Are Contact Angle Measurements Useful for Oxide-Coated Liquid Metals?

Area of Science:

• Materials Science
• Surface Science
• Physical Chemistry

Background:

• Gallium-based liquid metals are widely used in soft electronics and microfluidics.
• Contact angle measurements are crucial for understanding liquid-surface interactions.
• Static contact angles are commonly reported for liquid metals, but their validity is questioned.

Purpose of the Study:

• To investigate the utility of static contact angles for gallium-based liquid metals.
• To elucidate the role of the surface oxide skin in wetting behavior.
• To provide best practices for wetting experiments with liquid metals.

Main Methods:

• Contact angle measurements (advancing and receding) on various substrates.
• Analysis of the mechanical properties of the surface oxide skin.
• Investigation of the influence of substrate surface chemistry and roughness.

Main Results:

• Static contact angles are uninformative for gallium-based liquid metals due to a mechanically impeding oxide skin.
• Advancing angles are consistently high (>140°) due to oxide rupture.
• Receding angles exhibit significant hysteresis, varying from 0° to >140° due to oxide sac formation and collapse.

Conclusions:

• Static contact angle measurements are not reliable for gallium-based liquid metals.
• The surface oxide skin significantly influences wetting behavior and introduces substantial hysteresis.
• Recommendations for improved wetting experiments are provided for applications in soft electronics, composites, and microfluidics.