Drying in a capped capillary

Abstract

We use a model Density Functional, based on the White-Bear version of Fundamental Measure theory, to test recent predictions, due to Evans and co-workers, that capillary condensation in a capped capillary-slit is a continuous interfacial critical phenomenon related to the complete wetting transition. Using a model with a square-well intermolecular fluid–fluid attraction we first determine accurately the location of the first-order capillary evaporation transition in an infinite (open) hard-wall capillary slit. Extending the density functional model to allow for a two-dimensional order-parameter profile, we then study the adsorption as the chemical potential is reduced to capillary evaporation but now in a capillary-slit that is capped at one end. The equilibrium density profiles obtained show that, sufficiently close to the phase boundary, a meniscus separating liquid-like and vapour-like phases forms near the capped end, and that as capillary evaporation is approached, continuously unbinds from the capped end. Our numerical results indicate that the divergence of the adsorption due to the unbinding of the meniscus is logarithmic and is the same as for the complete wetting transition in systems with short-ranged forces.

Keywords:

• wetting and drying
• capillary condensation
• density functional theory

Acknowledgments

We are just two of the masses who were taught how to calculate by Bob (although any mistakes in this article are entirely our own). It is our heartfelt pleasure to thank him for his deep physical insight and the inexhaustible encouragement he has always given us.