Pressure driven transport of neutral macro-solute in microchannel with porous wall at high surface potential

Abstract

Mass transport of macro-solutes under streaming potential in a microchannel with porous wall is presented in this study. The streaming field under high wall potential and overlapping electric double layer with coupled velocity and concentration of neutral solutes is computed numerically in this work. Streaming potential increases by 7 folds as κH decreases from 0.7 to 0.1 at ζ=-50mV, where, κ-1 is Debye length, H is channel half height and ζ is wall zeta potential. Mass transport in terms of Sherwood number is calculated including osmotic effect of the solution and selective retention of macrosolutes. Electroviscous effects resulting from streaming potential and overlapping electric double layer at higher wall zeta potential have significant effect on velocity field and the mass transport of macrosolutes. Apparent viscosity due to this effect increases upto 3.2 times the solution viscosity at κH=0.2 for ζ=-50mV. This effect is intensified upto 4 times at higher wall potential, ζ=-200mV at κH=0.5. At these points, the velocity profile suffers the most adversely affecting the mass transfer reducing Sherwood number to 4 (about 5 times reduction compared to higher κH) at ζ=-200mV and κH=0.5. Sherwood number reduces by three to four folds at the critical κH values, representing the minimum. The induced electrical field due to the streaming potential is higher compared to axial pressure drop for |ζ|>50mV at κH<0.3. The electric double layer overlaps for the values of κH upto 0.7 corresponding to ζ=-50mV. In case of high ζ (−200mV), the centerline potential can be as high as 75% of the wall potential for κH=1.5. The minimum wall Peclet number (Pew‾) occurs at κH=0.5 for ζ=-200mV, which is five times less than the situation without electrokinetic effects.