Surface Acoustic Wave Microfluidics: Droplets, Pinned Liquid Films, and Biodetection

Fluid manipulation at the microscale presents enormous promise for chemical and biomolecular detection. The advent of such technology could allow complex laboratory functions to be performed on a microchip, which would reduce the quantity of chemicals consumed, improve portability and turnover time, and reduce manufacturing costs. In order to deliver on these promises, an effective actuation mechanism must be developed which can overcome the viscous and capillary forces that tend to dominate at small scales.

This document describes surface acoustic waves (SAWs), which are elastic compression waves generated on the surface of piezoelectric substrates, as a mechanism for such actuation. These waves can refract from the solid piezoelectric into a liquid film on the substrate surface, producing a highly non-linear acoustic forcing pressure. In addition, the electromechanical coupling inherent in SAWs produces an electric field at the solid-liquid interface and thus an electric Maxwell pressure. When applied to a pinned liquid film, the acoustic and electric pressures are shown to generate two sequences of surface droplets extending away from the bulk, spanning in from nanometers to tens of microns in size. The size distributions of these two sequences of droplets obey self-similar exponential scalings, with the two distinct exponents corresponding to the radiative decay length of the acoustic pressure and to the electric field leakage length scale. A high acoustic pressure has also been shown to rupture the bulk liquid film. Appropriate experimentation and analysis to predict the onset of film breakup and of global rapid aerosolization is presented by the author. This newfound understanding of SAWs is then applied to the study and diagnosis of pancreatic cancer, which has a five year survival rate of less than 6%. The aforementioned SAW pressures are shown to capable of lysing exosomes, secreted membrane vesicles ~30-200 nm in diameter that are present in blood, saliva, urine, and other bodily fluids. A microfluidics-based approach to the analysis of exosomal RNA is presented based on SAW exosome lysis and ion-exchange nanomembrane RNA sensing.