Imaging the shear flow of complex fluids using high-frequency ultrasound

Abstract

Complex fluids show fascinating properties under flow due to the existence of a mesoscopic scale located between the microscopic and the macroscopic scales. For instance, uncoiling of polymer molecules under flow leads to well-documented shear-thinning behaviors. More surprisingly, inhomogeneous flows of visco-elastic fluids are observed in simple shear experiments even at very low Reynolds numbers. Indeed, due to strong coupling between the flow and the fluid microstructure, shear may induce new structural organizations that coexist in the flow leading to shear localization or to shear bands. Whereas classical rheology only yields global data such as the viscosity averaged over the whole sample, the present work is devoted to local ultrasonic velocimetry in sheared complex fluids. A 1D high-frequency (36 MHz) speckle tracking technique is presented that allows the spatio-temporal study of various inhomogeneous unsteady flows. Moreover, preliminary results using a 2D ultrafast flow imaging technique at 12 MHz show that images of the flow can be obtained with an unprecedented temporal resolution.