What distinguishes hydrophilic from hydrophobic penetrants?

Hydrophilic penetrants are designed to attract water, which allows them to penetrate into defects or discontinuities on the surface of a test object more effectively. This characteristic of hydrophilic penetrants is crucial in enhancing their ability to draw in and retain liquid, making them particularly useful in different testing scenarios, especially when the surfaces being tested might be contaminated with oily residues or other materials that could obstruct inspection.

In contrast, hydrophobic penetrants repel water and do not readily mix with aqueous solutions. This distinction is critical in applications where the presence of moisture could interfere with the testing process or when inspecting surfaces that need to remain dry. While hydrophobic penetrants are often used for specific conditions, particularly in high-temperature environments, their inability to attract water does not define them in the same way as hydrophilic penetrants, which directly relate to their attraction to water.

Thus, the correct identification of hydrophilic penetrants as those that attract water promotes a better understanding of how different types of penetrants function in liquid penetrant testing.