Question

For which phenomenon colloidal particles do not settle down?

Answer 1

it is because the molecules of the solvent constantly hits the colloidal particle

Answer 2

In a colloidal solution the colloidal particles are small and they are being continuously hit upon by the molecules of depression medium.

It is very unlikely that the dispersed face particles will be hit upon by an equal force from all the sides.

Therefore there is a resultant force acting upon the colloidal particles in a particular direction.

And colloidal particles execute zigzag motion called Brownian movement.

Because of this the colloidal particles do not settle down under the influence of gravity.