A spatio-temporal study of rheo-oscillations in a sheared lamellar phase using ultrasound

TL;DR

This study uses ultrasound to investigate the complex flow oscillations in a sheared lamellar phase, revealing the roles of wall slip and bulk interface dynamics during shear-induced phase transitions.

Contribution

It provides the first detailed spatio-temporal analysis of rheo-oscillations in lamellar phases using high-frequency ultrasound, linking flow dynamics to phase coexistence and interface motion.

Findings

Identification of two distinct oscillation regimes in viscosity.

Correlation between interface displacement and flow dynamics.

Evidence of shear banding and phase coexistence during oscillations.

Abstract

We present an experimental study of the flow dynamics of a lamellar phase sheared in the Couette geometry. High-frequency ultrasonic pulses at 36 MHz are used to measure time-resolved velocity profiles. Oscillations of the viscosity occur in the vicinity of a shear-induced transition between a high-viscosity disordered fluid and a low-viscosity ordered fluid. The phase coexistence shows up as shear bands on the velocity profiles. We show that the dynamics of the rheological data result from two different processes: (i) fluctuations of slip velocities at the two walls and (ii) flow dynamics in the bulk of the lamellar phase. The bulk dynamics are shown to be related to the displacement of the interface between the two differently sheared regions in the gap of the Couette cell. Two different dynamical regimes are investigated under applied shear stress: one of small amplitude oscillations of the viscosity ($\delta\eta/\eta\simeq 3$%) and one of large oscillations ($\delta\eta/\eta\simeq 25$%). A phenomenological model is proposed that may account for the observed spatio-temporal dynamics