DYNAMIC LIGHT SCATTERING (PHOTON CORRELATION SPECTROSCOPY)
The photon-correlation spectrometers IKO Sizer are based on the Photon Correlation Spectroscopy (PCS) technique and designed for measurements of sub-micron particle sizes, diffusion coefficients, viscosities, molecular weights of polymers in basic and applied studies. The PCS method consists in determining the velocity distribution of particles movement by measuring dynamic fluctuations of intensity of 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. This in turn results in fluctuations of intensity of the scattered light. The linewidth of the light scattered spectrum (defined as the half-width at half-maximum) is proportional to the diffusion coefficient of the particles D (Eq.1):
where n is the refractive index of the medium, the laser wavelength, and the scattering angle. With the assumption that the particles are spherical and non-interacting, the mean radius is obtained from the Stokes-Einstein equation:
where kB is the Boltzmann constant, T the temperature, and the shear viscosity of the solvent. Information about the light-scattering spectrum can be obtained from the autocorrelation function of the light-scattering intensity. 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. Therefore, the diffusion coefficient and either particle size or viscosity can be found by fitting the measured correlation function to a single exponential function. A 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.