What two properties primarily determine a liquid's penetrating ability?

The penetrating ability of a liquid in liquid penetrant testing is significantly influenced by its surface tension and the cosine of the contact angle. Surface tension is the force that causes the liquid's surface to contract, which affects how well the liquid can spread and penetrate into surface discontinuities. A lower surface tension generally enhances the ability of the penetrant to enter tiny cracks and pores.

The contact angle, defined as the angle formed between the liquid's surface and the solid surface at the point of contact, plays a crucial role as well. When evaluating the cosine of the contact angle, a smaller contact angle (indicative of better wetting) correlates with improved penetration, allowing the liquid to cover and enter surface defects more effectively. Thus, the combination of surface tension and the cosine of the contact angle provides a comprehensive understanding of how a liquid interacts with the surface it is applied to, ultimately determining its penetrating ability in liquid penetrant testing.