Brownian motion
Einstein’s 1905 paper on Brownian motion is his most cited article. Einstein argued that the erratic path of a small particle suspended in a fluid resulted from the random collisions with the molecules of the fluid. This motion had been first described by the botanist Robert Brown, in 1828. Experiments by Perrin in 1909 confirmed this diffusive motion of the Brownian particle . Kappler repeated the experiments in1931 with a mirror in a dilute gas. Gaspard et al., showed the fluid in their experiment to have chaotic dynamics.
Nature volume 394/ 27th August 1998, p.831.