Capillarity is a phenomenon through which liquids have the ability to rise or fall through a capillary tube.
For its part, a capillary tube is an object, with different diameter measurements, through which liquids or fluids are conducted and, it is in these where the phenomenon of capillaryness occurs.
This phenomenon depends on the surface tension of the liquid, which causes it to face its resistance in order to increase its surface. Likewise, the surface tension also depends on the intermolecular force of the liquid and that will precisely allow it to rise or fall from the capillary tube.
In this sense, when a liquid rises through the capillary tube it is because the intermolecular adhesive force, between the liquid and the solid object, is greater than the intermolecular or cohesion force of the liquid molecules.
In this case, the liquid will rise until the surface tension equilibrium is reached and a concave curve will form on its surface, which will ultimately determine that it is a wetting liquid.
On the contrary, if the intermolecular force of the liquid is greater than the adhesion to the capillary tube, then the liquid descends such as, for example, the mercury that is characterized by forming a convex surface.
Now, these curves that form on the surfaces of the liquids in the capillary tubes, whether concave or convex, are called meniscus.
Below are several examples to explain how this phenomenon of capillary fluid occurs.
Glass capillary tube
When a glass capillary tube is placed in a container with water, the water level will rise inside the tube. However, if another tube is introduced but, with a larger diameter, the water that will enter it will be at a lower level with respect to the narrower tube and a concave curve or meniscus will be formed.
Mercury capillary tube
If a capillary tube is introduced into a container with mercury, this liquid will rise through the capillary tube but to a lesser extent than water, and an inverted meniscus or convex curve will form on its surface.
Capillarity in plants
Plants absorb water from the soil through their roots and then transfer to their leaves. This is possible because plants have capillary tubes through which liquids and nutrients are distributed throughout their parts.