Mercury on glass has a contact angle of about 132 degrees. What does this indicate about wetting?

• A. Poor wetting (low adhesion to surface)
• B. Excellent wetting
• C. Neutral wetting
• D. Super wetting

Answer: (A)

Wettability is judged by the contact angle between a liquid and a surface—the bigger the angle, the less the liquid spreads. A contact angle of about 132 degrees means mercury does not wet the glass at all; it beads up instead of spreading. This happens because the adhesive forces between mercury and glass are weak compared to mercury’s own cohesive forces, so the liquid remains on the surface with high surface tension and low spreading. In practical terms, this is poor wetting. If the angle were small, the liquid would spread more (excellent wetting). Very near 90 degrees would be neutral wetting, and extremely small angles could indicate super wetting.