Dependency of the sample turbidity on particle size/material

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Dependency of the sample turbidity on particle size/material

Postby MathiasReufer » Wed Nov 28, 2012 3:48 pm

DWS is applied to samples with high turbidity. It is thus important to understand which factors influence the turbidity of the sample. In general, for a given laser wavelength the turbidity of a sample consisting of dispersed particles depends on

    -particle concentration
    -particle size
    -refractive index of the particle relative to the solvent

The turbidity can be quantified by the transport mean free path l* (the distance a photons travels to lose its direction). For dilute conditions (concentrations < 5 % v/v) the turbidity is directly proportional to the particle concentration. However, the dependency on the particle size and refractive index is more complex. Figure 1 compares the calculated values for l* of different particles with a volume fraction of 1% dispersed in water. These calculations are made using the scattering calculator (

Figure 1: Transport mean free path versus particle diameter of different particle materials.
lstar_4.png (7.35 KiB) Viewed 9443 times

It shows that particles with high refractive index (TiO2) yield samples which are much more turbid (smaller value of l*) then with moderate (PS) and low (SiO2) refractive index. Moreover, there is a strong size dependency with maximal turbidity for particles with diameter in the range of 200 to 500 nm. The stability and size monodispersity is generally better for PS than for TiO2 particles. Therefore, as tracer particles in water based systems, we recommend PS particles with diameter of 200 to 600 nm. If your system is not water based, you must check if PS particles are stable in your specific system. Finally, PS particles in water are also recommended as reference sample for the DWS Rheolab due to the small size polydispersity.
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