An optical fiber based interferometer to measure velocity profiles in sheared complex fluids

TL;DR

This paper presents an optical fiber interferometer using heterodyne DLS to accurately measure velocity profiles in sheared complex fluids within a Couette cell, overcoming previous technical challenges.

Contribution

It introduces a heterodyne DLS-based interferometer setup capable of detailed velocity profiling in complex fluids under shear, with improved spatial and temporal resolution.

Findings

Heterodyne DLS effectively measures velocity fields in narrow-gap Couette geometries.

The setup achieves spatial resolution of 50-100 micrometers and temporal resolution of 1 second.

Simultaneous velocity and rheological measurements are demonstrated on emulsions.

Abstract

We describe an optical fiber based interferometer to measure velocity profiles in sheared complex fluids using Dynamic Light Scattering (DLS). After a review of the theoretical problem of DLS under shear, a detailed description of the setup is given. We outline the various experimental difficulties induced by refraction when using a Couette cell. We also show that homodyne DLS is not well suited to measure quantitative velocity profiles in narrow-gap Couette geometries. On the other hand, the heterodyne technique allows us to determine the velocity field inside the gap of a Couette cell. All the technical features of the setup, namely its spatial resolution ($\approx 50$--$100 \mu$m) and its temporal resolution ($\approx 1$ s per point, $\approx 1$ min per profile) are discussed, as well as the calibration procedure with a Newtonian fluid. As briefly shown on oil-in-water emulsions, such a setup permits one to record both velocity profiles and rheological data simultaneously