Critical behaviour of the contact angle within nonwetting gaps

Area of Science:

• Physical Chemistry
• Surface Science
• Thermodynamics

Background:

• Studies of fluid adsorption near walls with differing interaction ranges previously suggested critical point wetting might not occur.
• Nonwetting gaps were observed in surface phase diagrams, separating wetting and drying transitions up to the critical temperature.

Purpose of the Study:

• To clarify common features of surface phase diagrams for fluid adsorption, irrespective of interaction range and balance.
• To analyze the behavior of wetting and drying transition lines and constant contact angle lines near the critical temperature.

Main Methods:

• Analysis of density functional theory and simulation studies.
• Theoretical clarification of surface phase diagram features.
• Estimation of contact angle behavior based on interaction range dominance.

Main Results:

• Lines of temperature-driven wetting/drying transitions and constant contact angle (0 < θ < π) converge to an ordinary surface phase transition at the critical temperature (Tc).
• When nonwetting gaps exist, the contact angle vanishes or approaches π as T approaches Tc.
• Specific scaling behaviors for contact angle are derived: π - θ ∝ t^0.16 for long-ranged wall-fluid interactions and θ ∝ t^0.77 for short-ranged wall-fluid interactions (t = (Tc-T)/Tc).

Conclusions:

• The universal convergence of constant contact angle lines implies critical point filling always occurs for fluids adsorbed in wedges.
• Understanding these surface phase diagrams is crucial for predicting fluid behavior near interfaces in various physical systems.