Explain capillarity phenomenon

Capillarity is the rise or fall of a liquid in a narrow tube relative to the surrounding free surface, caused by surface tension and the contact angle between the liquid and the tube wall.

• If cohesion is weaker than adhesion (water-glass, $\theta<90^\circ$), the liquid wets the wall and rises.
• If cohesion is stronger (mercury-glass, $\theta>90^\circ$), the liquid is depressed.

Balancing the vertical pull of surface tension against the weight of the column in a tube of diameter $d$:

$$h=\frac{4\sigma\cos\theta}{\rho g d}$$

The rise is larger for smaller tubes. Examples: rise of water in soil pores and plant stems, wick action, and errors in piezometer and manometer readings when the tube is very narrow.