Nanoscale Mechanisms of Evaporation, Condensation and Nucleation in Confined Geometries

Capillary condensation, evaporation, and nucleation have been extensively studied for decades, but many fundamental questions remain unresolved. Compared to capillary condensation (e.g., pore filling), the reverse phenomenon of capillary evaporation (e.g., pore emptying) has received little attention, mainly due to a lack of suitable experimental techniques. We present the first detailed ångstrom-level measurement of evaporation of a cyclohexane condensate (liquid bridge) in a slitlike nanopore bounded by two mica surfaces of adjustable gap separation. As the separation is increased beyond the equilibrium (Kelvin) diameter, the liquid condensate evaporates, but we also find that the density becomes nonuniform across the condensate falling significantly over a few microns from the edge of the meniscus. As the evaporation proceeds in time, the nonuniform density falls gradually and continuously from liquid to vapor, ultimately attaining that of the vapor. If the gap distance is decreased before the condensate has fully disappeared, then recondensation takes place via a gradual and continuous increase in the density without hysteresis, i.e., reversibly. Our findings suggest a new mechanism for capillary evaporation and nucleation and, possibly, for the hydrophobic interaction.