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Diffusing-Wave Spectroscopy (DWS) treats the transport of photons throughturbid samples as a diffusion process, thereby making it possible to extractthe dynamics of scatterers from measured correlation functions. The analysis ofDWS data requires knowledge of the path length distribution of photonstraveling through the sample. While for flat sample cells this path lengthdistribution can be readily expressed in analytical form, no such expression isavailable for cylindrical sample cells. DWS measurements have thereforetypically relied on dedicated setups that use flat sample cells. Here we showhow DWS measurements, in particular DWS-based microrheology measurements, canbe performed in standard dynamic light scattering setups that use cylindricalsample cells. To do so we perform simple random walk simulations which yieldnumerical predictions of the path length distribution as a function of both thetransport mean free path and the detection angle. This information is used inexperiments to extract the mean-square displacement of tracer particles in thematerial, as well as the resulting frequency-dependent viscoelastic response.An important advantage of our approach is that by measuring at differentdetection angles, the average photon path length can be varied. Using a singlesample cell, this gives access to a wider range of length and time scales thanobtained in a conventional DWS setup. Such angle-dependent measurements alsooffer an important consistency check, as for all detection angles the DWSanalysis should yield the same tracer dynamics, even though the respective pathlength distributions are very different. We validate our approach by performingmeasurements both on aqueous suspensions of tracer particles and on solid-likegelatin samples, for which we find our DWS-based microrheology data to be inexcellent agreement with rheological measurements.

Diffusing-Wave Spectroscopy in a Standard Dynamic Light Scattering Setup