A liquid penetrant inspection uses capillary action to draw a penetrant liquid into surface-breaking flaws. Capillary action is caused by the balance of cohesive forces within the liquid and adhesive forces between the liquid and a solid surface. After removing excess penetrant, a developer is applied to draw the penetrant out of flaws by capillary action so they are visible.

1. Steps of a Liquid Penetrant Inspection

3. Capillary Action

4. Cohesive forces
Adhesive forces

5. Cohesion - Adhesion

6. Cohesive – Adhesive Forces

7. Surface Tension

8. Contact Angle

10. Capillary Action