Evaluation of macroscale wetting equations on a microrough surface.

Area of Science:

• Surface Science
• Materials Science
• Physical Chemistry

Background:

• Wettability of surfaces is critical in various applications, from microfluidics to coatings.
• Understanding droplet behavior on rough surfaces at the microscale is essential for designing advanced materials.

Purpose of the Study:

• To investigate the wettability of critical droplets on microscale geometric rough surfaces.
• To analyze the effect of surface roughness on nucleation free-energy barriers and interfacial free-energy variations.
• To derive vapor-liquid-solid contact line tensions and construct wetting diagrams.

Main Methods:

• Density functional theory (DFT) approach was employed.
• Local density fluctuations at liquid-solid interfaces were analyzed.
• Contact angles of nuclei were used to derive contact line tensions.
• Wetting diagrams were constructed for Cassie, Wenzel, and impregnation regions.

Main Results:

• Deviations between microscale and macroscale models were observed, increasing with roughness.
• These deviations were most pronounced in the Wenzel, impregnation, and Cassie-Wenzel transition regions.
• The cooperative effect of liquid-solid interfacial free energy and line tension explains these deviations.

Conclusions:

• Surface roughness significantly impacts droplet wettability at the microscale.
• DFT provides a fundamental understanding of ultrasmall droplet behavior on rough surfaces.
• The findings are crucial for designing surfaces with tailored wettability properties.