Surface acoustic wave microfluidic actuation
thesisposted on 15.01.2017, 23:03 by Tan, Ming Kwang
Fluid manipulation at the microscale presents significant challenges to the design of portable chip based diagnostic and biosensor devices. Various types of microactuation mechanisms have been proposed. However, few have been practically implemented in micro-electro-mechanical systems (MEMS) or Lab-on-a-Chip devices because they require a large external energy source to power them. This thesis primarily focusses on the development of high-frequency surface acoustic wave (SAW) driven microactuation of free droplets and micropumps for microchannel flow, necessary for the future development of integrated microfluidic platforms. A sessile drop responds differently depending upon the strength of an acoustic field; a weak acoustic field induces slow streaming whereas a strong acoustic field induces fast streaming. The droplet behavior was found to be associated with the characteristic length scale of the fluid motion Lf relative to the sound wavelength in the fluid Wf: Schlichting streaming is dominant when Lf/Wf << 1, Rayleigh streaming is dominant when Lf/Wf ~ 1, whereas Eckart streaming is dominant when Lf/Wf >> 1. We note here, in particular, that for Eckart and Schlichting streaming induced by a SAW in the form of a Rayleigh wave, the direction of flow is in, and opposite to, the direction of the propagating Rayleigh-SAW, respectively. These various types of acoustic streaming, arising from the interaction between the substrate undulation due to the SAW propagation along it and the fluid above the substrate, form a basic theoretical framework for understanding and analyzing the phenomena associated with the surface acoustic wave microactuation of fluids. The studies presented herein combine experimental and numerical investigations of SAW driven microactuation of fluids. An overview of the basic working principle of SAW devices, the transmission of acoustic waves and the generation of acoustic streaming in fluids are discussed. Three related studies have arisen out of this theme, the first two dealing with droplet actuation involving free surfaces and the last involving fluid flow in confined microchannels: (1) microparticle collection and concentration by SAW induced droplet translation, (2) high velocity microjet induced by focused SAWs, and (3) rapid fluid flow and mixing in microchannels induced by SAWs. As a prelude to the main body of work on the interaction of the SAW with fluids, however, we first report on the visualization of the SAW using smoke particles, which offers a cost-effective alternative visualization method to interferometry techniques. Finally, the thesis concludes with an experimental demonstration and a numerical analysis of the transmission of high power radio frequency acoustic radiation via fluid couplants into superstrates, which offers a method for carrying out microfluidics on surfaces other than the piezoelectric substrates that are required for the generation of the SAW.