Exploring particle dynamics in flowing complex fluids using differential dynamic microscopy

TL;DR

This paper introduces differential dynamic microscopy (DDM) adaptations to analyze particle dynamics in flowing complex fluids without resolving individual particles, revealing microscopic and bulk yielding behaviors.

Contribution

It develops novel echo-DDM and flow-DDM techniques to study highly-concentrated samples under flow, enabling insights into yielding processes without particle resolution.

Findings

Identified two-step transition from solid-like to liquid-like states.

Measured shear-induced droplet rearrangements and flow velocity.

Demonstrated techniques applicable to various complex fluids.

Abstract

Microscopic dynamics reveal the origin of the bulk rheological response in complex fluids. In model systems particle motion can be tracked, but for industrially relevant samples this is often impossible. Here we adapt differential dynamic microscopy (DDM) to study flowing highly-concentrated samples without particle resolution. By combining an investigation of oscillatory flow, using a novel "echo-DDM" analysis, and steady shear, through flow-DDM, we characterise the yielding of a silicone oil emulsion on both the microscopic and bulk level. Through measuring the rate of shear-induced droplet rearrangements and the flow velocity, the transition from a solid-like to liquid-like state is shown to occur in two steps: with droplet mobilisation marking the limit of linear visco-elasticity, followed by the development of shear localisation and macroscopic yielding. Using this suite of techniques, such insight could be developed for a wide variety of challenging complex fluids.