Diffusion-limited aggregation is the process whereby particles undergoing a random walk due to Brownian motion cluster together to form aggregates of such particles . This theory, proposed by Witten and Sander in 1981 , is applicable to aggregation in any system where diffusion is the primary means of transport in the system .
 http://en.wikipedia.org/wiki/Diffusion-limited_aggregation  T. A. Witten Jr, L. M. Sander, Phys. Rev. Lett. 47, 1400 (1981)