Numerical Simulation Of Microfluidic Mixing Under The Action Of Electric And Sound Field

The microfluidic system has a small size and good portability,which can reduce the operating area in the laboratory,energy consumption and reagent dosage.It has wide applications in materials research,pharmaceutical medicine and chemical preparation.However,laminar flow is dominant in a microfluidic channel because of the low Reynolds number,so it is difficult to mix the fluids in the microchannel quickly and effectively.In order to solve this problem,this paper mainly studies the effects of three kinds of active control methods,such as applied electric field,sound field driving bubble and ultrasonic standing wave,on fluid mixing in microchannel.The details are as followed:In chapter two,the mixing efficiency of fluids with different concentrations under applying DC signal or AC signals of four potential waveforms on the wall in two-dimensional microchannels were studied by finite element analysis.The velocity distribution and concentration distribution of the mixed fluids under DC and sinusoidal alternating current and the mixing efficiency with frequency under the four potential waveforms are analyzed.It has been found that when a direct current signal is applied to the electrode plates,the longitudinal electric field force generated in the microchannel induces disturbance of the fluid and mixing.When sinusoidal alternating current signal is applied to the electrode plates,two vortices with opposite rotation directions and periodically varying in size are formed in the vicinity of the electrode plates,therefore the flow instability between different fluids is greatly increased,which enhances mixing effect.In chapter three,the effect of acoustically actuated bubbles on fluid mixing is studied.The flow characteristic of micro-scale fluid under surface acoustic waves is explored.The flow situation and mixing efficiency of different microchannel height,inlet flow rate,bubble distance and arrangement are analyzed.The results show that the fluid pressure change caused by bubble under acoustic actuation would have better mixing in fluids when the microchannel height is low.When the inlet flow rate is small,the time that the fluid is disturbed near the bubble would longer,the mixing efficiency could higher.When the inlet flow rate increases,shorter time for fluid to flow through the microchannel,the two fluids change flow direction only near the bubble,and no mixing occurs.When the bubble radius is large,the vortex disturbance is enhanced and the mixing efficiency is improved.The mixing effect of two bubbles on the fluid is much greater than that of one single bubble,and the distance between the bubbles has no effect on the mixing efficiency.In chapter four,the acoustic flow phenomena of cross-section in two-dimensional microchannels are studied by using Reynolds stress method and limiting velocity method.It is found that both methods can well describe the classical Rayleigh streaming pattern,but the limiting velocity method uses a uniform mesh with a few numbers of meshes.Therefore,this method is used to explore the three-dimensional microchannels with different aspect ratios.It is found that the similar acoustic pressure distribution in two different three-dimensional microchannels.When h/w=1/20,four symmetric streaming vortexes are formed on the x-y cross-section in the microchannel under ultrasonic drive.When h/w=1/3,eight symmetrical acoustic vortices are created in the y-z cross-section of the microchannel.Eight symmetric vortices are generated in the Rayleigh-like acoustic streaming model,while four symmetric vortices are generated in the flat acoustic flow model.However,the mixing efficiency of the two models is close.the solution flow in the Rayleigh-like microchannel is seven times than that of the transducer-plane streaming model,therefore the turbulence of the Rayleigh-like acoustic streaming model is stronger.