Abatement of mixing in shear-free elongationally unstable viscoelastic microflows

Abstract

The addition of minute amounts of chemically inert polyacrylamide polymer to liquids results in large instabilities under steady electro-osmotic pumping through 2 : 1 constrictions, demonstrating that laminar flow conditions can be broken in electro-osmotic flow of viscoelastic material. By excluding shear and imposing symmetry we create a platform where only elongational viscoelastic instabilities, and diffusion, affect mixing. In contrast to earlier studies with significant shear that found up to orders of magnitude increase in mixing we find that inclusion of polymers excites large viscoelastic instabilities yet mixing is reduced relative to polymer-free liquids. The absolute decrease in mixing we find is consistent with the understanding that adding polymer increases viscosity while viscoelastic flows progress towards elastic turbulence, a type of mild (Batchelor) turbulence, and indicates that electro-osmotic pumped devices are an ideal platform for studying viscoelastic instabilities without supplementary factors.

Electroosmotic flow of non‐Newtonian fluids in a constriction microchannel

“…Bryce and Freeman [55] observed an unstable electroosmotic flow of polyacrylamide (PAA) solution through a constriction microchannel when the DC electric field reaches a threshold value. This extensional instability was, however, found [56] to not really enhance the electroosmotic mixing. Lu et al [57] reported an unexpected oscillation of polystyrene microparticles that travel along with the electroosmotic flow of polyethylene oxide (PEO) solutions through a constriction microchannel.…”

“…Bryce and Freeman observed an unstable electroosmotic flow of polyacrylamide (PAA) solution through a constriction microchannel when the DC electric field reaches a threshold value. This extensional instability was, however, found to not really enhance the electroosmotic mixing. Lu et al.…”

A coupled finite-volume solver for numerical simulation of electrically-driven flows

“…The second and last test case is the electroosmotic flow in a contraction/expansion device. This flow is numerically challenging due to the singularities developed at the reentrant corners and it also has experimental relevance [21][22][23]. The characteristics of the two grids used to discretize the computational domain are listed in Table 5.…”

“…This work addresses the implementation of a coupled solution method for EDF in order to improve the numerical stability and temporal accuracy of the segregated solver that we previously implemented in OpenFOAM ® [15], and that has been incorporated in rheoTool [20]. The coupling algorithm is developed considering Newtonian fluids and also more complex viscoelastic fluid models, which can show unusual behavior in EDF [21][22][23][24]. In addition, we also formulate and test the solver with the Poisson-Boltzmann 4 (PB) model, which is often used in replacement of the PNP system of equations [15,24,25].…”

Electro-osmosis of viscoelastic fluids and prediction of electro-elastic flow instabilities in a cross slot using a finite-volume method

“…Electrokinetic forcing can lead to the onset of flow instabilities, called electrokinetic instabilities. When combined with viscoelastic fluids, new instability phenomena arise, as recently discovered by Bryce and Freeman [34]. However, a sound understanding of the driving mechanism of these electro-elastic instabilities is still missing.…”