Diffusing-wave spectroscopy in a standard dynamic light scattering setup

Abstract

Diffusing-Wave Spectroscopy (DWS) extends dynamic light scattering measurements to samples with strong multiple scattering. 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 calculated and 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 corresponding frequency-dependent viscoelastic response. An important advantage of our approach is that by performing measurements at different detection angles, the average path length through the sample can be varied. Using measurements on 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 very good agreement with rheological measurements performed on the same samples.

Passive and Active Microrheology for Biomedical Systems

“…The experimental setup of DWS is similar to that of DLS. Two commonly used modes in DWS experiments are transmission and backscattering ( Pine et al, 1990 ; Fahimi et al, 2017 ). In the high multiple photon scattering regime, the details of individual single scattering events are no longer relevant.…”

Spectral Properties of Foams and Emulsions

“…Motivated by the continuous improvement of DWS techniques [113,202,203] and the need to rectify collective scattering effects in DWS of dense colloidal systems, which can adversely affect passive microrheology, a systematic experimental comparison of the plateau elastic shear moduli measured using both mechanical rheometry and modern DWS microrheology of disordered, microscale monodisperse emulsions, as a function of droplet volume fraction was reported [204]. In addition, DWS in a standard Dynamic Light Scattering setup was published [205].…”

Deconvoluting Emulsion Stabilization Mechanisms Using Diffusing Wave Spectroscopy

“…The method was first described by Fahimi et al studying suspensions in a similar setup to the one employed in this work. 18 The method was further adapted for the study of emulsions by Lorusso et al, and both a Brownian component and shear component were determined from the mean squared displacement of the emulsion droplets. 19 In this work, a similar methodology of l* determination is utilized that builds on both of these prior works and is discussed in detail in McMillin et al 15 This developed framework advances prior work by allowing for l*, extracted from a series of DWS experiments, to be connected with kinetics, providing unparalleled mechanistic information associated with destabilization in semisolid lubricating oil formulations.…”

“…Both s and P ( s ) are functions of the experimental geometry as well as l *, and in certain circumstances have an analytical form . However, if no analytical form can be constructed, the path length and path length distribution can be obtained via numerical Monte Carlo simulations specific to the geometry employed for a given value of l *. ,, For the experimental setup used in this work, Monte Carlo simulations were performed, as outlined in McMillin et al…”