The penetrant moves into tiny surface flaws during the dwelling period, and the penetrant stays in those cracks when you wipe off any excess penetrant. The penetrant seeps into the small cracks through a process called capillary action. What is capillary action?
Capillary action is the movement of water within the spaces of porous materials due to the forces of adhesion, surface tension, and cohesion. Adhesion is how the water molecules like to stick together. Surface tension is what makes water stay in a puddle on a surface. Cohesion is how the water molecules like to stick to other objects.
Capillary action occurs when the water's adhesion to the walls of a crack are greater than the cohesion between the molecules. Surface tension limits the depth to which penetrant will seep into a crack.
You can do some miniature experiments at home to see firsthand how capillary action works. If you stick the bottom of a paper towel in water, you will notice the water climb up the paper towel using cohesion and adhesion. If you spill a small amount of water on the table, you will see how surface tension keeps the water in one place. Another experiment you can do is to put a few drops of food coloring in a cup of water. Then, place a stalk of celery in the cup of water, and you will see how capillary action works with plants. The ends of the celery should turn the same color as the water after a few days.
Capillary action is how water moves through adhesion, surface tension, and adhesion.