PERIODICAL PRESSURE-DRIVEN FLOWS IN MICROCHANNEL WITH WALL SLIP VELOCITY AND ELECTRO-VISCOUS EFF

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Available online a w'ww.SC ncedi删.oom ~ Science Direct 

Journal of HydrOdynal11 es 2010,22(6):829—837 DOI:10.1016/S1001—6O58(09)60123—2 

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PERIoDICAL PRESSURE.DIUVEN FLoWS IN MICRoCHANNEL WITH WALL SLIP VELoCITY AND ELECTRo VISCoUS EFFECTS 

WANG Lei.WU Jian—kang Department of Mechanics,Huazhong University of Science and Technology,National Laboratory for Optoelectronics,Wuhan 430074,China,E—mail:wangleisabrina@yahoo.com.cn 

(Received October 27,2009,Revised December 25,2009) Abstraet:In a microfluidic system.the flow slip velocity on a solid wall can be the same order of magnitude as the average velocity in the microchanne1.The flow—electricity interaction in a complex microfluidic system subjected to a ioint action of wal】slip and electro—viscosity is an important topic.An analytical solution for the periodical pressure—driven flow i11 a two—dimensional uniform microchanne1.with consideration of wall slip and electro.viscous effect is obtained based on the Poisson—Boltzmann equation for the Electric Double Layer fEDL、and the Navier.Stokes equations for the liquid flow.The analytic solutions agree well with the numerical solutions.The analytical results indicate that the periodical flow velocity and the Flow—Induced Electric Field(FIEF) strongly depend on the frequency Reynolds number(Re:(oh / 1,that is a function of the frequency,the channel size and the kinetic viscosity of fluids.For Re<1.the flow velocity and the FIEF behave similarly to those in a steady flow.whereas they decrease rapidly with Re as Re>1.In addition.the electro—viscous effect greatly influences the periodical flow velocity and the FIEF.particularly。when the electrokinetic radius H is smal1.Furthermore,the wall slip velocity amplifies the FIEF and enhances the electro—viscous effect on the flow. 

Key words:electrokinetic flow,frequency Reynolds number,wall slip,electro—viscous effects,Flow—Induced Electric Field(FIEF) 

1.Introduction The rapid development of microfluidics in MEMS,Biochips and Lab—On.a.Chip fL0C1 devices requires a good understanding about the microscopic flow behaviors【 .The flow behaviors in microchannels can be significantly affected by the electrica1 charges whether on the channel wall or in the fluidsL ’ .Particularly.most solid surfaces would carry electrostatic charges when in contact with electrolyte solutions,which will interact with the counter ions in liquids.Specifically.an Electrical Double Layer(EDL)can be formed near the solid surface as the result of interactions between solid surface charges and counter ions in the fluids. Project supported by the National Natural Science Foundation of China(Grant No.50805059) Biography:WANG Lei(1 983一),Female,Ph.D.Candidate C0rresp0nding author:WU Jian—kang, E—mail:wujkang@mail.hust.edu.cn Moreover.the counter ions in the diffuse layer of the EDL can move in the fluids when the fluids move through a microchannel under an applied hydrostatic pressure gradient.This creates an electric field called the Flow.Induced Elec仃ic Field rFIEF) J.The FIEF builds up in microchanne1s and drives the counter ions in the diffuse layer of the EDL to move in a direction opposite to the flow.Consequently。the overall flow rate will be reduced and the liquid will have a higher apparent viscosity,as is referred as the electro—viscous effect_oJ|The electro—viscous effect in a steady flow was studied both theoretically and experimentally ’ . The periodical pressure—driven flow and the FIEF in microchannels were also studied In all these studies ,the non—slip boundary condition was specified on the channel wal1.which may not be appropriate in some cases of microscopic flows.In macroscopic flows,the non—slip boundary condition on wall is widely accepted.A number of recent studies ’ J found evidences of slip velocities for 

一 830 liquid flows over hydrophobic walls(say PDMS polymer materials).The walI slip velocity for liquid flows was first proposed by Navier jn 1 823.generally expressed as u 一 “/ .The parameter is the slip length ranging from several micrometers to several microns,and is assumed to be a property parameter of the wal1 material and the working fluid. The ratio of the wall slip velocity to the average velocity is about 0( / )Ll 3J,the ratio of the slip length to the flow length scale fsay the channel width).The wall slip may play a significant role in the flow-electricity interaction in a microfluidic system.Finite difference method was used to investigate the flow behavior in a rectangular microchannel with consideration of wall slip and electro—viscous effects【J/1.The eflfect of wall slip on the electrokinetic energY conversion in a microfluidic system was examined based on the classica1 Onsager relationL“J.However.it is not clear how the wal1 slip affects the periodical flow.electricity interaction in a microfluidic system with electro— viSCOUS ef.fecC.上n order to better understand the f1ow behavior in a microfluidic system,this article presents a possible analytic solution for the periodical pressure--driven flows in a two--dimensional uniform microchannel with consideration of wall slip and electro—viscous effects. 2.Problem formulation A two—dimensional uniform microchanne1 with an isolated wall js schematically shown in Fig.1. where H is the half width of the channe1.Because of the symmetry,only the half region of the microchannel will be considered. Fig.1 Schematic representation of a two—dimensional uniform microchannel 2.1 Governing equations and boundary conditions For a symmetric binary electrolyte solution,both the electrical potential and the net charge density Pe are described by Poisson-Boltzmann equation V ——2noze sinn( ] (1) 一占 。 2 ze sinh (z e  ̄/・) (2) where and z are the bulk ionic concentration and the valence of ions.respectively.e is the elementary charge. is the dielectric constant of the solution, is the Boltzmann constant,and T is the absolute temperature.Due to the symmetry,the boundary conditions related to Eq.f1)are as follows