Surface acoustic wave devices for microfluidic applications

• Main Author: Du, X.
• Published: University of Cambridge 2009
• Subjects: 612
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598662

This project investigates the use of surface acoustic waves (SAWs) for applications in low cost, low voltage, digital microfluidic systems. To be able to produce surface acoustic waves, the substrate of the microfluidic device needs to be a piezoelectric material. This study explored the use of two different substrates: 128° Y-cut lithium Niobate (LiNbO₃) and RF magnetron sputtered Zinc Oxide(ZnO) on Silicon (Si) (100). The SAW device incorporates aluminium InterDigital Transducers (IDTs) on LiNbO₃ and ZnO/Si piezoelectric material that acts as an excitation agent to create a surface wave on the substrate. When the signal through the IDT matches the correct frequency, a mechanical wave propagates away from the IDT on the substrate surface. Droplet mixing and movement experiments demonstrate a linear relationship between the applied voltage and droplet movement. Other factors tested are the surface treatment effect on droplet movement and surface temperature effects caused by the SAW mechanical wave. Before droplets could be moved a hydrophobic coating had to be deposited on the surface. The surface coating utilizes the octadecytrichlorosilane (OTS) for both its chemical inertness and bio-compatibility. The OTS coating is smooth and thin and does not effect the propagation of the SAW. The propagation mode of the acoustic wave is determined by the structure of the SAW devices and materials. A higher order harmonic mode wave appears in addition to the fundamental Rayleigh wave for LiNbO₃ samples.  The Rayleigh mode and higher mode- Sezawa mode can be induced for the ZnO/Si SAW devices. These different wave modes have been utilized to induce streaming and manipulate liquid droplets for microfluidic application.