What is the Brownian motion | salient features of Brownian movement

What is the Brownian movement ??

British botanist Robert Brown observed in 1827 a peculiar phenomenon while working with pollen grains. He observed that when the pollen grains are suspended in water and the under a high power microscope, they show a continuous, random, erratic,motion, consisting of trauslations and rotations, and performing, as it were, a wild fantastic dance with no sign of abatement or stoppage. Subsequently, it transpired that the phenomenon is not restricted to pollen grains alone; neither it is connected with the biological origin of the particles, nor specifically water has anything to do with it. For instance, such chaotic movements are performed by particles of smoke in still air.
In fact, if any colloidal solution is observed under high power microscope (better, ultramicroscope), the suspended particles exhibit such an eternal, irregular, to-and-fro motion. In the field of view of the nicroscrope, each particle is seen to spin, rise, sink and rise again. This phenomenon of chaotic motion of colloidal particles suspended in a liquid or a gas was termed Brownian motion after Brown who was neither the first to observe it, nor could interpret it.
Brownian movement experiment



●Characteristics of Brownian movement 

(1)  The motion is continuous, eternal, irregular and random. No two particles ose vicinity the same direction at the same time. This shows that the cause does not lie in eddies, convection or streaming motion of the fluid.

(2)  The motion is independent of the mechanical vibration, say the shaking of the container.

(3)  The motion is independent of the mechanical vibration, say the shaking of the container.

(4)  The higher the temperature, greater is the motion and conversely. Two particles of the sane size move equally fast at the same temperature.

●Explanation :-

                             It is thus established that the brownian motion can not but due to the contanuous impact received by the suspended particles from moving molecules of the liquid or gas due to their ever-existing heat motion. Naturally, every particle is subjected to such impacts from all sides, every instant and, due to the complete disorder of moleculer movements, it is expected that the number of irmpacts received by a particle from one direction should, on the average, equal to the number of impacts  from the opposite direction.  The particles should therefore remain stationary. This is exactly what will occur if the particles are too large. But if the particle-size be small enough (~ 10^-4 to 10^-5 cm) - colloidal particles-matters are different. Since the molecular motion is chaotic, the number of impacts averaged over a sufficiently long time from different directions. Deviations from average values however are inevitable in a statistical system such as a fuid. Such deviations from mean values which occur in a small volume or during a small time-interval are called fluctuations. For a body of ordinary size in a fluid, the number of impacts received is too large to note either separate impacts or a random prominence of impacts in one direction over another. For too small particles, the total number of impacts is comparatively small and the prevalence first in one direction and then in another becomes noticeable. It is exactly such fltuctuations that give the particles a perceptible motion, opposed only by the viscous drag of the fluid. prominence in one direction would be randomised. But the molecular motion is chaotic; so the prominence in one direction would be randomised. This therefore is the motion of a Brownian particle.

●Impotent topics regarding Brownian movement :- 

(1)  The importance of the Brownian motion is that it provided the direct proof of the reality of motion of molecules- the corner stone of the Kinetic Theory.

(2)  Brownian movement is not the movement of the molecules. What we observe is not the result of the impact of one molecule, but rather the result of a number of impacts in one direction prevailing over those in the opposite direction.

(3)  Brownian movement is explained by a resultant force in a different direction owing to a chance difference between the number of impacts against a particle from different direction. Since fluctuations usually last for only a short while, the direction of the resultant will change very rapidly, and along with it the direction of motion of the particle. This give the chaotic nature of Brownian movement which in its turns reflects the chaotic of molecular motion.


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