Visualization of polymer relaxation in viscoelastic turbulent micro-channel flow

Abstract

In micro-channels, the flow of viscous liquids e.g. water, is laminar due to the low Reynolds number in miniaturized dimensions. An aqueous solution becomes viscoelastic with a minute amount of polymer additives; its flow behavior can become drastically different and turbulent. However, the molecules are typically invisible. Here we have developed a novel visualization technique to examine the extension and relaxation of polymer molecules at high flow velocities in a viscoelastic turbulent flow. Using high speed videography to observe the fluorescein labeled molecules, we show that viscoelastic turbulence is caused by the sporadic, non-uniform release of energy by the polymer molecules. This developed technique allows the examination of a viscoelastic liquid at the molecular level, and demonstrates the inhomogeneity of viscoelastic liquids as a result of molecular aggregation. It paves the way for a deeper understanding of viscoelastic turbulence, and could provide some insights on the high Weissenberg number problem. In addition, the technique may serve as a useful tool for the investigations of polymer drag reduction.

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

“…Earlier investigations focused on the onset of purely-elastic instabilities in shear flows with curved streamlines, such as the Couette flow between rotating cylinders 2 , 3 and the swirling flow between two plates, 4 both relevant in rheometry. More recently, extensionally-dominated flows have also been investigated with emphasis on contraction–expansion microgeometries 5 , 6 and stagnation point flows, such as the T-channel 7 and the cross-slot device. 8 – 12 Elastic instabilities occurring in these extensional flows are related with the molecular coil-stretch transition that occurs when the Weissenberg number (Wi) exceeds a critical value, Wi c ≈ 0.5.…”

Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging

“…Viscoelastic surfactants have been widely utilised in applications related to the oil and gas industry, such as drilling and reservoir stimulation, heating and cooling applications, as well as household, e.g. detergents, and personal care products 23 , 24 . The dilution of polymers in concentrations of parts-per-million in the base liquid has been demonstrated to reduce the levels of turbulence and hence fluid drag in single-phase flows 25 .…”

Lane change in flows through pillared microchannels

“…Non-Newtonian fluids sometimes exhibit time dependent fluctuations in their flow fields that are reminiscent of turbulence, yet they occur under conditions where Newtonian fluids (with equivalent viscosity) display steady laminar flow. [1][2][3][4][5][6][7] The fluctuations occur when polymers, or other mesoscale objects present in viscoelastic fluids, are unable to respond sufficiently fast to changes in the fluid velocity field, leading to an elastic response. To quantify the flow conditions of viscoelastic fluids, two non-dimensional numbers play a significant role.…”

“…Elastic instabilities have been observed by Poole et al 1 and Arratia et al 2 in cross-channel flow, by Pan et al 3 in long straight a) Author to whom correspondence should be addressed: J.T.Padding@ tudelft.nl microchannels with obstructions close to the inlet, and even in simple straight channels as reported by several researchers. [4][5][6] These observations have led to a number of numerical and theoretical works that try to reproduce or explain the instabilities. For example, Berti et al 7 analyzed the Lyapunov exponent to characterize elastic instabilities, Morozov and Van Saarloos 8 performed a nonlinear stability analysis for planar Couette flow, and Pakdel and McKinley 9 developed a dimensionless criterion that characterizes the critical conditions for the onset of elastic instabilities in (two-dimensional) viscoelastic flows.…”