High-frequency ultrasonic speckle velocimetry in sheared complex fluids

TL;DR

This paper introduces a high-frequency ultrasonic speckle velocimetry technique for measuring detailed velocity profiles in sheared complex fluids, revealing flow inhomogeneities and transient behaviors.

Contribution

The study develops a novel ultrasonic velocimetry method with high temporal and spatial resolution for complex fluids under shear.

Findings

Detection of wall slip and shear bands near layering transition

Observation of complex spatio-temporal flow behaviors

Capability to resolve transient flow regimes

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 $\mu$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.