Integration And Application Of Surface Acoustic Wave Technique In Microfluidics

Taking the advantage of Micro Electronic Mechanical System (MEMS) technologies, the study on micro total analysis systems (uTAS) or lab on-a-chip systems (LOC) has gained constantly progress in the past 20 years. This new scientific discipline, which involves physics, chemistry, biology, machining, micro-electronics and even clinical medicine, aims at miniaturization and integration of traditional analysis equipments into one coin size micro-chip, and finally realization of portable device for analysis and individual diagnosis. Obviously, the scaling down the size of these analytic apparatus allows not only minimizing the consumption of samples and reagents but also enhancing the preparation of nano-materials, chemical synthesis as well as single cell analyses.Among many others, manipulation of microscale particles, fluids and droplets using LOC systems is an important task for numerous applications in advanced chemistry and biology, such as nanoparticle synthesis, blood circulation tumor cells detection, etc. Acoustically driven microfluidic actuation mechanisms, often considered a subset of acoustofluidics, have been explored in microfluidics, which have advantages as non-invasive, have high energy density, compatible with soft lithography micromachining, and work for near all kinds of microscale objects. However, such powerful driven mechanisms received little attention compared to the other mechanisms, such as electrokinetically driven mechanisms. Indeed, in microfluidics, low Reynolds number hydrodynamics essentially pose significant challenges in the low actuation speeds for micropumping and the difficulty in generating turbulent vortices for micromixing. So that, we believe it is significant to study the knowledge of behavior of small volume of liquid under the influence of acoustic wave, which can be contributed to the development of microfluidics.Accordingly, this thesis is organized as follows.In chapter one, we provide an overview of the research area and the main objective of this thesis work. Firstly, we recall the history of microfluidics and describe some basic concepts which would be used in our investigations, device fabrications and experiments. Then, we review the methods of SAW technique as well as the state of the art of the related research topics.In chapter two, we introduce the knowledge of surface wave and also the types of surface wave. Then, we discuss how to choose the piezoelectric materials which are used as the substrate of surface acoustic wave (SAW) devices. After that, the fundamental notions of interdigital transducers (IDTs) are presented which would be the most important part in SAW generation. Finally, we introduce SAW microfluidic devices fabrication process, including IDTs substrate production, microfluidic channels production and device encapsulation techniques.In chapter three, IDTs modeling results are presented by using software COMSOL multiphysics for the simulation of two kinds of SAW generation and propagation on piezoelectric substrate. Two types of IDTs models are compared, which show focused SAW (F-SAW) having enhanced intensity and high beam-width compression ration at specific region, while the parallel SAW would propagation in a straight path. Then, two bigger size IDTs models are built to illustrate the potential SAW propagation in real SAW devices. The enhance radiation of F-SAW will be analyzed based on the F-IDTs model simulation result.In chapter four, we present SAW driving and mixing small liquid droplet atop a flat surface. At first, the transform of wetting angle of droplet by the influence of SAW and F-SAW is compared. Then, an internal streaming induced micromixing is demonstrated. After, we present the behavior of microparticles in a droplet under the influence of acoustic force.In chapter five, a simple application is described, showing the potential of manipulation all kinds of suspension microparticles in microfluidic channel. First, we design and fabricate SAW devices with integrated Bragg reflectors for enhanced particle focusing. Then, two types of standing surface acoustic wave (SSAWs) microfluidic chips are built to study particle trapping and patterning in microfluidic channel. The damage of high intensity radiation input in the channel will also be discussed at the end. In chapter six, a SAW based microfluidic mixer is reported which could efficiently mix two kinds of liquid in very short time in the channel. Three types of channel structures and the number of F-IDTs are discussed for efficient mixing. In addition, the mixing efficiency as a function of applied voltage is analyzed. Afterward, the ultrafast mixing will be demonstrated using deionized water and fluorescent solution. Finally, a simple application will described, showing an enhanced enzymatic reaction using the SAW micromixer.