Fluid flow and microparticle manipulation in surface acoustic wave microfluidic devices

Surface acoustic wave (SAW) devices have emerged over the last two decades as a very promising platform for microfluidics applications. Indeed, they can provide almost all the key functional units required for complete lab-on-a-chips, such as fluid actuation, mixing and cell sorting just to mention a few. Despite the wide range of applications and phenomena demonstrated in both digital and continuous- flow devices, there has been very little effort devoted to device miniaturization and automation, two points of paramount importance for the design of handheld diag- nostic tools. This thesis targets these issues by looking at the scaling of SAW-driven fluid flows and microparticle dynamics with respect to device geometries and acoustic length scales, focusing our attention on both acoustic counterflow and digital microfluidic devices . We demonstrated firstly that, in order to downsize acoustic counterflow devices, it is necessary to consider the ratio between the microchannel height and the ultra- sound wavelength in the fluid as well as the ratio between the microchannel width and the SAW wavelength. The former allows switching between two alternate parti- cle dynamic regimes and processing capabilities of whole-blood samples, while the latter limits pumping efficiency. Furthermore, by increasing the device operating frequency from standard values (typically in the range of ∼20 – 200 MHz) to the ul- tra high frequency range (UHF, 300 – 3000 MHz), we showed both significant device miniaturization and downscaling of internal flow patterns. By similarly shifting SAW driven digital microfluidic devices to the UHF regime, we have shown that the internal flow types and microparticle behavior within the droplets depends both on the ratio of the reactor height with respect to the damping length of ultrasounds into the fluids, and on the ratio of the droplet diameter to the SAW damping length underneath the fluid. By operating the device at ∼1.1 GHz, we demonstrated fast mixing in reactors of the order of the nanoliter, a reactor volume approximately 1000 times smaller than what was typically reported. Whole device miniaturization requires not only the downscaling of on-chip com- ponents, but also minimizing the off-chip equipment. Towards this aim, we pro- posed a novel automated fluid routing scheme that does not need any external feed- back circuits and paves the route for integrated logic gates and sequencers based on instantaneous liquid configuration. This technology exploits the resonant inter- action between travelling SAWs and the stationary modes of SAW cavities and we demonstrated this principle showing automated fluid positioning in digital micro- fluidics devices. Among the different experimental methods used throughout this thesis, spatio- temporal image correlation spectroscopy deserves a particular mention. This was recently proposed as a novel method to measure the internal streaming using a standard bright-field inverted microscope and a high-fps camera. Here, we es- tablished for the first time its accuracy for these applications with respect to the interrogation-area size, acquisition frame rate, exposure time, observation time, and seeding-particle concentration as well as general formulas to transfer our results to different experimental set-ups.