Capillary Condensation in Nanogaps: Nucleation or Film Coalescence?

Abstract: Nucleation and film coalescence represent two fundamentally different pathways for capillary condensation. Yet, both have so far been proposed as the processes driving the condensation in nanometric confinements, leading to a long-standing and overlooked ambiguity. Here, we delineate the dichotomy between these mechanisms and test their validity using an experimental method capable of absolute distance measurement during capillary condensation. We show that the molecular content of the capillary meniscus given by the first nucleation theorem is far smaller than what the confinement geometry and the Kelvin equation require. In contrast, the analysis based on film coalescence reproduces the experimental observations and describes the final meniscus formation as a barrierless process, while allowing for an intermediate, first-order-like film-thickening transition prior to the meniscus formation.