Theory

When laser light hits small particles suspended in a liquid, it scatters in all directions and one may observe time dependent intensity fluctuations in the scattering caused by the particle interaction with the surrounding solvent molecules. The intensity fluctuations are caused by Brownian Motion of the particles and solvent which produce constructive and destructive interference of the light and information about the hydrodynamic size of the scatterers lies therein.

By observing the time dependent fluctuations and correlating the information at very short time intervals (typically around 1µs), an exponential decay related to the diffusion coefficient of the scatterers is generated. This is the second order correlation function, g2 shown below

where A is an amplitude correction factor and Γ is the decay rate, related to the properties of the observed particles.  Having knowledge of Γ we can determine the diffusion coefficient, Dt and from this, the average hydrodynamic radius, Rh of the particles.

where: K is Boltzman’s constant, T is the absolute temperature, η is the solvent viscosity, Ris the hydrodynamic radius, λ is the laser wavelength