Surface acoustic wave actuated rotations for microfluidics

Microfluidics and the lab-on-a-chip concept promises to open up new possibilities in miniaturised portable fluid systems and point-of-care diagnostics. Centrifugal microfluidics, most commonly recognised in the lab-on-a-CD concept, has already shown many applications in chemical and biological systems by exploiting the radial forcing associated with high rotational velocities. Bulk rotation of a 100 mm diameter plastic disc is however a cumbersome approach to driving nano-litres of fluid, and furthermore the rotations require large external motor systems for actuation. Surface Acoustic Wave (SAW) driven microfluidics can address these issues by exploiting a very efficient mechanism of energy transfer to the fluids themselves. SAWs have been shown to actuate a large range of microfluidic processes on a truly miniaturised scale, owing to the very efficient coupling that occurs between the surface waves and fluids that are in contact with them. By clever use of wave blocking and substrate geometries it has been shown that SAWs can generate rotational flows with applications in micromixing and centrifugation [Biomed. Microdev. 9, 647-656 (2007)]. In this thesis we report on the use of focussed SAWs to generate very high rotational velocities in microfluids, and an associated reduction in process time for micromixing and particle concentrations. We then further explore the process by which SAWs promote fluid mixing and show for the first time that the mixing process is in fact dominated by chaotic advection. The last part of the thesis explores these ultra-fast fluid rotations for use as a coupling layer for a high speed miniaturised motor system that could be used as a truly portable centrifugal microfluidics platform, akin to the larger lab-on-a-CD concept. At even further reduced scales, we show that by removing the fluid coupling layer we can drive 1 mm diameter rotors at ∼ 10, 000 rpm by direct contact with focussed SAWs. In this thesis we aim to demonstrate some of the vast potential that SAW driven ultra-fast rotational flows have in actuating centrifugal microfluidics on truly portable, solid state devices.