Parametrical Study On Electrokinetic Micromixer Based On Laser Induced Fluorescence

Microfluidic technology is the technology that manipulates or processes small volume fluid on microscale.It has a wide range of applications and high development potential in the fields of chemical industry,biology,medicine,environment and energy,etc.Micromixer is one of the core components of microfluidic system and experiences rapid development in recent years.To realize rapid,high-throughput,controllable mixing,many methods have been developed.However,in conventional micromixers,the mixing was primarily realized by laminar or chaotic flow.The mixing is relatively slow.Recent researches have shown that turbulence could be generated electrokinetically in a "Y" type micromixer with side walls as electrodes at low Reynolds number,to significantly enhance mixing.Nevertheless,previous reports have not systematically studied the influence of many important physical and geometrical parameters,e.g.intensity and frequency of AC electric field,electric conductivity ratio and geometrical dimensions of micromixer,on mixing effect,scalar transport process of scalar turbulence and flow transition conditions,etc.In this investigation,based on laser induced fluorescence flow visualization technique,we systematically studied the influence of aforementioned physical and geometrical parameters on the mixing effect,the scalar transport process of scalar turbulence and the flow transition conditions of the electrokinetic flow in the micromixer.1.The mixing effect evaluated by degree of mixedness has been investigated in a relatively large parameter space to find the optimal mixing conditions of the electrokinetic turbulence micromixer.It is found the optimal parameters are 100 kHz and 1.14×105 V/m AC electric field,1:3000 conductivity ratio and 350 μm wide electrokinetic turbulence micromixer.With these parameters,the mixing efficiency and the mixing coefficient are the highest.A 0.93 mixing index can be achieved at 84 μm away from the entrance,and the mixing time is 0.1 s.2.By analyzing the critical mixing length between the linearly increasing mixing region and the nonlinear saturation mixing region at different electric Rayleigh number Rae,the universal expression of critical mixing length has been preliminarily revealed.It is found that the dimensionless critical mixing length(xc*)is decreased with Rae in power-law relationship.In the 350 μm wide micromixer,xc*～43.83Rae-0.2942.In the 650 μm wide micromixer,xc*～13.38Rae-0.2619.3.The scalar transport process in scalar turbulence has been investigated by analyzing the influence of the above parameters on the power spectra density of concentration.It is found that:(1)Obukhov-Corrsin-5/3 spectra have been observed in both 350 μm and 650 μm wide electrokinetic turbulence micromixer,in a broad parameter space.Compared to that in the 350μm wide electrokinetic turbulence micromixer,the-5/3 spectra measured in the 650 μm wide electrokinetic turbulence micromixer show wider bandwidth towards the lower wavenumber region.This is consistent with the cascade theories of kinetic energy and scalar variance of electrokinetic turbulence advanced by Zhao and Wang.The observation of-5/3 spectra from the spatial fluctuation of concentration supports the existence of inertial subrange in this electrokinetic scalar turbulence.One more time,it shows the feature of high Reynolds number turbulence can be realized via electrokinetic turbulence in microchannel flow.(2)With the increase of the intensity and frequency of AC electric field,we surprisingly observed a-7/3 spectra of concentration over a wide region.This spectral slope has never been theoretically predicted.It indicates there exist some unknown and complicated scalar transport process in electrokinetic scalar turbulence to be investigated in future.4.We study the critical electric field(Ec)and the critical electric Rayleigh number(Rae,c),corresponding to the transition of flow from a steady state to an unsteady state.Rae,c is also the critical parameter for linear electrokinetic instability.The results show that with the increase of AC electric field frequency,Ec and Rae,c both decrease gradually.There is an approximately linear relationship between Rae,c and AC electric field frequency.Interestingly,although a smaller Rae,c implies the electrokinetic flow can have stronger linear instability under a lower electric field,it is insufficient to induce a fast and uniform mixing under a higher electric field.This means,to predict the optimal parameter of an electrokinetic turbulence micromixer,applying linear electrokinetic instability theory is insufficient.The investigations on the mixing effect,the scalar transport process of scalar turbulence and the flow transition conditions of the electrokinetic flow in the micromixer can not only promote the application of the electrokinetic turbulence micromixer and its industrialization,but also reveal the difference between electrokinetic turbulence and other turbulence counterparts.The investigations can deepen our understanding of electrokinetic turbulence,and promote researches on the universality of turbulence.