Capillary contact angle in a completely wet groove.

Area of Science:

• Physical Chemistry
• Materials Science
• Thermodynamics

Background:

• Understanding fluid behavior in confined geometries is crucial for various applications.
• Capillary condensation is a key phenomenon in porous materials and microfluidics.
• Wetting phenomena are governed by interfacial interactions and surface properties.

Purpose of the Study:

• To investigate the phase equilibria of a fluid confined in a capillary groove with dissimilar wall materials.
• To analyze the emergence and behavior of capillary contact angles in such systems.
• To determine the impact of these angles on capillary condensation transitions.

Main Methods:

• Utilizing density functional theory (DFT) to model fluid behavior at the nanoscale.
• Employing interfacial models to describe interactions between the fluid and different wall materials.
• Analyzing the capillary contact angle as a function of groove width and material properties.

Main Results:

• A capillary contact angle (θ(cap)(L)) arises due to differences in Hamaker constants between the side and bottom walls.
• Even with complete wetting of individual surfaces, θ(cap) > 0 when wall attractions differ, leading to first-order capillary condensation.
• The capillary contact angle vanishes at macroscopic and microscopic groove widths, indicating distinct capillary wetting transitions.
• The microscopic transition is marked by large fluctuations and critical singularities due to marginal interfacial interactions.

Conclusions:

• Confined fluid phase equilibria are significantly influenced by the heterogeneity of wall materials.
• The study identifies novel capillary wetting transitions driven by interfacial interactions in confined systems.
• These findings have implications for designing materials with controlled capillary condensation properties.