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Photon Correlation Spectroscopy
for particle size analysis from 1 nm to 10 µm
in highly diluted suspensions

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Photon Correlation Spectroscopy (PCS) is based on Dynamic Light Scattering. The time decay of the near-order of the particles caused by the Brownian motion is used to evaluate the size of nanoparticles via the Stokes-Einstein relation. At constant temperature T the method only requires the knowledge of the viscosity h of the suspending fluid for an estimation of the average particle size and its distribution function (and for volume fractions the refractive index n).

Brownian motion with speckle pattern changing in time t Intensity fluctuation due to Brownian motion Decay of the near order displayed by the slope of the auto-correlation function

The Brownian motion results from impacts of the thermal movement of the molecules of the suspending fluid on the particles.

The intensity in the diffraction pattern observed under a scattering angle Q fluctuates with time t.

As the near-order of fine particles dissolves faster than the near-order of coarse particles, the particle size can be deducted e. g. from the slope of the auto-correlation function G (with known h and T).

The well established theory for PCS is only valid for light being scattered once. Any contribution of multiple scattered light leads to erroneous PCS results and misinterpretations. As a consequence, PCS requires highly diluted suspensions in order to avoid multiple scattering. The low concentration of particles makes this method sensitive to impurities in the liquid. So usually very pure liquids and a clean room environment have to be used for the preparation and the operation. For further details please refer to ISO 33321 (1996).

In contrast to PCS, Photon Cross-correlation Spectroscopy (PCCS) as offered by Sympatec can be used for the measurement of particle size and stability of nanoparticles in high concentrated opaque supensions and emulsions, as multiple scattering is completely eliminated.

symbol: wet dispersion for suspensions or emulsions