Brownian Movement

Brownian movement is the random erratic zigzag movement of particles in a suspended medium. There is a constant collision among particles in the medium. The pollen grains floating in steady water or the movement of dust particles in a room mainly by the air currents are examples of Brownian movement. Brownian motion is a microscopic phenomenon observed in almost all colloidal systems. The Brownian movement is of great importance in various fields of science as it supports the modern atomic theory. It is independent of the external factors and mainly depends on the internal factors like particle size and number, and viscosity of the fluid medium.

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What is the Brownian movement?

Brownian movement is the uncontrolled and rapid movement of microscopic particles in a suspended fluid due to the constant collision of the particle. This phenomenon can be observed in all colloidal suspensions like solid in liquid, solid in gas, liquid in liquid, liquid in gas, and gas in a liquid. Colloidal particles immersed in a liquid-like environment exhibit Brownian motion. These molecules are in a continual, unpredictable motion with a velocity proportional to the square root of the temperature.

The Brownian movement is also called pedesis, the Greek word for “leaping”.

Brownian motion is observed for particles with a diameter of less than 0.001 mm, which are small enough to participate in the thermal activity but large enough to be seen with a microscope or ultramicroscope. 

The Brownian motion is independent of all the external factors. This effect is attributed to the thermal mobility of the fluid molecules.

History of the Brownian Movement

The Brownian movement is named after the Scottish botanist Robert Brown who, in 1827, observed the movement of the plant spores floating in the water, but he was not able to explain it. Then, in 1905, Albert Einstein published a paper regarding the movement of pollen grains by water molecules. Further, Jean Perrin verified the results, and he was awarded the Nobel Prize in Physics in 1926.

Brownian movement of sol particles

Colloidal particles in a sol are constantly assaulted from all sides by the molecules of the dispersion medium. However, the effects are not equivalent in all directions. As a result, the sol particles travel in zigzag or random patterns. Brownian movement refers to colloidal particles’ random or zigzag motion in a sol. Because of the mobility, the larger colloid particles are constantly bombarded by the liquid’s smaller particles.

Brownian motion is the motion and bombardment that keeps the colloid particles suspended.

All colloidal systems exhibit this type of movement. Under ultra microscopes, such random motion can be seen, and for larger particles, it can even be seen under regular microscopes.

As the viscosity of the liquid increases or the particle’s size increases, the Brownian motion becomes less prominent. The rate of motion increases by higher temperature, increased particle number, tiny particle size, and low viscosity.

Causes of Brownian movement

Particle Size – As the particle size decreases, the collision and movement increase. The Brownian movement is inversely proportional to particle size.

Viscosity – Viscosity is the resistance offered to the flow of the liquid. So when the viscosity of the liquid increases, the Brownian motion decreases. They are also inversely related.

Momentum and Velocity of Particles – The momentum transfer is less as the particle size increases. The momentum imparted is inversely related to the mass of the particle. The lighter particles possess more velocity on collision than the heavier particles.

Particle Number – As the number of the particles increases, the collision increases, and hence the Brownian motion is directly proportional to the particle number.

Importance of Brownian motion

The Brownian motion supported the modern atomic theory. Brownian motion’s mathematical models are now employed in maths, economics, engineering, physics, biology, chemistry, and various other fields. 

We can also distinguish a colloid and true sol with the help of the Brownian movement. Due to the constant movement of particles, the colloid particles do not settle down, and the stability is maintained.

Examples of Brownian movement

The typical examples of the Brownian movement are

• The movement of plant spores or pollen grains in steady still water

• Diffusion of calcium in bones

• The motion of dust particles in a room by the air currents

• The movement of pollutants (like smoke and soot particles) in the air.

Conclusion

The Brownian movement, also called pedesis, is the random and discontinuous movement of particles in a suspended medium. The motion is due to the bombardment among the particles and is observed in all types of colloidal suspension. It is a microscopic phenomenon and is independent of external factors. The main factors depend on viscosity and particle size. The Brownian motion becomes less visible as the particle size or viscosity increases. The Brownian movement is essential in many aspects as it supports the modern atomic theory and is used in many fields of science and research. One of the main factors that helps stabilise a colloidal solution by not letting the dispersed phase particles settle down. Some examples of Brownian motion include the floating of plant spores on steady water, movement of soot and smoke particles, which are pollutants in the air, and the activity of dust particles in a room by the air currents.