J. Chem. Phys. \textbf{140}, 224318 (2014)], the structure of water/nonane nanodroplets was investigated using classical molecular dynamics (MD) simulations of SPC/E water and a unified atom model of nonane. Because nonane and water are essentially immiscible fluids that only partially wet each other, one might expect unusual nanodroplet structures to arise. Indeed, nonspherical, phase-separated Russian Doll (RD) structures were found to occur for these nanodroplets over the entire temperature range studied in the MD simulations, 220K -- 300K. An idealized, but realistic lens-on-sphere model for the observed RD structures consists of a spherical nonane lens that partially wets a spherical water droplet. This model was used to analyze the experimental small angle x-ray scattering measurements. The simulated contact angle of nonane on water was found to be quite sensitive to the value of the Lennard-Jones energy parameter $\varepsilon _{OC}$ for the cross-interaction between oxygen and carbon atoms. The standard geometric mean approximation for $\varepsilon_{OC\, }$yielded contact angles in the range 70$^{o}$- 80$^{o}$, while a 19{\%} increase in $\varepsilon_{OC\, }$reduced the simulated contact angle close to the experimental value of 33.6$^{o}$ at 295 K.