Viscosity Effects In Dynamic Light Scattering Measurements

Dynamic light scattering measurement of particle size is strongly influenced by the viscosity, both in the calculation and in how the sample behaves. Care must be taken to provide correct values for viscosity and to prepare the sample so that the viscosity does not unduly influence the results.

Dynamic light scattering (DLS) actually measures the translational diffusion of particles due to Brownian motion. The relationship between the diffusion coefficient and particle size can be expressed by the following Stoke- Einstein equation: D = k T / (3 πηa) where k is the Boltzmann constant, T the absolute temperature, η the viscosity coefficient of the solvent, a the particle size, and D the diffusion coefficient.

If a particle is free to exhibit Brownian motion, its size can be related to the viscosity of the fluid, as indicated in the above formula. When the temperature at the time of measurement and the viscosity of the dispersant are known, it is possible to determine the particle size by measuring the diffusion coefficient.

The viscosity required is the dynamic viscosity at zero shear rate.