What is the term used to define the tendency of certain liquids to penetrate into small openings such as cracks or fissures?

The term that defines the tendency of certain liquids to penetrate into small openings such as cracks or fissures is capillary action. This phenomenon occurs due to the adhesive forces between the liquid and the walls of the small openings, which may be stronger than the cohesive forces among the liquid molecules themselves. As a result, the liquid can move upward or laterally against the force of gravity, filling small cracks and fine spaces.

In contexts such as soil science, capillary action plays an important role in the movement of water and nutrients through soil pores, affecting plant growth and groundwater replenishment. It is a critical concept in understanding how liquids behave in porous materials and the interactions between fluids and solids.

Surface tension pertains to the elastic tendency of liquids that makes them acquire the least surface area possible, while vapor pressure refers to the pressure exerted by a vapor in equilibrium with its liquid or solid phase. Hydraulic pressure relates to the pressure exerted by a fluid in motion, typically in a confined space, and is not directly related to the ability of a liquid to penetrate small openings. Hence, capillary action is the most accurate term associated with the liquid's penetration into small openings.