Removing the penetrant from the surface of the sample, without removing it from the flaw, is one of the most critical operations of the penetrant inspection process. The penetrant must be removed from the sample surface as completely as possible to limit background fluorescence. In order for this to happen, the adhesive forces of the penetrant must be weak enough that they can be broken by the removal methods used. However, in order for the penetrant to have good surface wetting characteristics, the adhesive forces (forces of attraction between the penetrant and the solid surface being inspected) must be stronger than the cohesive forces (forces holding the liquid together). Proper formulation of the penetrant materials provides the correct balancing of these forces.
Another consideration in the formulation of the penetrant liquid is that it should not easily commingle and become diluted by the cleaning solution. Dilution of the penetrant liquid will affect the concentration of the dye and reduce the dimensional threshold of fluorescence.