Brownian Motion
Brownian Motion is the erratic and constant movement of tiny particles when they are suspended in a fluid or gas. For example, if you sprinkle tiny grains of dust into some water, and then look at the dust particles under a microscope, the dust particles will appear to dance around, quite randomly.
 This zig-zag motion happens regardless of how still the water is.
The phenomenon was discovered in 1827 by the British botanist Robert Brown. He was investigating pollen grains in water, and noticed that they wouldn't sit still under his miscroscope. At first he thought the pollen was moving because it was alive. But even hundred-year old pollen grains danced around. When he looked at non-living particles, they moved too, so he knew there had to be some other explanation.
Futher experiments by Brown and others showed that the motion became more rapid and the particles moved farther in a given time interval when the temperature of the water was raised, when the viscosity of the fluid was lowered, or when the size of the particles was reduced.
This motion makes sense if you imagine the pollen grain or dust mote being bombarded on all sides by particles too tiny to see, that are in constant motion.
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The many moving tiny particles are of course molecules of the liquid. They were too small to see under a microscope when Brownian motion was discovered, but it was obvious they were there.
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You can see the molecules of liquid hitting the bigger particle in the animation on the left. (The size of the molecules has been dramatically increased in order to make them visible).
The atoms or molecules that make up a liquid or gas are in constant thermal motion, and their velocity distribution is determined by the temperature of the system. The motion of the molecules of the fluid, due to the fact that the fluid contains heat, causes the molecules to strike the suspended particles at random. The impact makes the particles move ... the net effect is an erratic, random motion of the particle through the fluid.
Obviously, the molecules of the liquid will move faster when the liquid is heated, causing more agitated Brownian movement of the big particle. Similarly, if you make the liquid less viscous, the molecules can move more easily, also resulting in more particle motion. And of course, if the particle is too big, the random bumps by molecules won't be noticed at all. Albert Einstein in 1905 arrived at a mathematical explanation of the phenomenon, and integrated it into kinetic theory. One of the earliest estimates of the value of Avogadro's number was made by the French scientist Jean Baptiste Perrin by a study of Brownian motion.
The mean kinetic energy of a molecule in the liquid is given by:
E = ½·m·v2 = (3·k·T) ÷ 2
where m is the mass of a particle, v is the velocity, k is the Boltzman constant, and T is the temperature. From this formula you can see that the mean kinetic energy of Brownian motion is proportional to the temperature. You can also use it to find the velocity of a particle.
Collision with fluid molecules can also make a suspended particle rotate. This phenomenon, called rotational Brownian motion, has also been observed and explained.
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