Question

What causes capillarity?

Answer 1

1. Capillarity is the rise or depression of a liquid in a tiny passageway, such as a tube with a small cross-sectional area, such as the spaces between towel fibers or the pores in a porous substance.
2. The causes of capillarity are:
   2.1 Adhesive force: Adhesive force is the force between two dissimilar molecules or substances.
   2.2 Cohesive force: When two similar substances or molecules face the force of attraction this force is known as a cohesive force.
   2.3 Surface tension: Surface tension is the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimise surface area.
3. The liquid in the capillary rises to the level where the gravitational pull of gravity precisely balances this upward force if the cohesive forces are weaker than the adhesive forces, as they are for water.
4. When cohesive forces outweigh adhesive forces, as they do when mercury and glass are in contact, the liquid drags itself down into the capillary beneath the surface of the bulk liquid to reduce contact with the glass.
5. Capillary action is also boosted by a decrease in surface tension.