Tracer particles
Particles are often naturally present in a sample (e.g. droplets in a concentrated emulsion, or particles in a suspension). If the system is sufficiently turbid, it can be measured with DWS as is.
Transparent or mildly turbid systems require the addition of tracer particles, in order to reach sufficient turbidity for DWS to be applicable. Tracer particles are added at a minimum concentration of 0.1 % to 0.5 % in volume, depending on the type of particles and the thickness of the cuvette used.
Typically, TiO2 and Polystyrene colloidal particles are used, although other commercially available particles such as melamine, are suitable as well.
The size of the tracers should be between 0.2 to 2 micrometers. Typically tracers around 0.4 to 0.6 micrometers in diameter are used. An important requirement is that the refractive index difference between the tracers and the sample must be sufficiently large (at least 0.1) and that the tracer size be larger than the characteristic size of the sample microstructure.
For a monomodal size distribution, up to a polydispersity of 30-50 %, DWS results are only weakly affected by polydispersity. The main consequence will be that frequency-dependent features in the viscous or elastic moduli will be obtained with lower precision – however with the same accuracy.