Monday, March 7, 2011

Proving Atoms Exist with Brownian Motion

In 60 BC, the Roman poet Lucretius published a poem entitled De rerum natura, or On the Nature of Things. Its contents were mostly philosophical, focusing on themes of life, death, love, the soul, and such. However, the first two sections of the six-book poem were focused on a very different subject matter: atoms. The poem read:
“Observe what happens when sunbeams are admitted into a building and shed light on its shadowy places. You will see a multitude of tiny particles mingling in a multitude of ways . . . their dancing is an actual indication of underlying movements of matter that are hidden from our sight. It originates with the atoms which move of themselves [i.e., spontaneously]. Then those small compound bodies that are least removed from the impetus of the atoms are set in motion by the impact of their invisible blows and in turn cannon against slightly larger bodies. So the movement mounts up from the atoms and gradually emerges to the level of our senses, so that those bodies are in motion that we see in sunbeams, moved by blows that remain invisible.” 
This is a phenomenon called “Brownian Motion,” named after Botanist Robert Brown. Although it was Albert Einstein that finally described the physics behind this, it was named after Brown because of his being the first to test this theory by observing pollen grains bouncing off of water molecules. But it was ultimately Einstein who brought this to the attention of the physics community, and in doing so, proving the existence of what we now think of as atoms and molecules.

We decided to reproduce Brown’s experiment, adapting directions from Dave Walker.

What you need:
  • student microscope with 200X or 400X magnification
  • milk (we used 2%)
  • microscope slides and coverslips
  • thin needle or wire
  • water (preferably distilled, although we used tap water)
  • Vaseline petroleum jelly (optional)
To prepare the slide:

1. Place a very small drop of water in the middle of the slide (use a dropper).

2. Dip the needle in the milk, then dip and stir in the water drop. We picked up a drop in the eye of the needle and stirred it with the needle almost flat in the water drop.

3. Gently lower the coverslip onto the diluted milk drop.

4. Make sure no water is near the edge of the cover slip. This is important to ensure you observe Brownian motion and not liquid movement caused by evaporation).

5. Optional: To make a slide that lasts longer, seal the coverslip on with a thin line of Vaseline to minimize evaporation.

Still photo of fat molecules taken with standard microscope and point-and-shoot camera.

We tried this experiment with both our computer microscope and our standard microscope. Although we were unable to get decent pictures with either microscope, we were able to see decent results with the basic microscope.

We placed a droplet of water on a glass slide, and then, using a pin, we placed a smaller droplet of milk (we used 2% instead of whole) inside the water. We then placed a coverslip over the droplet, and sealed it with vasoline. The first few times we did this, we were unable to observe, or even locate the water droplet with our computerized microscope. However, after a few tries, we found that making adjustments to the experiment (such as not using vasoline and placing smaller droplets on the slide) were effective, allowing us to at least see the water. Switching from the computerized microscope to our lower-tech but more high-powered microscope produced much more positive results. The higher magnification allowed us to actually observe the Brownian Motion of the milk fat particles bouncing off the water molecules.

We also decided to test a simplified version of Brown’s experiment by dropping food coloring into a large bowl of water. While this was not as precise as the milk experiment, observing the cigarette smoke-like movements of the dye was more effective at demonstrating Brownian Motion. Above, you can see a video of a similar experiment, conducted by GeekMom writer Kay Holt and her son Bastian.

Other resources:

Einstein Year
University of Virginia
Discover Magazine