What causes the penetrant applied to a test specimen's surface to be drawn into discontinuities?

The correct answer is based on the principle of capillary action, which is the ability of a liquid to flow in narrow spaces without the assistance of external forces. When the penetrant is applied to the surface of the test specimen, it enters into any discontinuities or defects such as cracks or pores due to this phenomenon.

Capillary action occurs because of the adhesive forces between the liquid and the solid surface, which enables the liquid to "climb" into the small openings and effectively fill them. This property is particularly important in non-destructive testing, where the detection of flaws is achieved by observing how the penetrant is absorbed into these discontinuities, allowing for identification of defects without compromising the integrity of the material.

The other options do not accurately describe the mechanism by which penetrants are drawn into surface discontinuities. Air pressure would provide a different method of application and not influence the liquid's absorption into tiny cracks. Heat application could modify the viscosity of a liquid but is not the primary reason for penetration. Surface tension, while it plays a role in the behavior of liquids, is not the main driving force that facilitates the penetrant's movement into discontinuities, as capillary action encompasses how both surface tension and adhesion affect the behavior of liquids in confined