In heterodyne measurements, which are most suitable for small intensities, the scattered light is mixed coherently with a static light source at the incident wavelength, and the static field is added to the scattered fields at the photodetector. There exist two techniques of measuring the correlation function: heterodyning and homodyning. Therefore, the diffusion coefficient and either particle size or viscosity can be found by fitting the measured correlation function to a single exponential function.Ī characteristic autocorrelation function of the scattered light is shown below where the baseline b is proportional to the total intensity I, and it can be determined experimentally. For the simplest case of spherical monodisperse non-interacting particles in a dust-free fluid, the characteristic decay time of the correlation function is inversely proportional to the linewidth of the spectrum. Information about the light-scattering spectrum can be obtained from the autocorrelation function G(τ) of the light-scattering intensity. Where k B is the Boltzmann constant, T the temperature, and η the shear viscosity of the solvent. With the assumption that the particles are spherical and non-interacting, the mean radius is obtained from the Stokes-Einstein equation N is the refractive index of the medium, λ the laser wavelength, and θ the scattering angle. The linewidth of the light scattered spectrum Γ (defined as the half-width at half-maximum) is proportional to the diffusion coefficient of the particles D (Eq.1): This in turn results in fluctuations of intensity of the scattered light. The disperse particles or macromolecules suspended in a liquid medium undergo Brownian motion which causes the fluctuations of the local concentration of the particles, resulting in local inhomogeneities of the refractive index. The PCS method consists in determining the velocity distribution of particles movement by measuring dynamic fluctuations of intensity of scattered light. Photocor particle size analyzers is based on the Dynamic light scattering (DLS) technique and designed for measurements of sub-micron particle sizes, diffusion coefficients, viscosities, molecular weights of polymers in basic and applied studies.
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