Diffusing Wave Spectroscopy
Diffusing Wave Spectroscopy (DWS) is an advanced light scattering technique, which is primarily applied for microrheology  and particle sizing . Both applications utilize the capacity of DWS to extract the mean square displacement (MSD) of particles dispersed in a solvent from measurements of the temporal fluctuation of backscattered or transmitted light . The transmitted light intensity fluctuates over time, as shown by the evolving speckle pattern in Fig.1. This is a direct result of both the motion of the scattering particles within the sample and the coherent nature of the light.
Fig. 1: Above: Basic set-up of a DWS experiment in transmission geometry on a sample containing particles performing Brownian motion. Below: From the measured intensity fluctuations, the intensity correlation function (ICF) and the mean square displacement (MSD) of the particles are calculated.
The particles in the sample perform Brownian motion, which is sensitive to the rheology of the particles' local environment. Particle motion constitutes, for this reason, a reliable probe to characterize the medium's rheological properties.
The measurement and analysis of temporal intensity fluctuations of scattered light is similar to dynamic light scattering (DLS); the ICF is calculated first, from which the particle MSD  is subsequently extracted. In DWS, however, each detected photon is scattered by many particles; DWS is therefore more sensitive to small particle displacements compared with DLS, where photons are scattered only once. Whilst DLS typically can detect displacement of several nanometers, DWS can measure sub nanometer displacements. As a result, DWS is an excellent tool to study slow dynamics and non-ergodic samples such as gels, foams, and highly concentrated suspensions. Moreover, combined with fast photon detectors, DWS can measure particle displacements at high frequencies up to 106 Hz; a frequency range that cannot be accessed by other techniques.
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 Scheffold, F. Particle Sizing with Diffusing Wave Spectroscopy, Journal of Dispersion Science and technology 23, 591-599 (2002).
 Weitz, D. A.; Pine, D.J. Diffusing-wave Spectroscopy. In Dynamic Light Scattering; Brown, W., Ed.; Oxford University Press: New York, 652-720 (1993).