Dye penetrant is based on the capillary action to detect flaws.

Capillary action is the rise of a liquid up a narrow vertical channel (can also occur horizontally), as the molecular forces between the liquid and the tube (called interfacial tension) are stronger than the corresponding forces within the liquid (called surface tension).

Dye penetrant test:

3.Liquid penetrant is applied to the surface being examined and is drawn into the cavities by capillary action.A dye (commonly red) is added to the liquid to make it clearly visible. The surface tension of the liquid draws it from the wet surface into dry cavities.

4.The surface is then wiped clean of the liquid.

5, 6.After excess penetrant has been removed, a white developer is applied to the sample.The developer draws penetrant from defects out onto the surface to form a visible indication, commonly known as bleed-out. Any areas that bleed out can indicate the location, orientation and possible types of defects on the surface.