Capillary condensation under atomic-scale confinement (2009.11238v1)

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.

Capillary Condensation Under Atomic-Scale Confinement: A Detailed Examination

The paper "Capillary Condensation Under Atomic-Scale Confinement" presents a paper on the phenomenon of capillary condensation within atomic-scale confinements. Unlike the classical understanding that the century-old Kelvin equation ceases to apply when the confinement approaches the molecular size, the authors explore this extreme domain using van der Waals (vdW) assembly of two-dimensional (2D) materials, specifically mica and graphite, to form atomic-scale capillaries.

Key Findings and Methodology

In the paper, capillaries fabricated with heights less than 4 Å were used, capable of holding just a monolayer of water. Contrary to expectations, the macroscopic Kelvin equation, with parameters characteristic of bulk water, accurately described the capillary condensation transition in these environments. This unexpected concordance in strongly hydrophilic mica capillaries and qualitative agreement in weakly hydrophilic graphite capillaries is attributed to the elasticity of capillary walls which mitigates expected commensurability-induced oscillations.

For experimental investigation, atomic-scale capillaries were created using exfoliated mica and graphite to construct 2D channels, with graphene spacers setting the separation. Atomic Force Microscopy (AFM) was employed to measure condensation by observing changes in sagging of the uppermost crystal under varying humidity. This sagging diminished as condensation occurred, suggesting mechanical stabilization through water molecules intercalation, reducing vdW adhesion effects.

Implications and Theoretical Contributions

The accurate description of condensation by the Kelvin equation at this scale challenges the assumption that macroscopic thermodynamic descriptions breakdown under angstrom-scale confinement. The demonstration that elastic deformation compensates for molecular commensurability alters the fundamental understanding of phase transitions in confined geometries. The paper indicates that the absence of hysteretic behavior and the rapid transition states observed highlight far-reaching implications for fabricating nanostructures in various industrial applications, including microelectronics and pharmaceuticals.

Additionally, molecular dynamics simulations provided insights into solid-liquid interaction energy changes under extreme confinement, explaining the oscillatory potential energy profiles and reinforcing the theoretical basis for the experimental observations. These simulations, coupled with experimental results, showcase the capacity for elastic accommodation within confined systems, suggesting potential utility in designing nanofluidic devices where control of vapor-liquid equilibria is essential.

Future Directions and Speculations

The findings encourage exploration into other material systems at similar scales, potentially broadening applicability beyond hydrophilic/hydrophobic extremes studied. Future research could focus on incorporating self-consistent elastic response models into molecular dynamics simulations to accurately predict behavior in varying material contexts. Moreover, the methodology's implications on understanding rapid water transport dynamics could offer novel insights into biological systems and enhance the development of biomimetic filtration and osmotic systems.

In summary, this paper provides a comprehensive paper reaffirming the Kelvin equation's utility even at atomic confinements, challenging preconceived notions about condensation at molecular scales. By addressing the role of elasticity and its impact on phase transitions, the research opens new avenues in the paper of confined fluids, with implications that transcend both fundamental physics and practical engineering solutions.