Combined viscoelectric and steric effects on the electroosmotic flow in nano/microchannels with heterogeneous zeta potentials

The present study analyzes the combined viscoelectric and steric effects on the electroosmotic flow of Newtonian fluids in nano/microchannels. The governing equations that describe the flow field are the modified Poisson-Boltzmann equation for the electric potential, the mass conservation and momentum equations, which are solved numerically. On the channel walls, arbitrary and heterogeneous zeta potentials modulated via sinusoidal functions are imposed, being controlled by a phase angle, an amplitude and a wave number. The use of arbitrary zeta potentials leads to the study of the viscoelectric effect due to the fact that the viscosity near the walls of the channel increases drastically. Also, the presence of high ionic concentrations and large effective ionic sizes causes an excluded volume of the crowding of ions into the electric double layer; in this manner, the study of the steric effect is relevant in the present work. The results show that the viscoelectric effect produces a reduction in the magnitude of the velocity profiles when zeta potentials are magnified. On the contrary, the steric effect counteracts the aforementioned effect, increasing the velocity of the flow. In addition, the heterogeneous zeta potentials at the walls generate an induced pressure and recirculations on the flow. In particular, when the wave number is even leads to a condition with null volumetric flow rate; while for an odd wave number, a favorable volumetric flow rate is generated. This study extends the knowledge of electroosmotic flows under field effects for future mixing applications.