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Dynamic Light Scattering
(Photon Correlation Spectroscopy)
Dynamic light scattering (DLS) is a technique that analyses the sample dynamic and measures diffusion coefficients. It is used to determine size and size distribution of all kinds of polymerical and colloidal systems. In concentrated dispersions, it enables the measurement of dynamic quantities and the study of the nature of dynamic processes.
experimental setup speckle pattern
If you observe the scattered light under a certain angle via two pinholes, you receive an aperiodic speckle pattern (see figure above). In this pattern, each speckle corresponds to one coherence area (the area, where the scattered light, coming from different scattering centers, has a fixed phase relation). The scattering points are moving in the sample and therefore the amplitude of the scattered light in one speckle fluctuates:
These fluctuations can be correlated and one gets the intensity autocorrelation function, that can be related to the intermediate scattering function:
The analysis of the slope of this curve allows the determination of particle radius and radius distribution in dilute samples. The concentration dependence of the intermediate scattering function enables important insight in the physics of colloidal dispersions.