How can a dyed or fluorescent penetrating liquid reveal surface cracks in metal?

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Liquid penetrant inspection is a common nondestructive testing method used to identify surface-level defects in metals. The process works through the following steps:

Application: A brightly dyed or fluorescent penetrating liquid is applied to the surface of the metal.
Penetration: The liquid is allowed to soak into any tiny surface cracks or flaws present in the material.
Removal: Excess liquid is wiped off the surface, leaving the penetrant trapped only within the cracks.
Visualization: Because the liquid remains in the flaws, it highlights their location, making them readily visible for inspection.

This technique is highly effective for surface analysis; however, it is important to note that it cannot detect internal flaws within the material.1