Wall slip, shear banding, and instability in the flow of a triblock copolymer micellar solution

TL;DR

This study investigates the flow behavior of a triblock copolymer micellar solution, revealing wall slip at lower temperatures and shear banding with flow instability above a critical temperature, using rheological and velocity profile measurements.

Contribution

It provides new insights into the flow regimes, wall slip, and shear banding phenomena in triblock copolymer solutions across a transition temperature.

Findings

Wall slip observed below transition temperature.

Shear banding with high shear rate near walls above transition temperature.

Flow instability associated with the high shear branch.

Abstract

The shear flow of a triblock copolymer micellar solution (PEO--PPO--PEO Pluronic P84 in brine) is investigated using simultaneous rheological and velocity profile measurements in the concentric cylinder geometry. We focus on two different temperatures below and above the transition temperature $T_c$ which was previously associated with the apparition of a stress plateau in the flow curve. (i) At $T=37.0^\circ$C $<T_c$, the bulk flow remains homogeneous and Newtonian-like, although significant wall slip is measured at the rotor that can be linked to an inflexion point in the flow curve. (ii) At $T=39.4^\circ$C $>T_c$, the stress plateau is shown to correspond to stationary shear-banded states characterized by two high shear rate bands close to the walls and a very weakly sheared central band, together with large slip velocities at the rotor. In both cases, the high shear branch of the flow curve is characterized by flow instability. Interpretations of wall slip, three-band structure, and instability are proposed in light of recent theoretical models and experiments.