Studies Of Liquid Mixing Based On Surface Acoustic Wave Technology

The microfluidic systems have the advantages of a smaller size and portability.They can reduce the unnecessary volume of the entire system,and can also reduce the consumption of both energy and reagents,hence they have been widely used in biochemical research,medical diagnosis,chemical synthesis,and so on.However,the Reynolds number of the fluid in such systems is so low that it becomes difficult to mix different fluids.To solve this problem,several methods have been proposed,among which acoustic techniques have attracted more and more attention owing to their perfect biocompatibility and non-invasiveness.In this thesis,we studied how surface acoustic wave(SAW)can efficiently accelerate liquid mixing.The main contents can be divided into three parts:(1)COMSOL software was used to simulate the distribution of acoustic pressure inside the liquid as the surface acoustic wave on the substrate penetrate into the fluid.(2)With experiments,on the one hand,we studied the mixing efficiency of different fluids under the action of acoustic streaming driven by the surface acoustic wave.On the other hand we compared the mixing efficiency of the fluids when the interdigital transducer(IDT)substrate is inserted into the liquid with different inclination angles.The thesis is divided into four chapters:In Chapter One,we introduce the background of the microfluidic systems and describe the advantages of acoustic techniques compared to dielectrophoresis,magnetism,and optical techniques when applied in the microfluidic systems.In Chapter Two,we describe how the IDT work and how to set up the device that drives surface acoustic wave.With the chosen parameters,we simulate the acoustic field inside the fluid with the software COMSOL,using the "pressure-acoustic,transient" module.We obtain the evolution of the pressure distribution inside the fluid when surface acoustic wave on the substrate penetrate into the fluid.In Chapter Three,we performed a series of experiments by changing the driving voltage of the interdigital transducer,the shape of the interdigital transducer,and the inclination angle of the interdigital transducer substrate inside the fluid to see how they affect the mixing efficiency of the fluids.It is revealed that when the driving voltage reaches a threshold,an Eckart flow is formed inside the fluids,which can efficiently accelerate the mixing of the fluids.With the same driving voltage,the focused interdigital transducer can generate stronger acoustic streaming,hence the mixing timescale is significantly reduced.It is also found that,when the single-phase unidirectional transducer(SPUDT)is used,the mixing time of the fluids can be reduced by up to 92%.For lower viscosity liquids,the mixing efficiency by use of the focused interdigital transducer is increased by almost 100%compared to the single-phase unidirectional transducer.When changing the inclination angle of the interdigital transducer substrate inside the fluids,we found that the mixing efficiency reaches maximum when the inclination angle is 45°,irrespective of the interdigital transducer type.Finally,in Chapter Four,we give a summary of the thesis and the outlook for future work.