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

Centre de Recherche Paul Pascal, avenue Schweitzer, 33600 Pessac, France

Received: 31 October 2002
Revised: 7 January 2003
Accepted: 9 January 2003
Published online: 25 February 2003

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 (≈ 50 − 100 µm) and its temporal resolution (≈ 1 s per point, ≈ 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.