Mechanical Properties (Brownian Movement)
Colloidal particles present in a colloidal solution exhibit a very important property called Brownian movement. When a colloidal solution is viewed under an ultra microscope, the colloidal particles are seen continuously moving in a zigzag path. The property was discovered by a botanist Robert Brown in 1827, when he observed that pollen grains suspended in water exhibit random zigzag motion. After the name of the discoverer, the property was named as Brownian movement. It may be defined as follows.
The continuous zigzag movement of the colloidal particles in the dispersion medium in a colloidal solution is called Brownian movement.
(a) Brownian movement (b) Unequal bombardments of the molecules of dispersion medium on a colloidal particle.
Brownian movement is shown diagrammatically in the above figure.
Cause of Brownian movement: Brownian movement is due to the unequal bombardments of the moving molecules of dispersion medium on colloidal particles. The moving molecules of the dispersion medium continuously attack on colloidal particles from all sides and impart momentum to them. Since the chances of their collisions are unequal, the net driving force on a colloidal particle forces it to move a particular direction. As the particle moves in that direction, other molecules of the medium again collide with it and the particle changes its direction. The process continues. This results in a random zigzag movement of the colloidal particle.
The Brownian movement decreases with an increase in the size of colloidal particle. This is why suspensions do not exhibit this type of movement. Brownian movement plays an important role in imparting stability to a sol. This is because Brownian movement opposes the gravitational forces acting on colloidal particles and prevents them from getting settled down.