Study On Numerical Simulation Of Electroosmotic Flow In Trapezoidal Microchannels

The microfluidic chip based on the MEMS (Microelectro-mechanical Systems)technology is the main direction of research in the μ ?TASsystem and causestremendous impact on the analytical science. During these recent years, themicrofluidic chip has presented expansive development trend in the domain ofsample separation, chemical analysis and biomedicine and its influence will beobviously strengthened.With the rapid development of mircofluidic chip, the electroosmotic flow(EOF) has been extensively used as the driving force to manipulate the liquid flowand transportation in many microfluidic devices. As a potential micropump, theelectroosmotic flow is significant in the domain of microfluidic chip due to itsoutstanding merits, especially popular in the application of capillaryelectrophoresis for DNA sequencing.In this paper, the electroosmotic flow in trapezoidal microchannels issimulated by the CFD software. The main contents of this paper are following:In the first part of this paper, the development background of microfluidic chipis stated and the advantages of electroosmotic flow as micropump is analyzed. Alsothe researching actuality of electroosmosis with different section figure issummarized in this part. Though a great deal of information about electroosmoticflow in microchannels of various geometry have been found in the literature, butthe actual geometry of microchannels is always trapezoidal which is not regularand the research in this domain is not specific. So, having summarized thedevelopment situation about electroosmotic flow, this paper confirms theresearching object and focuses on the simulation of electroosmosis in trapezoidalmicrochannels.In the second part, the foundation of numerical simulation for electroosmoticflow in trapezoidal microchannels is introduced and the reasons forming electricaldouble layer (EDL) and electroosmosis are presented. Also the mathematicalmodels about EOF are analyzed.Most solid surfaces obtain electric charges when contacting with theelectrolyte solution and the electrostatic charges will influence the distribution ofions in the electrolyte solution near the solid surfaces. Then the redistribution of theelectric charges and the ions forms the EDL. In the research of macrochannel flow,the influence of EDL is often ignored because it is much thinner comparing withthe size of channels. But when the dimension of the channel is small enough, evenat the magnitude of micro or nanometer, the effect of EDL is obvious and cannot beignored.Electroosmotic flow is induced by an applied external electrical field and EDLWithin the diffuse layer, the net charge density is not zero. If an electric potential isapplied along the channel, the body force will be exerted on the ions in the diffuselayer making them move to the opposite electrode. The moving of ions willinfluence the liquid near the solid surface and pull the liquid to move with them.Also, the liquid at the inner of microchannel can be moved by the viscid force ofthe liquid and results in the electroosmotic flow. Obviously, the electroosmoticflow is the whole motion of liquid relative to the stationary surface.Electroosmotic flow induced by an applied external electric field is animportant electrokinetic phenomenon and is quite different from the liquid flowdriving by the external pressure. The velocity profile of the full developedelectroosmotic flow is flat like a plug, whereas the pressure-driven flow is typicallyparabolic shape. The flat velocity profile will not broaden the sample section andbe propitious to the sample separation.The theoretical model of electroosmotic flow in microchannels mainlyincludes three aspects, which are the analysis of EDL field, applied external electricfield and velocity field. They can be expressed respectively by thePoisson-Boltzmann equation, the Laplace equation, the continuous equation andthe modified Navier-Stokes equation which include the body force caused by theapplied external electric potential and the net charges density in the EDL. Theseequations are strongly coupled with each other.There are many factors that can influence the electroosmotic flow includingthe effects of electrolyte component and concentration, the pH value of electrolyte,the applied external electric potential, the material of microchannels and thetemperature. In the domain of microfluidic chip, the electroosmotic flow is anelementary operation, and it is necessary to control it taking the above factors intoaccount. In allusion to these factors, this paper simulates the electroosmotic flow intrapezoidal microchannels.In the third part, the electroosmotic flow in trapezoidal microchannels issimulated in detail using CFD software. The characteristic of electroosmotic flowunder different circumstances is analyzed comparing with the electroosmotic flowin rectangular microchannls. The influences of microchannel geometry to thevelocity, the average velocity, the volumetric flowrate and the induced pressure arediscussed.Firstly, in the same microchannel with uniform zeta potential, the velocity ofelectroosmotic flow increases when the concentration of electrolyte decreases, also,the velocity increases with the applied external electrical field and the velocity isplug-like. Compared with the electroosmotic flow in rectangular microchannelswith the same area under the same circumstances, the average velocity andvolumetric flowrate of electroosmotic flow in trapezoidal microchannels are allsmaller, but the disparity is quite small. In the trapezoidal microchannels, theaverage velocity and volumetric flowrate increase when the ratio ofbottom-substrate and upper-substrate, i.e. Wb / Wa, increases, but the disparity is notobvious.Secondly, when the materials of the bottom-substrate and upper-substrate aredifferent, the zeta potentials are often different. Under this circumstance, thevelocity of electroosmotic flow often deviates from the plug-like profile. In thevicinity of zeta potential that is higher, the velocity is higher. Conversely, thevelocity is smaller near the zeta potential that is smaller. So the velocity is notequal in the microchannels. When keeping the area of microchannel unchanged, theaverage velocity and volumetric flowrate increase with the ratio of Wb / Waincreases and the disparity is also small.Thirdly, electroosmotic flow in heterogeneous microchannels is also discussedin this paper. When the zeta potential is nonuniform, the velocity profile is changedthrough the microchannels. When the zeta potential is step-changed, the velocityprofile is distorted a lot. There is an induced pressure in the microchannel with thenonuniform zeta potential. In the zeta-existing region, a positive pressure gradientis generated, which decreases the velocity and leads to a distortion of the velocityto be a cupped profile. Conversely, in the non-zeta region the pressure gradient isnegative which increases velocity and causes the velocity profile to assume aparabolic shape. i.e., similar to that noted in pressure-driven flow.When we change the parameter Wb with W aunchanged, the velocity and theinduced pressure are also influenced. The induced pressure increases with the ratioof Wb / Wa decreased. So the cupped profile and the parabolic profile are extended,even the backflow phenomenon is found in the zeta-existing region.In the fourth part, the research results are summarized and the futuredeveloping trends are also prospected. The research will enrich the fundamentaltheory of microfluidic chip, and can be used to designing the relative product, suchas microfluidic chip etc., especially to optimizing the design parameters forelectroosmotic micropump.