Numerical Simulation Of Electroosmotic Flow And Mixing Driven By AC Electric Field In Microchannels

The microfluidic chip can integrate many functions of analysis Lab, which has favorable application prospect and it has become one of focuses of MEMS. Fluid transfer and mixing in microchannels as two important operations, will directly affect the performance of microfluidic chips. In this paper, on the basis of analyzing internal and foreign situation of study, electroosmotic flow and mixing driven by AC electric field in microchannels were researched systematically, using numerical simulation methods.The main jobs and innovations of this paper as follows:Firstly, starting with the electrokinetic phenomena of microfluid, the formation mechanisms of electric double layer and electroosmotic flow were studied. The physical fields of electroosmotic flow and correlative equations were also analyzed. This paper solved the analytical solution of the potential distribution of electric double layer and electroosmotic flow speed in 2D microchannels with smooth surface based on Debye-Hiickel approximation. The analytical solution was compared with numerical simulative solution to validate the dependability of COMSOL Multiphysics.Secondly, electroosmotic flow driven by AC electric field in 2D smooth microchannel were simulated using COMSOL Multiphysics based on finite element method, and the electric field frequency, the electric field strength, the solution concentration and the microchannel height impact on electroosmotic flow were analyzed. The results show that the velocity distribution of alternant electroosmotic flow is "wave-like", the transformation of velocity does not keep pace with the transformation of electric field. With the increase of the electric field frequency, the "wave-like" velocity distribution is visible, and the slip velocity of electric double layer decreases. Especially, the slip velocity of electric double layer rapidly decreases when the electric field frequency exceeds 3000Hz. It means that the high frequency AC electric field will reduce the drive effect in microchannel. The velocity of electroosmotic and the slip velocity of electric double layer are proportional to the electric field strength, and nonlinearly decrease with the increasing of the solution concentration. The thickness of electric double layer is attenuation when the solution concentration and the microchannel height increase, and the wave crest of "wave-like" velocity distribution is more spiculate. So we can control the velocity and the velocity distribution of electroosmotic by changing factors above. It provides theoretical reference for accurately controlling the electroosmotic flow driven by AC electric field in microchannels.Thirdly, the electroosmotic flow driven by AC electric field was modeld in 2D hydrophilic and hydrophobic microchannels. The results show that the hydrophobic microchannel is different from the hydrophilic microchannel, because of slip velocity on the walls of hydrophobic microchannel, the velocity of electroosmotic and the slip velocity of electric double layer increase obviously, and they are proportional to the slip length. With increasing of the solution concentration, the slip velocity of electric double layer decrease firstly, then increases, because the increasing of slip velocity on boundary is the main influence factor of the slip velocity of electric double layer. The productons above make the research on electroosmotic flow more comprehensive.Finally, a method for initiative enhancing fluid mixing in microchannels was proposed. The electric field frequency, the electric field strength, the quantities of electrodes and the electrodes polarity impact on micromixing were studied using numerical simulation. The results indicate that the AC electric field which is perpendicular to the axial of microchannels can induce chaos flow effectively and improve the mixing efficiency in microchannels. Overall, the mixing efficiency is on the increase with increasing of the electric filed strength and the electric field frequency, however, as the frequency of 4-8Hz, the mixing efficiency at outlet of microchannels is unstability, it has greater volatility. While the quantities of electrodes on the walls of microchannels increase, the mixing efficiency decreases originally, then increases. Compared with the electrodes have the same polarity, when the electrodes have the different polarity, the mixing efficiency increases obviously. According to the regularities of these influencing factors impact on micromixing, the parameters of model were optimized. This paper obtains perfect mixing effect which can make the mixing efficiency relatively remain stable at 99% after 0.5 seconds. The mixing effect increased by 46% than no AC electric field. It offers a new way for enhancing the mixing efficiency of the fluid in microscale, and provides theoretical reference for optimization design of micromixer.