Phase transitions of capillary-held liquids in a slit-like pore

Dynamics of capillary held liquids plays important roles in a wide range of systems including adhesion, printing of paints and inks, the behavior of wet granular materials, and the mass transfer through porous media. Recent study suggested the presence of two distinct modes for the disappearance of capillary-held liquids in a slit-like pore of adjustable slit width that depended on the slit-opening rates. In contrast to the first mode that is well-documented in terms of the Young-Laplace equation, a novel and unexpected mode was observed when the liquid bridge was held in the vicinity of the thermodynamic phase boundary (equilibrium Kelvin length). Here we extended the study to three new compounds that have a wide range of vapor pressures. An evaporating liquid bridge developed large refractive index gradients that extended over a few micrometers from the edge of the meniscus once the slit width was increased beyond the equilibrium Kelvin length. This interfacial region with depleted refractive index retreated inward as the meniscus shrank with time, and the refractive index of the entire bridge subsequently fell from that of the liquid once the interfacial regions from the opposite sides of the shrinking bridge met at the center. The refractive index recovered to that of the liquid when the slit width was closed to below the Kelvin length and the vapor was allowed to recondense. The time scale of the evaporation and condensation depended on the size of the surface gap, and, when the surfaces were placed at a separation very close to the Kelvin length, it was possible to detect a stage in which the system was in an apparent kinetic equilibrium between two physical states-with and without the liquid connecting the two surfaces.