Brownian Motion - Observation, Theory and Experiment

Our modelling of diffusion will start with Brownian motion. The Scottish botanist Robert Brown was one of the pioneers of using microscopes to study plants. During his studies, observed that pollen suspended in water "danced" or "jittered" around incessantly. He repeated the observation with inorganic minerals to make sure the pollen weren't alive somehow and propelling themselves. In 1827, Brown described this phenomena (later called Brownian motion in his honor). However, the reason for Brownian motion remained a mystery for almost eighty years after its discovery.

In 1905, Albert Einstein published a paper that explained Brownian motion as the result of the pollen particles colliding with randomly moving atoms. One of his insights was that you could simply model the pollen particles as very large atoms. His mathematical models predicted how far particles of various sizes would move, on average, in a given amount of time. If his predictions turned out to be correct, it would both explain the why behind Brownian motion and - perhaps more importantly - provide extremely strong evidence that atoms themselves were real. (As late as 1905 there were still accomplished and prominent scientists who did not think atoms existed.)

Three years later, in 1908, the French physicist Jean Baptiste Perrin, along with a team of research students, began the work that experimentally confirmed Einstein's predictions. The confirmation of Einstein's theoretical predictions is what convinced the last skeptics that atoms were real and not just a mathematically useful analogy. For more on the history of Brownian motion and how it confirmed atomic theory see Einstein’s random walk.

Figure 7.3.1 shows three traces of the motion of colloidal particles in water seen under a microscope from Perrin's 1901 article Mouvement brownien et réalité moléculaire (Brownian motion and molecular reality).