High-frequency ultrasonic speckle velocimetry in sheared complex fluids

Details High-frequency ultrasonic speckle velocimetry in sheared complex fluids High-frequency ultrasonic speckle velocimetry in sheared complex fluids

2003-11-04

arxiv.org/abs/cond-mat/0311072v1

Eur. Phys. J. AP, vol. 28, 361-373 (2004)

Physics

Condensed Matter

Soft Condensed Matter

15 pages, 18 figures, submitted to Eur. Phys. J. AP

Scientific paper

10.1051/epjap:2004165

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 $\mu$m 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 $\mu$m. 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.

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