Capillarity is the phenomenon in which a liquid is sucked through a narrow channel, without an external force being applied and regardless of the direction of gravity.
For example, water can be sucked through the narrow spaces between the fibers of a towel, of a wall, a glass tube, tissue paper… and this in any direction: upwards, horizontal, oblique…
This phenomenon is used in passive hydroponics: the nutrient solution is sucked up through the tiny channels of the substrate and reaches the roots of the plant.
Why does this happen?
The capillary force is the result of the cohesion or attraction between the liquid particles and the adhesion or attraction between the liquid particles and the walls of the tube.
When a very fine glass tube is plunged in water, the water rises in the tube above the water level outside the tube.
The water particles are more attracted by the glass tube than by air and therefore try to augment the contact surface with the glass. This results in the water crawling up, till the force of gravity compensates the forces of adhesion and cohesion and the liquid level remains stable.
The height of the liquid in the tube also depends on the diameter of the tube: the narrower the tube, the higher the liquid is sucked up.
Other liquids can have the opposite effect: with mercury, the attraction of the glass tube is smaller than the attraction of air and the level in the glass tube drops, compared to the level of the liquid outside the tube.