Probing shear-induced rearrangements in Fourier Space. II. Differential Dynamic Microscopy

TL;DR

This paper explores how Fourier-space measurements, specifically Differential Dynamic Microscopy under shear, can be used to connect microscopic dynamics with mechanical properties in soft matter, providing theoretical insights and experimental validation.

Contribution

It introduces a theoretical framework and practical guidelines for analyzing DDM under shear, distinguishing affine and non-affine displacements, and highlights the importance of source coherence.

Findings

Validated theoretical analysis with experiments on 2D samples and 3D gels.

Identified the role of source coherence in determining the probed sample thickness.

Provided practical guidelines for separating displacement contributions in DDM.

Abstract

We discuss in two companion papers (arXiv:1802.03737 and the present manuscript) how Fourier-space measurements may be coupled to rheological tests in order to elucidate the relationship between mechanical properties and microscopic dynamics in soft matter. In this second companion paper, we focus on Differential Dynamic Microscopy (DDM) under shear. We highlight the analogies and the differences with Dynamic Light Scattering coupled to rheology, providing a theoretical approach and practical guidelines to separate the contributions to DDM arising from the affine and the non-affine part of the microscopic displacement field. We show that in DDM under shear the coherence of the illuminating source plays a key role, determining the effective sample thickness that is probed. Our theoretical analysis is validated by experiments on 2D samples and 3D gels.