Which statement defines surface wetting?

• A. It describes how easily a surface repels liquids
• B. If the penetrant wets the surface it will enter into voids
• C. The surface tension of the liquid
• D. It is unrelated to penetrant testing

Answer: (B)

Wetting describes how well the penetrant spreads across and adheres to the surface of the part. When a penetrant wets the surface, adhesive forces between the liquid and the material are strong enough to overcome the liquid’s cohesive forces, allowing the liquid to spread and enter tiny surface openings by capillary action. Because of this, the penetrant can flow into voids or defects and remain there after excess is removed, making flaws visible under development.

So the statement that if the penetrant wets the surface it will enter into voids captures the practical effect of good wetting in LPI. The other ideas—relying on surface repellency, focusing only on the liquid’s surface tension, or claiming wetting isn’t related to penetrant testing—do not describe how wetting enables defect detection.