  When two fluids of different electrical conductivities are transported side
by side in a microfluidic device under the influence of an electric field, an
electrokinetic instability (EKI) is often generated after some critical values
of the applied electric field strength and conductivity ratio. Many prior
experimental and numerical studies show that this phenomenon results in a
chaotic flow field inside a microdevice, thereby facilitating the mixing of two
fluids if they are Newtonian in behaviour. However, the present numerical study
shows that this chaotic convection arising due to the electrokinetic
instability can be suppressed if the fluids are viscoelastic instead of
Newtonian ones. In particular, we observe that as the Weissenberg number (ratio
of the elastic to that of the viscous forces) gradually increases and the
polymer viscosity ratio (ratio of the solvent viscosity to that of the
zero-shear rate viscosity of the polymeric solution) gradually decreases, the
chaotic fluctuation inside a T microfluidic junction decreases within the
present range of conditions encompassed in this study. We demonstrate that this
suppression of the chaotic motion occurs due to the formation of a strand of
high elastic stresses at the interface of the two fluids. We further show that
this suppression of the chaotic fluctuation (particularly the span-wise one)
inhibits the mixing of two viscoelastic fluids. Therefore, one needs to be
cautious when the EKI phenomenon is planned to use to mix such viscoelastic
fluids. Our observations are in line with that seen in limited experimental
studies conducted for these kinds of viscoelastic fluids.
