Manipulation in microfluidic systems using surface acoustic waves (SAW)

Lab-on-a-chip microfluidic systems hold substantial promise for a wide range of diagnostic and therapeutic applications. By shrinking down conventional laboratory processes and replicating their functions on-chip, the size, cost, required time, and amount of reagent and sample needed can be drastically reduced. However, because these devices operate at length scales orders of magnitude smaller than conventional fluid processes different physical phenomena become dominant, meaning new forces and techniques must be developed to perform them. Acoustic forces have the potential to be useful at small length scales, though, their use has for the most part been limited by the relatively small force magnitudes and low frequencies at which they have been generated, thereby limiting the promise of rapid acoustic manipulation on microfluidic scales. However, a developing technology relying on the application of surface acoustic waves (SAW) has shown the potential to overcome these limitations, especially due to the high frequencies (10-2000 MHz) and correspondingly small length scales (2-300 µm), on the order of the bacteria and eukaryotic cells, that are characteristic of this method. In this thesis, SAW is used in a range of applications that emphasize these advantages, specifically with respect to the large and localized forces that can be generated on interfaces, both between two immiscible phases and on particles within a single fluid phase. In the studies presented here, SAW is used to (1) actuate a fluid-air interface for the production of water-in-air droplets with tunable diameters in the range of ~0.5-50 µm for the purpose of targeted nebulization therapy, (2) actuate a water-oil interface for the tunable production of picoliter-sized water-in-oil droplets with simultaneous particle pre-concentration and encapsulation for application in digital microfluidic systems, (3) perform controlled concentration and release of particles using a novel microfabricated channel structure and (4) deterministically sort particles over a large size range, demonstrated between 0.3-7 µm with potential application in cell sorting systems where high sorting efficiency or sorting based on only small size differences is required. Finally the case is made that acoustic fields, especially those produced by SAW, are optimal for many, if not most, applications where manipulation of microfluidic species is required.