Metastable Vapor in a Janus Nanoconfinement

We study the competition between liquid and vapor states of water confined between diametrically different surfaces, one hydrophilic and the other strongly hydrophobic (Janus interface). Using atomistic simulations and a generalization of the capillarity approximation, we demonstrate that vapor bubbles can persist in the confinement in long-lived metastable states. In contrast to the well-known metastability of confined liquid with respect to capillary evaporation, a strongly metastable vapor phase has so far not been anticipated, as infiltration does not involve the formation of new liquid–vapor interfaces. In the case of a Janus interface, however, we show that the interfacial free energy passes through a pronounced maximum during infiltration. This counterintuitive phenomenon provides a new mechanism for water-mediated attraction (capillary force) between polar and nonpolar surfaces in biophysical systems and in dispersions of heterogeneous nanoparticles.