Purely-elastic flow instabilities and elastic turbulence in microfluidic cross-slot devices

Sousa, P.C.; Pinho, F.T.; Alves, M.A.

doi:10.1039/c7sm01106g

Cited by 57 publications

(66 citation statements)

References 31 publications

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“…By increasing the flow rate, we are able to change the Weissenberg number from 0.1 up to 200. Our results suggest that beyond a critical value of the Weissenberg number,Wi cr ≈ 0.46, the symmetry of flow is broken in agreement with previous studies (Arratia et al 2006;Sousa et al 2018). In the standard cross-slot geometry, figure 13 shows the flow distribution in symmetric Newtonian flows and a steady asymmetric flow for 190 ppm PAA in 70 : 30 glycerine-water viscoelastic fluid atWi = 0.8 and Re = 6 × 10 −6 .…”

Section: Resultssupporting

confidence: 88%

“…Previous experimental studies with the standard cross-slot geometry (Arratia et al 2006;Pathak & Hudson 2006;Haward et al 2012a;Sousa, Pinho & Alves 2018) have shown that by increasing the Weissenberg number to higher values, one can potentially trigger a second time-dependent instability. A numerical study of shear-thinning sPTT fluids conducted by Cruz et al (2016) has also shown that the critical values of Weissenberg number for the onset on instability for both the steady symmetry-breaking and the time-dependent instabilities are a function of the cross-section aspect ratio (AR = height/width) and the shear-thinning properties of the fluid (i.e.…”

Section: Resultsmentioning

confidence: 99%

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Control of a purely elastic symmetry-breaking flow instability in cross-slot geometries

2019

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The cross-slot stagnation point flow is one of the benchmark problems in non-Newtonian fluid mechanics as it allows large strains to develop and can therefore be used for extensional rheometry measurements or, once instability arises, as a mixing device. In such a flow, beyond a critical value for which the ratio of elastic force to viscous force is high enough, elasticity can break symmetry even in the absence of significant inertial forces (i.e. creeping flow), which is an unwanted phenomenon if the device is to be used as a rheometer but beneficial from a mixing perspective. In this work, a passive control mechanism is introduced to the cross-slot by adding a cylinder at the geometric centre to replace the ‘free’ stagnation point with ‘pinned’ stagnation points at the surface of the cylinder. In the current modified geometry, effects of the blockage ratio (the ratio of the diameter of the cylinder to the width of the channel), the Weissenberg number (the ratio of elastic forces to viscous forces) and extensibility parameters ($\unicode[STIX]{x1D6FC}$ and $L^{2}$) are investigated in two-dimensional numerical simulations using both the simplified Phan-Thien and Tanner and finitely extensible nonlinear elastic models. It is shown that the blockage ratio for fixed solvent-to-total-viscosity ratio has a stabilizing effect on the associated symmetry-breaking instability. The resulting data show that the suggested modification, although significantly changing the flow distribution in the region near the stagnation point, does not change the nature of the symmetry-breaking instability or, for low blockage ratio, the critical condition for onset. Using both numerical and physical experiments coupled with a supporting theoretical analysis, we conclude that this instability cannot therefore be solely related to the extensional flow near the stagnation point but it is more likely related to streamline curvature and the high deformation rates towards the corners, i.e. a classic ‘curved streamlines’ purely elastic instability. Our work also suggests that the proposed geometric modification can be an effective approach for enabling higher flow rates to be achieved whilst retaining steady symmetric flow.

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Section: Resultssupporting

confidence: 88%

Section: Resultsmentioning

confidence: 99%

Control of a purely elastic symmetry-breaking flow instability in cross-slot geometries

2019

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“…Ref. [13] provides proof of the inequality and in experiments values of α = 3.3 − 3.5 [6,33] and α = 3 − 4 [34] have been observed. However, these values correspond to 3D geometries.…”

Section: B Power Spectral Density Of Velocity Fieldmentioning

confidence: 79%

Elastic turbulence in two-dimensional Taylor-Couette flows

Buel¹,

Schaaf²,

Stark³

2018

EPL

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We report the onset of elastic turbulence in a two-dimensional Taylor-Couette geometry using numerical solutions of the Oldroyd-B model, also performed at high Weissenberg numbers with the program OpenFOAM. Beyond a critical Weissenberg number, an elastic instability causes a supercritical transition from the laminar Taylor-Couette to a turbulent flow. The order parameter, the time average of secondary-flow strength, follows the scaling law Φ ∝ (Wi − Wic) γ with Wic = 10 and γ = 0.45. The power spectrum of the velocity fluctuations shows a power-law decay with a characteristic exponent, which strongly depends on the radial position. It is greater than two, which we relate to the dimension of the geometry.

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“…Purely elastic instabilities manifest as spatiotemporal chaotic flow and elastic turbulence [3,4] in a wide range of natural and industrial applications: Elasticity generates secondary flows of DNA and blood suspensions in biological systems [5,6], hydrodynamic resistance increases [7] along with power consumption and cost in polymer processing, and elastic instabilities enhance mixing and dispersion in microfluidic and porous media flows [8][9][10]. Experimental [11][12][13][14][15] and numerical [16][17][18] efforts have characterized the onset and impact of elastic instabilities in well-defined geometries including cross slot [13,17], Couette [19,20], Poiseuille [8,21], and ordered pillar array flows [12,22,23]. In contrast to the high degree of symmetry in such systems, how geometrical disorder affects the onset of elastic instability remains an open question.…”

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confidence: 99%

Disorder Suppresses Chaos in Viscoelastic Flows

2020

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Viscoelastic flows transition from steady to time-dependent, chaotic dynamics under critical flow conditions, but the implications of geometric disorder for flow stability in these systems are unknown. Utilizing microfluidics, we flow a viscoelastic fluid through arrays of cylindrical pillars, which are perturbed from a hexagonal lattice with various degrees of geometric disorder. Small disorder, corresponding to ∼ 10% of the lattice constant, delays the transition to higher flow speeds, while larger disorders exhibit near-complete suppression of chaotic velocity fluctuations. We show that the mechanism facilitating flow stability at high disorder is rooted in a shift from extension-dominated to shear-dominated flow type with increasing disorder.

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Purely-elastic flow instabilities and elastic turbulence in microfluidic cross-slot devices

Abstract: Strong extensional flows of viscoelastic fluids generate purely-elastic instabilities and elastic turbulence at high Weissenberg numbers.

Cited by 57 publications

References 31 publications

Control of a purely elastic symmetry-breaking flow instability in cross-slot geometries

Control of a purely elastic symmetry-breaking flow instability in cross-slot geometries

Elastic turbulence in two-dimensional Taylor-Couette flows

Disorder Suppresses Chaos in Viscoelastic Flows

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