Capillary condensation is a well-known example of finite-size effects referring to a shift in the condition for condensation of fluids confined by solid walls. The phenomenon is ubiquitous in nature and plays an important role in many technological applications. Although well-described for simple models (e.g., planar or cylindrical geometry), the phenomenon is more unexplored for more complex systems, where its behavior reveals novel types of phase behavior.
It was the purpose of the study led by Prof. Alexandr Malijevský, recently published in Physical Review E, to investigate theoretically the process of condensation of fluids confined between a pair of walls of finite length H that are non-parallel, making an angle α with the vertical plane (say). We showed that the rotation of the walls by the angle α, breaking the reflection up-down symmetry, unexpectedly enriches the phase behavior of the confined fluid. In particular, the system experiences two types of condensation, termed single and double pinning, which can be characterized by one (single-pinning) or two (double-pinning) edge contact angles describing the shape of the condensed state. For both types of capillary condensation, the Kelvin-like equation was formulated, as well as the conditions under which the given type of condensation occurs. Furthermore, the global phase diagram reveals a reentrant phenomenon pertinent to the change of the capillary condensation type upon varying the inclination of the walls (see Fig. 1). The system also reveals close links with other related phase phenomena in different systems, the connection with which was shown a described analytically.
Fig. 1: A sequence of equilibrium condensed states for varying wall inclination. For small angles α, the system is in a double-pinned state but then adopts a single-pinned configuration, in which case the upper meniscus depins from the edges and slides down. Eventually, the upper meniscus reaches its minimum and moves up again until it reaches the wall’s edges and returns to a double-pinned state
- Malijevský A., Janek J.: Capillary condensation between nonparallel walls. Physical Review E 2024, 109, 054801. doi: 10.1103/PhysRevE.109.054801