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

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.

References in corpus (3)

• Edge contact angle and modified Kelvin equation for condensation in open pores
• Bridging of liquid drops at chemically structured walls
• Point tension in adsorption at a chemically inhomogeneous substrate in two dimensions

Cited by in corpus (4)

• Phase Field Simulation of Liquid Filling on Grooved Surfaces for Complete, Partial and Pseudo-partial Wetting Cases
• Phase behaviour of fluids in undulated nanopores
• Capillary Condensation and Depinning Transitions in Open Slits
• Beyond capillary condensation: Shear-induced bridging transitions in patterned slits