  Diffusing-Wave Spectroscopy (DWS) treats the transport of photons through
turbid samples as a diffusion process, thereby making it possible to extract
the dynamics of scatterers from measured correlation functions. The analysis of
DWS data requires knowledge of the path length distribution of photons
traveling through the sample. While for flat sample cells this path length
distribution can be readily expressed in analytical form, no such expression is
available for cylindrical sample cells. DWS measurements have therefore
typically relied on dedicated setups that use flat sample cells. Here we show
how DWS measurements, in particular DWS-based microrheology measurements, can
be performed in standard dynamic light scattering setups that use cylindrical
sample cells. To do so we perform simple random walk simulations which yield
numerical predictions of the path length distribution as a function of both the
transport mean free path and the detection angle. This information is used in
experiments to extract the mean-square displacement of tracer particles in the
material, as well as the resulting frequency-dependent viscoelastic response.
An important advantage of our approach is that by measuring at different
detection angles, the average photon path length can be varied. Using a single
sample cell, this gives access to a wider range of length and time scales than
obtained in a conventional DWS setup. Such angle-dependent measurements also
offer an important consistency check, as for all detection angles the DWS
analysis should yield the same tracer dynamics, even though the respective path
length distributions are very different. We validate our approach by performing
measurements both on aqueous suspensions of tracer particles and on solid-like
gelatin samples, for which we find our DWS-based microrheology data to be in
excellent agreement with rheological measurements.
