Dynamic Light Scattering

Today, dynamic light scattering, also referred to as quasielastic light scattering or photon correlation spectrocopy, is one of the most popular methods for determination of particle sizes.

Basically, the method's advantages are the following:

  • short experimental duration
  • high level of automatation
  • for routine measurements, no extensive experience is required
  • modest experimental effort
  • modest method development costs.

These points are stated for commercial "particle sizing" systems, mostly operating at only one angle (90°) and using red light (675 nm). The dependence on concentration is also commonly neglected.

Using more sophisticated experimental equipment (goniometer, short-wavelength light source), the methods can be considerably extended. However, there are two basic limitations in all scattering methods:

  • the particles in question must not be too small in relation to the incident wavelength,
  • in general, no mixtures of multiple species can be analyzed.

As light scattering is not a fractionating method, an overlay/convolution of scattering functions due to multiple populations must be described theoretically. There are more or less sophisticated approaches to solve this problem, but a basic physical limitation cannot be overcome by evaluation work: scattering intensity increases with the sixth power of particle size; larger particles will always dominate in scattering methods.

Classical DLS evaluation using the autocorrelation function (ACF) yields a particle size and polydispersity for a monomodal system with some reliability. Often, modern instruments allow the monitoring of the building up and stabilisation of the ACF during the experiment, as shown in Figure 1.

Autocorrelation in Dynamic Light Scattering
Figure 1: Autocorrelation in Dynamic Light Scattering

Enhanced evaluation procedures make use, for example, of inverse Laplace transformation rather than autocorrelation. As with any such enhancements, however, one has to keep track of the assumptions and prerequisites introduced by an increasing level of data analysis. The need for interpretation may grow. In any case, no however sophisticated evaluation procedures can overcome basic physical limitations of a method.

On a simpler level, some incertainty is introduced if semiautomatic measurements are used and data analysis will produce somewhat convincing results from any artefacts or unsuitable systems.

In our laboratory, an argon-ion laser (488 nm) is used as a light source. Two Watts of incident radiation power supply sufficient intensity also for small particles and grazing angles. For evaluation, we use cumulant fits as well as inverse Laplace transformations and apply optimised regularisation methods.