About 1905 Einstein published a work about diffusion of hard spheres and Brownian motion. One effluence of that is the so called "viscosity equation" which was/is very important for determining the molecular weight of macromolecules by the viscosity of solutions.
http://www.ias.ac.in/initiat/sci_ed/resources/chemistry/Viscosity.pdf(Link dead now, archived here)
What I'd like to know: Was there any relation to the other topics Einstein worked on? Is something known on Einstein's reason to work on viscosity?