Two step micro-rheological behavior in a viscoelastic fluid

TL;DR

This study reveals two distinct shear thinning regimes in a viscoelastic fluid using micro-rheology, with the second regime occurring at very low shear rates and indicating different underlying length scales.

Contribution

It introduces a micro-rheological analysis revealing a second shear thinning process at low shear rates, not observed in macro rheology, and proposes a model linking these to different length scales.

Findings

Two shear thinning regimes with Newtonian-like plateaus

Second shear thinning process at very low Weissenberg numbers

Model linking shear thinning to different length scales

Abstract

We perform micro-rheological experiments with a colloidal bead driven through a viscoelastic worm-like micellar fluid and observe two distinctive shear thinning regimes, each of them displaying a Newtonian-like plateau. The shear thinning behavior at larger velocities is in qualitative agreement with macroscopic rheological experiments. The second process, observed at Weissenberg numbers as small as a few percent, appears to have no analog in macro rheological findings. A simple model introduced earlier captures the observed behavior, and implies that the two shear thinning processes correspond to two different length scales in the fluid. This model also reproduces oscillations which have been observed in this system previously. While the system under macro-shear seems to be near equilibrium for shear rates in the regime of the intermediate Newtonian-like plateau, the one under micro-shear is thus still far from it. The analysis suggests the existence of a length scale of a few micrometres, the nature of which remains elusive.