Issue 9, 2019

Flow of wormlike micellar solutions around microfluidic cylinders with high aspect ratio and low blockage ratio

Abstract

We employ time-resolved flow velocimetry and birefringence imaging methods to study the flow of a well-characterized shear-banding wormlike micellar solution around a novel glass-fabricated microfluidic circular cylinder. In contrast with typical microfluidic cylinders, our geometry is characterized by a high aspect ratio α = H/W = 5 and a low blockage ratio β = 2r/W = 0.1, where H and W are the channel height and width, and the cylinder radius r = 20 μm. The small cylinder radius allows access up to very high Weissenberg numbers 1.9 ≤ Wi = λMU/r ≤ 3750 (where λM is the Maxwell relaxation time) while inertial effects remain entirely negligible (Reynolds number, Re < 10−4). At low Wi values, the flow remains steady and symmetric and a birefringent region (indicating micellar alignment and tensile stress) develops downstream of the cylinder. Above a critical value Wic ≈ 60 the flow transitions to a steady asymmetric state, characterized as a supercritical pitchfork bifurcation, in which the fluid takes a preferential path around one side of the cylinder. At a second critical value Wic2 ≈ 130, the flow becomes time-dependent, with a characteristic frequency f0 ≈ 1/λM. This initial transition to time dependence has characteristics of a subcritical Hopf bifurcation. Power spectra of the measured fluctuations become complex as Wi is increased further, showing a gradual slowing down of the dynamics and emergence of harmonics. A final transition at very high Wic3 corresponds to the re-emergence of a single peak in the power spectrum but at much higher frequency. We discuss this in terms of possible flow-induced breakage of micelles into shorter species with a faster relaxation time.

Graphical abstract: Flow of wormlike micellar solutions around microfluidic cylinders with high aspect ratio and low blockage ratio

Supplementary files

Article information

Article type
Paper
Submitted
15 Oct 2018
Accepted
12 Dec 2018
First published
18 Jan 2019
This article is Open Access
Creative Commons BY license

Soft Matter, 2019,15, 1927-1941

Flow of wormlike micellar solutions around microfluidic cylinders with high aspect ratio and low blockage ratio

S. J. Haward, N. Kitajima, K. Toda-Peters, T. Takahashi and A. Q. Shen, Soft Matter, 2019, 15, 1927 DOI: 10.1039/C8SM02099J

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