Brownian Motion

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What is the Brownian Motion?

"Brownian motion in chemistry is a random movement. It can also be displayed by the smaller particles that are suspended in fluids. And, commonly, it can be referred to as Brownian movement"- the Brownian motion results from the particle's collisions with the other fast-moving particles present in the fluid.

A particle will change its path when the other particle collides with it. Also, further collisions cause the particle to follow the random method, which is a zigzag motion. It involves an exchange or transfer of momentum or energy between the particles.

An illustration that describes the random movement of the fluid particles can be given as follows.

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Note: The Brownian motion was named after the Scottish Botanist Robert Brown, who first observed that when placed in water, pollen grains move in random directions.

Brownian Movement

The Brownian Movement in chemistry can be a random zig zag motion of a particle, which is observed usually under a high-power ultra-microscope. This specific movement resembles the exact motion of pollen grains in the water, which is explained by Robert Brown, and hence, it is named the Brownian movement.

More significantly, later, Albert Einstein, in his paper, has explained the Brownian movement more clearly, stating that the water molecules moved the pollen. This discovery has served as great evidence of the existence of molecules and atoms.

Understanding the Brownian movement is crucial because it forms a base for modern atomic theory. Also, the kinetic theory of gases is based on the Brownian motion model of particles. The mathematical models that describe the Brownian motion are used in various disciplines such as Physics, Maths, Economics, Chemistry, and more.

What is Brownian Movement in Chemistry?

The Brownian movement in chemistry, which is also called Brownian motion, can be defined as the erratic or uncontrolled movement of particles in fluid because of their constant collision with other fast-moving molecules.

In general, this random movement of a particle can be observed to be stronger in the less viscous liquid, smaller sized particles, and at a higher temperature. There also exist other factors that affect the movement of particles in a fluid.

One of such most common examples of the Brownian motion can be given as diffusion. The cases where calcium diffused in bones or pollutants are diffused in the air can be considered examples of this effect.

Brownian Movement in Colloids

We can see the Brownian motion effect in all types of colloidal sol. On the other side, this phenomenon explains the sol particles' random motion clearly and indicates that these particles are not static. Nevertheless, the major reason for this type of motion in the sol particles is the unequal bombardment of the depressed phase particle, leading to a non-uniform movement in native because of the particle's size difference.

Meanwhile, the Brownian movement can not be seen in the true sol because it is homogeneous, and there lies a uniform bombardment. However, considering colloids, the system is heterogeneous, and the bombardments are non-uniform, leading to a random measurement.

One of the major advantages of this effect is that it keeps the sol particles in continuous motion like the particles do not settle at the bottom by further preventing the lyophobic sols' coagulation. So, this type of motion increases the stability of a sol.

Brownian motion can also be observed in the cell's plasma, where the particles in the cell also exist in the random motion without making plasma in the cell dry.

Cause of Brownian Motion

The primary causes of the Brownian Motion can be listed as follows.

  • The particle's size is inversely proportional to the motion's speed, which means the small particles exhibit faster movements.

  • This is due to the momentum transfer is inversely proportional to the particles' mass. At the same time, lighter particles obtain greater speeds from collisions.

  • The Brownian motion's speed is inversely proportional to the viscosity of the fluid: the lower the fluid's viscosity, the faster the Brownian movement.

  • Viscosity can be given as a quantity that expresses the internal friction magnitude in a liquid. It is the resistance’s measure for a fluid's flow.

Effects of Brownian Motion

  • The Brownian movement causes fluid particles to be in constant motion.

  • This prevents the particles from settling down, leading to the colloidal sol's stability.

  • We can distinguish a true sol from a colloid with the help of this motion.

The paper of Albert Einstein on Brownian motion was vital evidence of the existence of molecules and atoms. The kinetic theory of gases that explains the temperature, volume, and pressure of gases is based on the particles of the Brownian motion model. 

FAQ (Frequently Asked Questions)

1. What Causes Brownian Movement in a Colloidal Solution?

Ans: Colloidal “solutions” consist of minute particles (solid or liquid) which cannot be seen by the naked eye but can be seen through a microscope, suspended in a liquid medium (solvent). They are not true solutions. For example, milk. Milk consists of minute fat particles suspended in water. The fat particles scatter light and can be seen. Now the question is why milk appears to be “milky”? (opaque and white) and not like water? So, you can observe Brownian motion when the fat particles are kicked here and there by the water molecules.

2. What is the Difference Between Brownian Motion and Motility?

Ans: It can be quite complex to differentiate between a movement due to Brownian motion and movement due to other effects. In biology, for example, an observer needs to be able to tell whether a specimen is moving because it is motile (capable of movement on its own, perhaps due to cilia or flagella) or because it is subject to Brownian motion. Typically, it's imaginable to segregate the processes because Brownian motion appears jerky, random, or like a vibration. True motility appears often as a path, or else the motion is twisting or turning in a specific direction. In microbiology, motility can be established if a culture inoculated in a semisolid medium wanders away from a stab line.

3. How does the Brownian Motion affect Temperature?

Ans: Let us discuss the cause of the Brownian movement. Indeed, the temperature is what we feel at the macroscopic (otherwise human) scale, which is caused by the Brownian motion at the microscopic or molecular scale.

The more energetic the molecule's Brownian motion, the higher the temperature we sense. Absolute temperature is proportional to the Brownian motion' kinetic energy per unit mass in a more specific way.

So, if we raise the temperature, the Brownian motion becomes more energetic.

4. How come the Uniform Electric Field gives a Zig-Zag Motion to the Electron Present in the Wire?

Ans: In any of the non-superconducting conductors having a potential difference over its endpoints, the band electrons of mobile conduction move towards more positive potential (via conventional current flow in the opposite direction). Not only are all these electrons affected by the potential difference across the wire, but also they are affected by all the charged particles present in the conductor.

Whereas the other charged particles are also in motion (some more so than others). So, the resultant effect is for the conduction band's electrons to have some random amount (or "zig-zag") motion in addition to their movement from one conductor end to another. The average distance traveled by a single electron is greater than the length of a physical conductor.