![]() The translational diffusion coefficient measured by PCS and the rotational diffusion coefficient measured by depolarized FPI may be combined to obtain the dimensions of non-spherical particles. This broadening is proportional to the particle rotational diffusion coefficient, which is in turn related to the particle dimensions. In FPI the frequency broadening of laser light scattered by the particles is analyzed. Depolarized Fabry-Perot interferometry (FPI) is a less common dynamic light scattering technique that is applicable to optically anisotropic nanoparticles. PCS is one of the most commonly used methods for measuring radii of submicron size particles in liquid dispersions. For a spherical particle, the hydrodynamic radius is essentially the same as the geometric particle radius (including any possible solvation layers). ![]() For dilute dispersions of spherical nanoparticles, the decay rate of the time autocorrelation function of these intensity fluctuations is used to directly measure the particle translational diffusion coefficient, which is in turn related to the particle hydrodynamic radius. In photon correlation spectroscopy (PCS), the time fluctuations in the intensity of light scattered by the particle dispersion are monitored. Similar to electron microscopy and differential centrifugal sedimentation, dynamic light scattering is considered as a confirmatory technique following the European Commission’s guidelines on nanomaterial identification, providing direct evidence of the presence of nanoparticles and the nanoparticle size distribution.Dynamic light scattering (DLS) techniques for studying sizes and shapes of nanoparticles in liquids are reviewed. Analysis of the size distribution on the nanoparticles released into the air as airborne nanoparticles can be obtained by the EN 17199-4 small rotating drum methodology in which an electronic impactor and a nanoparticle counter are being used. The DLS particle size analysis in the liquid phase provides information on the hydrodynamic particle size of the nanoparticles. Nanoparticles down to 3 nanometers can be identified and measured. The upper limit of the technique is in the size range of approximately 10 micrometers and the strong focus of dynamic light scattering is clearly on the nanoparticle size range. The measurement range of the dynamic light scattering technique is therefore also limited on the upper range since larger particles are simply to heavy to display Brownian motion and will also settle too fast. The faster movement of the smaller particles is due to the energy transfer of liquid molecules to the solid particles and nanoparticles will experience a greater effect of this energy transfer than larger and heavier particles. The Brownian motion of particles in the liquid phase occurs randomly in every direction and smaller particles will typically travel faster than larger particles. The technique uses a laser to track the Brownian motion of particles and in particular nanoparticles. Analysis of the nanoparticle size distribution and nanoparticles identification can be obtained by means of Dynamic Light Scattering (DLS), sometimes also referred to as Photon Correlation Spectroscopy.
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