Improved diffusing wave spectroscopy based on the automatized determination of the optical transport and absorption mean free path

TL;DR

This paper introduces an automated method for accurately determining the optical transport and absorption mean free paths in diffusing wave spectroscopy, simplifying measurements and reducing errors in studying complex fluids.

Contribution

The paper presents a novel automated approach for determining optical properties in DWS, eliminating the need for calibration samples and improving measurement accuracy.

Findings

Automated method accurately determines $l^*$ and $l_a$.

Simplifies DWS measurements by removing calibration step.

Demonstrated on monitoring foam coarsening.

Abstract

Diffusing wave spectroscopy (DWS) can be employed as an optical rheology tool with numerous applications for studying the structure, dynamics and linear viscoelastic properties of complex fluids, foams, glasses and gels. To carry out DWS measurements, one first needs to quantify the static optical properties of the sample under investigation, i.e. the transport mean free path $l^\ast$ and the absorption length $l_a$. In the absence of absorption this can be done by comparing the diffuse optical transmission to a calibration sample whose $l^\ast$ is known. Performing this comparison however is cumbersome, time consuming and prone to mistakes by the operator. Moreover, already weak absorption can lead to significant errors. In this paper, we demonstrate the implementation of an automatized approach, based on which the DWS measurement procedure can be simplified significantly. By comparison with a comprehensive set of calibration measurements we cover the entire parameter space relating measured count rates (CR$_t$, CR$_b$) to ($l^\ast$, $l_a$). Based on this approach we can determine $l^\ast$ and $l_a$ of an unknown sample accurately thus making the additional measurement of a calibration sample obsolete. We illustrate the use of this approach by monitoring the coarsening of a commercially available shaving foam with DWS.