Difference between DLS and DWS microrheology
In both techniques, the measurement and analysis of temporal intensity fluctuations of scattered light are conducted similarly. However, in DWS, photons must be scattered multiple times whereas in DLS multiply scattered photons must be avoided. This means that in DWS, only highly concentrated and turbid samples are measured.
The fact that DWS is based upon the analysis of the collective motion of a large number of particles through multiple scattering results in a greatly improved spatial resolution compared to DLS. DWS is, therefore, more sensitive to small particle displacements: whilst DLS typically can detect displacements of several nanometers, DWS can measure sub-nanometer displacements. As a result, DWS is an excellent tool to study slow dynamics, highly viscous samples, and even arrested (non-ergodic) samples such as gels. In highly viscous, rubbery, or gelled materials, microparticles move by only a few nanometers, such motion is virtually impossible to be observed with a microscope. While DLS-based microrheology probes purely liquid samples and viscoelastic liquids, DWS extends this characterization capability to viscoelastic solids: please see the section “Echo DWS” for more information Since DWS probes much larger sample volumes than DLS, it also yields enhanced statistics at shorter measurement times: as a consequence, the sensitivity of the technique is much higher than with DLS or mechanical rheometry. Very weak samples can be measured, and the frequency range accessible through DWS (up to 106 rad/s) is much higher than with DLS (limited to 103 rad/s).
Fig. 8: Illustration of the amplitude of moduli and frequency range of various rheology techniques.