Apparent Contact Angle Calculated from a Water Repellent Model with Pinning Effect

Area of Science:

• Surface Science
• Materials Science
• Physics

Background:

• Contact angle measurements are crucial for understanding liquid-surface interactions.
• Existing models like Cassie-Baxter and Wenzel equations have limitations in predicting behavior on microstructured surfaces.
• The pinning effect at three-phase contact lines on fine structures is often overlooked.

Purpose of the Study:

• To propose novel theoretical equations for apparent contact angles.
• To accurately model contact angles on microstructured surfaces, considering interfacial tension equilibrium.
• To validate the new equations against experimental data and compare them with existing models.

Main Methods:

• Derivation of new theoretical equations based on interfacial tension equilibrium at a pinned three-phase contact line.
• Experimental validation using 2 μL water droplets on poly(methyl methacrylate) microstructured samples (square pillars and holes).
• Comparison of predicted contact angles from new equations, Cassie-Baxter, and Wenzel equations with experimental results.

Main Results:

• The proposed theoretical equations show reasonable agreement with experimental contact angle measurements.
• The new equations successfully account for the pinning effect at the edges of microstructures.
• The Cassie-Baxter and Wenzel equations failed to accurately predict contact angles on pillar-patterned surfaces due to their neglect of pinning.

Conclusions:

• The developed theoretical equations provide a more accurate prediction of apparent contact angles on microstructured surfaces.
• Accounting for the pinning effect is essential for precise contact angle modeling in such systems.
• The new equations offer an improved theoretical framework for surface wettability studies on engineered materials.