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

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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

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Publication details

The article was received on 15 Oct 2018, accepted on 12 Dec 2018 and first published on 18 Jan 2019


Article type: Paper
DOI: 10.1039/C8SM02099J
Citation: Soft Matter, 2019, Advance Article
  • Open access: Creative Commons BY license
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    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, Advance Article , DOI: 10.1039/C8SM02099J

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