Electroviscous effects in electrolyte liquid flow through an oppositely-charged contraction-expansion microfluidic slit device

Electrokinetic flows in microchannels with opposite charge asymmetry, i.e., unequal and contrasting surface charges on opposing channel walls, significantly influence microfluidic hydrodynamics and can be exploited for enhanced control of mass transfer, mixing, and heat transport in microfluidic applications. This study numerically investigates electroviscous flow of a liquid electrolyte through an oppositely charged non-uniform microslit. The governing Poisson, Nernst-Planck, and Navier-Stokes equations are solved using the finite element method to determine the coupled electrokinetic fields for a wide range of dimensionless parameters: Reynolds number (Re = 0.01), Schmidt number (Sc = 1000), inverse Debye length (K = 2-20), top-wall surface charge density (St = 4-16), surface charge density ratio (Sr = -2 to 0), and contraction ratio (dc = 0.25). For completeness, results for like-charged devices, where both walls carry surface charges of the same sign (Sr = 0-2), are also included. The results show that maximum enhancement in total electrical potential (|{\Delta}U|) and pressure drop (|{\Delta}P|) reaches 296.82% at K = 20, St = 4, -2 \le Sr \le -1.25, and 14.57% at St = 16, Sr = 0, K = 2-20, respectively. The electroviscous correction factor Y exhibits maximum increases of 14.02% at K = 2, St = 16; 11.81% at K = 2, Sr = 0; and 14.57% at Sr = 0, St = 16, when Sr increases (-0.75 to 0), St increases (4 to 16), and K decreases (20 to 2), respectively. The overall maximum increment in Y is 15.13% at K = 2, Sr = 0, St = 16, relative to the non-electroviscous flow case. These findings demonstrate that opposite charge asymmetry, though weaker than similar asymmetry, still strongly influences electroviscous flow in non-uniform microchannels.