Breaking Cassie's law for condensation in a nano-patterned slit

TL;DR

This paper investigates phase transitions of fluids in nano-patterned capillary slits, revealing new two-step condensation behavior and corrections to classical theories, validated by microscopic modeling.

Contribution

It introduces a novel two-step condensation mechanism in narrow, patterned slits and derives new Kelvin equations accounting for interfacial pinning effects.

Findings

Two-step condensation with intermediate bridging phase

New Kelvin equations with different contact angles

Validation via microscopic density functional model

Abstract

We study the phase transitions of a fluid confined in a capillary slit made from two adjacent walls each of which are a periodic composite of stripes of two different materials. For wide slits the capillary condensation occurs at a pressure which is described accurately by a combination of the Kelvin equation and the Cassie law for an averaged contact angle. However, for narrow slits the condensation occurs in two steps involving an intermediate bridging phase, with the corresponding pressures described by two new Kelvin equations. These are characterised by different contact angles due to interfacial pinning, with one larger and one smaller than the Cassie angle. We determine the triple point and predict two types of dispersion force induced Derjaguin-like corrections due to mesoscopic volume reduction and the singular free-energy contribution from nano-droplets and bubbles. We test these predictions using a fully microscopic density functional model which confirms their validity even for molecularly narrow slits. Analogous mesoscopic corrections are also predicted for two dimensional systems arising from thermally induced interfacial wandering.