Xinya Lei, a post doctoral associate in my laboratory, took these pictures of 40 micron diameter polystyrene particles settling in a solvent called PAG. The picture on the upper left shows a random collection of particles immediately after stirring the sample. The picture on the upper right is the superposition of five frames taken at 15 second intervals. This frame shows large turbulent like swirls during the initial settling, due to the fluctuations in particle density for random configurations. However, eight hours later the lower right frame shows these fluctuations become much reduced. But the frame on the lower left still looks random. It is not! Determining particle occupancy statistics shows that large scale density fluctuations become highly suppressed.

Settling Statistics