Physicists test 'forgotten' Brownian motion theory

October 28, 2006

Researchers have finally tested a theory started by Einstein in 1906 and predicted in the 1930s on Brownian motion, the movement of small objects in water or air. The new results show if the tiny objects are rods, their actions are markedly different from the type of Brownian motion previously known.

The research was published in the Journal Science this week and is direct proof of the behavior. Brownian motion has also been used as a model to predict random behavior, such as the stock market. "It is such a profound and fundamental phenomenon that, as a physicist, I want to learn everything about [Brownian motion]," said Arjun Yodh, Physics professor at the University of Pennsylvania.

Einstein first described Brownian motion in a paper in 1906, but only concentrated on the movement of little balls. He concluded that if a small ball, like a dust particle or equivalent in water, spun, that action didn't affect where the ball traveled. It turned out that its location could be predicted with statistics. The University of Pennsylvania scientists took this further and photographed teeny plastic rods moving in water instead. Using a charge coupled device, or CCD camera, they took millions of pictures at a time, making a sort of movie of the motion. This way they could analyze the motion more directly. They found that the path of a spinning rod is directly related to its spin.

Post-doc Yilong Han explained this by saying, "since ellipsoids are longer than they are wide, they experience more water resistance going in one direction than the other... It gives rise to the weird behavior we observed."

In terms of the practicality of the experiments, team physicist Tom Lubensky stated that the research was directed at obtaining fundamental understanding of a "ubiquitous and important phenomenon." It is not clear yet what practical applications this research will lead to. But it may help with understanding the mechanisms in cells, because proteins in living cells undergo Brownian motion in response to bumps by smaller molecules.

After discovering the strange movement, the lab rediscovered the work done by the French physicist Francis Perrin in the 1930s, predicting the exact motion the researchers had observed. "One of the exciting aspects of this work is the precise agreement between a relatively simple theory and experiments," said Tom Lubensky, Penn's chair of the physics department.