1) Theory

Brownian motion is the result of collisions between suspended particles and the fast moving molecules of the solution in which they are suspended. Stokes showed that the mobility of spherical particles under these conditions is given by:

where µ is mobility, η is viscosity and r is the hydrodynamic radius of the sphere.

Einstein later showed that:

is the diffusion coefficient, is the Boltzmann constant and is the temperature. These equations were combined to form the Stokes Einstein relation:

Scattering light from a coherent light source by particles results in coherently scattered light. If these particles are in motion, the phases of the scattered light vary, leading to correlated intensity fluctuations at short time scales. Quantifying this correlation gives

, which is then used to determine the viscosity, η.