Gaussian and non-Gaussian speckle fluctuations in the diffusing-wave spectroscopy signal of a coarsening foam

TL;DR

This study tests the Gaussian assumption in Diffusing-Wave Spectroscopy of foams, confirming its validity in typical geometries but identifying breakdowns in specific localized backscattering setups.

Contribution

It provides the first direct experimental verification of the Gaussian field assumption in DWS for foams and identifies conditions where this assumption fails.

Findings

Gaussian field assumption holds in typical geometries

Breakdown of Gaussianity in localized backscattering geometries

Implications for interpreting DWS measurements in foams

Abstract

All prior applications of Diffusing-Wave Spectroscopy (DWS) to aqueous foams rely upon the assumption that the electric field of the detected light is a Gaussian random variable and that, hence, the Siegert relation applies. Here we test this crucial assumption by simultaneous measurement of both second and third-order temporal intensity correlations. We find that the electric field is Gaussian for typical experimental geometries equivalent to illumination and detection with a plane wave, both for backscattering and transmission through an optically-thick slab. However, we find that the Gaussian character breaks down for point-in / point-out backscattering geometries in which the illumination spot size is not sufficiently large in comparison with the size of the intermittent rearrangement events.