Capillary condensation under atomic-scale confinement

TL;DR

This study investigates capillary condensation in atomic-scale water channels, revealing that classical macroscopic theories surprisingly still apply at the atomic scale due to elastic effects, advancing understanding of nanoscale fluid behavior.

Contribution

The paper demonstrates that the Kelvin equation remains valid at atomic-scale confinement, supported by experimental evidence using 2D crystal assemblies, and explains the role of elastic deformation in this phenomenon.

Findings

Kelvin equation accurately predicts condensation at sub-4 Å capillaries.

Elastic deformation of capillary walls suppresses expected oscillatory behavior.

Atomic-scale capillaries can host a monolayer of water consistent with macroscopic theory.

Abstract

Capillary condensation of water is ubiquitous in nature and technology. It routinely occurs in granular and porous media, can strongly alter such properties as adhesion, lubrication, friction and corrosion, and is important in many processes employed by microelectronics, pharmaceutical, food and other industries. The century-old Kelvin equation is commonly used to describe condensation phenomena and shown to hold well for liquid menisci with diameters as small as several nm. For even smaller capillaries that are involved in condensation under ambient humidity and, hence, of particular practical interest, the Kelvin equation is expected to break down, because the required confinement becomes comparable to the size of water molecules. Here we take advantage of van der Waals assembly of two-dimensional crystals to create atomic-scale capillaries and study condensation inside. Our smallest capillaries are less than 4 angstroms in height and can accommodate just a monolayer of water. Surprisingly, even at this scale, the macroscopic Kelvin equation using the characteristics of bulk water is found to describe accurately the condensation transition in strongly hydrophilic (mica) capillaries and remains qualitatively valid for weakly hydrophilic (graphene) ones. We show that this agreement is somewhat fortuitous and can be attributed to elastic deformation of capillary walls, which suppresses giant oscillatory behavior expected due to commensurability between atomic-scale confinement and water molecules. Our work provides a much-needed basis for understanding of capillary effects at the smallest possible scale important in many realistic situations.