Brownian Motion:-
The random or zig-zag motion of small particles suspended in a fluid is called Brownian motion.
The irregular motion of smoke and dust particles in air are the examples of Brownian motion (fig) Brownian motion never stops.
Brownian motion was discovered by Brown, an English botanist in 1827. He performed a simple experiment which presented a direct evidence for the existence of molecular motion. Using a simple microscope, he observed that tiny particles or the pollen grains of flowers constantly moved about in the water in which they were immersed. He also found that the zig-zag motion of pollen grains never stopped. This motion is known as Brownian motion. He thought that this rapid movement was possible because of life in pollen grains. But he found that pollen grains even in boiling water continued to move. Finally, he found that a small particle never stands still and the random motion is not concerned with the life of pollen grains.
Reason for zig-zag motion:-
The suspended particles are extremely large in size as compared to the molecules of the fluid and are being continually bombared on all sides by them. If the particles are sufficiently large, equal number of molecules strike the particles on all sides at each instant. For smaller particles, the number of molecules striking the various sides of the particle at any instant may not be equal. Hence the particles at each instant suffer an unbalanced force causing it to move this way or that way. This is the reason of the zig-zag motion of the particles.
Factors affecting the Brownian motion:-
(i) Temperature of the medium- Brownian motion increases with the increase of the temperature of the medium in which the tiny particles are suspended. On the other hand, it decreases with the decrease of the temperature of the medium.
(ii) Viscosity of the medium- The Brownian motion decreases with the increase in the viscosity of the medium in which particles are suspended and vice-versa.
(iii) Density of the medium- The Brownian motion increases if the density of the medium decreases and decreases if the density of the medium increases.
(iv) Size of the suspended particles- The Brownian motion of particles which are bigger in size is less than the particles which are smaller in size.
