What phenomenon causes a liquid to rise spontaneously in a capillary tube?

The phenomenon that causes a liquid to rise spontaneously in a capillary tube is best explained by capillary attraction. This effect occurs when the adhesive forces between the liquid and the walls of the tube are stronger than the cohesive forces within the liquid itself. As a result, the liquid climbs up the sides of the tube, forming a meniscus, in a process influenced by the combination of the tube's internal diameter and the properties of the liquid.

In applications such as liquid penetrant testing, understanding capillary action is crucial because it allows penetrant materials to effectively seep into surface defects, ensuring accurate inspection and detection of flaws. This principle is fundamental when selecting the right penetrant for testing, as the ability of a liquid to infiltrate tiny cracks is a key factor in ensuring thorough and reliable results.

While gravity, viscosity, and surface tension are important concepts in fluid mechanics, they do not directly account for the spontaneous rise of liquid in a capillary tube as comprehensively as capillary attraction does.