Electrokinetic Flow Study Of Microfluidic System With Wall Slippage

Microfluidic flow control is one of the key technical problems to make biochemical analysis of microfluidic chips accomplished and optimized. A deep understanding of micro-flow behavior is of great significance for effectively guiding microfluidic design and development. Flow behavior in microfluidic system is different from that in macroscopic system. And micro-scale effect, electric double layer of liquid-solid interface, wall slippage and multi-physics interaction play an important role in microflow of microfluidic system. This work.mainly studies electrokinetic flow in microfluidic system subjected to wall slippage and electric effects by employing theoretical analysis and numerical solutions. Micro-electrokinetic flow in microchannel made of different materials (polymer PDMS and GLASS) is also investigated. This work includes following studies:(1) By employing theoretical analysis and numerical solutions, this study investigates effect of solid wall slip and electric effect on pressure-driven flow behavior in two-dimensional microchannel. Both analytic solution of Poisson-Boltzmann equation and numerical solution of Poisson-Nernst-Planck equation are presented and made comparison. The results indicate that electro-vicous effect decreases flow velocity, flow rate and flow-induced electric field, while wall slip inceases flow velocity and amplifies electro-vicous effect. The solutions of Poisson-Boltzmann equation and Poisson-Nernst-Planck equation are in good agreement in the cases without either wall slip or electro-viscous effect. In the case of considering both wall slip and electro-viscous effect,the solutions of Poisson-Nernst-Planck equation is smaller than that of Poisson-Boltsmann equation due to difference of physical meaning between P-B equation and P-N-P equation. The solution difference increases with the increase of wall slip length and electro-viscous number.(2) This work presents an analytic solution of pressure-driven liquid flow velocity and flow-induced electric field in a two-dimensional microchannel made of different materials (PDMS-GLASS) with wall slip and electro-viscous effects. The Poisson-Boltzmann equation and the Navier-Stokes equation are solved for the analytic solutions. The analytic solutions agree well with the numerical solutions. The study found that the wall slip amplifies flow-induced electric field and enhances electro-viscous effect on flow. Thus electro-viscous effect can be significant in a relatively wide microchannels with relatively largeκh, the ratio of channel width to thickness of electric double layer, in comparison with the channel without wall slip.(3) This work presents an analytic solution of pressure-driven liquid flow velocity and flow-induced electric field in a rectangular microchannel made of polymer PDMS and GLASS with wall slip and electro-viscous effects. The Poisson-Boltzmann equation and the Navier-Stokes equation are solved for the analytic solutions. The analytic solutions well agree with the numerical solutions. The study results indicate that wall slip increases flow velocities in microchannel, the electro-viscous effect decreases flow velocities. The slip velocities on wide wall are larger than those on narrow wall. It is also found that the wall slip amplifies flow-induced electric field and enhances electro-viscous effect on flow.(4) An analytical solution for periodical pressure-driven flow in a two-dimensional uniform microchannel, with wall slip and electro-viscous effect is presented based on the Poisson-Boltzmann equation for electric double layer (EDL) and the Navier-Stokes equations for liquid flow. The analytic solutions agree well with the numerical solutions. The analytical results indicate that the periodical flow velocity and flow-induced electric field (FIEF) strongly depend on the frequency Reynolds number (Re=ωh2/v that is a function of the frequency,the channel size and the kinetic viscosity of fluids). For Re< 1, the flow velocity and FIEF behaves similarly to that of steady flow, whereas decreases rapidly with Re as Re>1. In addition, the electro-viscous effect greatly influences the periodical flow velocity and FIEF, particularly when the electrokinetic radiusκH is small. Furthermore, wall slip velocity amplifies FIEF and enhances electro-viscous effect on flow.