Brownian Motion – An Experimental Investigation.

Shamli Manasvi, April 2020

The Particle Theory of Things, is –

Everything can be broken down into smaller particles (analysis view). In other words, everything is made up of small particles (synthesis view). These particles have two fundamental properties –

  1. They are in constant random motion,
  2. And they all attract each other.

But, really? I’m not buying this without any evidence.

Here is some supporting evidence for the claim made in statement 1 of the Particle Theory of Things (see video below).

The Video –What you see in the video, is footage of milk under a microscope at about ×1000 magnification. The milk has been mixed with a little bit of green dye, to be able to see the particles better. The large particles are clusters of fat molecules, that are embedded in water (space between fat globules) to form a suspension that we see as milk. The motion we are discussing here, is the jiggling of each of the milk particles (Brownian Motion). Try to imagine the water as being made up of millions tiny water particles (they are too small to be seen even under a microscope), that are moving all around, and bombarding the milk particles, which causes the Brownian Motion.

Sizes table

The Story –Today, this seems simple. But like everything else, it has a story behind it. The understanding of this motion has taken hundreds of years to become what it is today. Here is some of its history –

Jan Ingenhousz in 1785, and Robert Brown in 1827, described this peculiar, random, jiggling motion of large particles, as seen under a microscope. Ingenhousz was observing coal dust particles, while Brown was studying pollen grains. They were puzzled by this unusual jiggling movements; Brown even suggested that these particles might be alive.

brownian motion simulation gif (2)

Almost 80 years later in 1905, this motion was modeled mathematically, by Albert Einstein. What Einstein calculated, and thus showed, was that these larger particles (pollen, dust etc.) are not themselves the source of the motion, they are embedded in a sea of much smaller water particles. These water particles, he imagined, are moving continuously and randomly. The pollen or dust grains are being bombarded by the water particles from all sides. The imbalance in the bombardment results in the jiggling of the grains.

This explanation was convincing evidence that even though we cannot see them, atoms and molecules exist.

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