Dynamic light scattering (DLS) is the general designation for a method of determining the size of extremely small particles in the submicron range. These particles are found in Brownian motion in suspensions or emulsions.
The diffusion speed of this motion is inversely proportional to the dimension dp (hydrodynamic diameter) of the particles:
To determine the particle size accurately, the precise value of parameter T (temperature) and η (viscosity) of the liquid must be known.
Interference occurs within the light which is scattered by the different particles. This interference can be either constructive (intensification) or destructive (cancellation). Because particles are not bound to a location in liquids, the interference changes over time and this leads to variation of the scattered light intensity. This intensity is recorded by a highly sensitive photomultiplier (PMT).
The time dependency of the scattered light intensity therefore depends on the motion speed of the particles and thus also on particle size, which is usually analyzed via autocorrelation. Hence, the method is also called photon correlation spectroscopy.
Analyzers using this method record the scattering light at 90° angle to the incident laser light or they record the back scattering (173°-177°). Which angle works best depends on scattering intensity and concentration of the sample.
The nano particle analyzer SZ-100 includes two high-performance photomultipliers, which determine high as well as low concentrations with the greatest precision.