What is the contact angle?

• A. The angle of refraction of light in the liquid.
• B. The angle between two liquid drops.
• C. The measured angle that a drop of liquid makes with a solid surface.
• D. The angle of surface roughness.

Answer: (C)

The contact angle is the angle formed at the point where a liquid drop sits on a solid surface, measured inside the liquid between the tangent to the liquid surface and the solid. It tells you how well the liquid wets the surface: a small angle means the liquid spreads and wets the surface well, while a large angle means it beads up and wets poorly. This concept matters in liquid penetrant inspection because good wetting helps the penetrant enter small flaws. The other options describe light refraction, the angle between two drops, or surface roughness, none of which define this contact angle.