For simple fluids adsorbed at a planar solid substrate, critical-point wetting, that is, the vanishing of the contact angle θ at a temperature Tw lying below that of the critical point Tc, need not occur. While critical-point wetting necessarily happens when the wall-fluid and fluid-fluid forces have the same range (e.g., both are long-ranged (LR), or both short-ranged (SR) nonwetting gaps appear in the surface phase diagram when there is an imbalance between the ranges of these forces.
In the study, which resulted from a long-term collaboration between Prof. Alexandr Malijevský and Prof. A. Parry from Imperial College London, it was shown that in contrast to wetting on a planar wall, fluids adsorbed in wedges (and cones) always exhibit critical-point filling (wedge wetting or wedge drying) regardless of the range and imbalance of the forces. We illustrate the necessity of critical-point filling, even without critical-point wetting, using a microscopic model density functional theory of fluid adsorption in a right-angle wedge. The location and order of the filling phase boundaries are determined and shown to be in excellent agreement with exact thermodynamic requirements and predictions for critical singularities based on interfacial models.
Fig. 1. Schematic surface diagrams for filling transitions for fluid adsorption in a wedge for the four possible scenarios SR-SR, LR-LR, LR-SR, and SR-LR for the ranges of the wall-fluid and fluid forces. Phase boundaries of continuous (blue) and first-order (red) wedge filling are shown
- Malijevský A., Parry A.O.: Critical-point wedge filling and critical-point wetting. Physical Review E 2024, 109(2), 024802. doi: 10.1103/PhysRevE.109.024802