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Observing Brownian motion

Found in: Theory

Brownian motion is the random movement of particles. This motion can be observed under the microscope, when looking at very small particles. In this case we will have a look at the fat droplets in milk.

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All particles that have a temperature, and this is always the case above absolute zero, will move. Particles in a solid will vibrate as well, but are locked into place. To observe Brownian motion, therefore, it is necessary to suspend the particles either in a fluid or a gas. Particles which are larger are also subjected to these forces, but because of their larger mass, their movement is not visible and they tend to stay put. If the particles are small enough, however, then they can be seen vibrating under the microscope.

If you want to observe Brownian motion, then you need to have suspended particles in water. Because of the small movement, it is necessary to use a high magnification, such as 400x. Brownian motion is also responsible for diffusion effects. In diffusion, particle move from an area of high concentration to low concentration. Brownian motion is named after botanist Robert Brown. He observed, while observing pollen grains under the microscope, that particles trapped in the water move about. This was in 1827. He could not explain the cause of this motion, however. Much later, in 1905, Albert Einstein published a paper, in which he explained the causes of this motion. Brownian motion therefore already hinted towards the now well established fact, that matter is made of atoms and molecules, which bump into each other.




Dilute one to four drops of milk in about 5ml of water. If you use undiluted milk, then the concentration of the fat droplets is to high to see them move freely. Place one drop of the dilute milk on the microscope slide and place a cover slip on top of it. Observe under the microscope in bright field and in dark field. It is also possible to change the temperature by placing the slide into the refrigerator or by warming it up. The Brownian motion is temperature dependent and should increase with increasing temperature.


Brownian Motion. Wikipdia, the free Encyclopedia.

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