Brownian motion

The zig-zag, random, irregular motion exhibit by small particles of matter suspended in a fluid is called as Brownian motion or Brownian movement. This type of movement can observed in almost all type of colloidal suspensions such as liquid in liquid, gas-in-liquid, solid in liquid, liquid-in-gas and solid-in-gas.

Let’s define Brownian motion. Brownian motion definition can write as the random motion of particles in all possible directions.

Brownian motion was first observed by botanist Robert Brown in 1827 in pollen grains floating in water. The movement of particles is not depends on any external force. The velocity of particles is proportional to the square root of the temperature. Generally particles of about 0.001 mm in diameter can show Brownian movement as these particles are small enough to share in the thermal motion, yet large enough to be seen with an ultra-microscope.

In 1905 Albert Einstein explained, ‘What is Brownian Motion’ and purposed the theoretical treatment of Brownian motion. Further Jean Perrin made a quantitative experimental study and proved the dependence of Brownian motion on temperature and particle size and also provided verification for Einstein's mathematical formulation.

Brownian movement can easily observe in colloidal solution. The colloidal particles and molecules of dispersion medium are in continuous collisions due to their constant motion. Due to their motion, particles pass their kinetic energy and strike in all sides results into zigzag movement. That is the reason when colloidal solutions are viewed under an ultra microscope; particles are seen continuously moving in a zigzag path. The main cause of Brownian movement is the unequal bombardments of the moving molecules with other particles. Like in colloidal solution, the molecules of dispersion medium continuously attack on colloidal particles from all sides and impart momentum to them.

Due to collisions between molecules, colloidal particles change its direction and again move in another direction to colloid with another molecule. The collision between molecules remains continuous and results in a random zigzag movement of particle. As the size of colloidal particles increases, the Brownian movement decreases and that is the reason suspensions cannot exhibit this type of movement.

The stability of colloidal sol is mainly depends upon the Brownian movements of particles. As Brownian movement opposes the gravitational forces acting on colloidal particles, therefore particles would not settle down. These movements are also helpful to explain the force of gravity on colloidal particles.

So we can briefly describe Brownian moment of a particle as the zig zag motion of the particles in simple words. Brownian moment hence plays a significant role when we describe the characteristics of the particles.