High-frequency ultrasonic speckle velocimetry in sheared complex fluids

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

Corresponding author: sebm@crpp-bordeaux.cnrs.fr

Received: 3 November 2003
Accepted: 11 May 2004
Published online: 5 August 2004

Abstract

High-frequency ultrasonic pulses at 36 MHz are used to measure velocity profiles in a complex fluid sheared in the Couette geometry. Our technique is based on time-domain cross-correlation of ultrasonic speckle signals backscattered by the moving medium. Post-processing of acoustic data allows us to record a velocity profile in 0.02–2 s with a spatial resolution of 40 μm over 1 mm. After a careful calibration using a Newtonian suspension, the technique is applied to a sheared lyotropic lamellar phase seeded with polystyrene spheres of diameter 3–10 μm. Time-averaged velocity profiles reveal the existence of inhomogeneous flows, with both wall slip and shear bands, in the vicinity of a shear-induced “layering” transition. Slow transient regimes and/or temporal fluctuations can also be resolved and exhibit complex spatio-temporal flow behaviors with sometimes more than two shear bands.