Acoustofluidic droplet generation: physical understanding and applications of jetting and atomization
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Acoustofluidic droplet generation: physical understanding and applications of jetting and atomization


Ultrasonic waves generated by piezoelectric devices produce forces on fluids and particles that can be controlled to produce useful functions. The research field of acoustic microfluidics has developed fundamental understanding of these forces and demonstrated some useful functions, but it has yet to break through in industry and commercial use. We describe acoustic radiation force and acoustic streaming as well as some less common forces and review piezoelectric materials. Then we review some fundamental work in fluid mixing, translation, jetting, and atomization. Particle manipulation will also be touched on briefly, but it is not the focus. Instead, the focus of our work has been on producing droplets of liquid in air, either one at a time via jetting or many at a time via atomization We present a focused surface acoustic wave system in order to investigate the fundamental physics of jetting and to explore the possibility of some applications. We show that jetting must account for surface tension effects. Acoustic streaming is well understood to be the driving force behind jetting and its amplitude and direction are not dependent on surface tension in most literature because it is typically considered far from a deformable interface. However, jetting to produce a droplet is fundamentally an interfacial phenomenon and we show quantitatively how it should be accounted for in order to control droplet ejection angles. Our experimental system uses only two transducers, but it is not hard to imagine a system with four transducers that could control droplet ejection in two dimensions. This capability could be useful in printing by reducing the amount that the printing head has to translate. With this same focused surface acoustic wave device we demonstrate how frozen liquid can be melted and subsequently atomized. We also demonstrate a through-hole liquid supply that allows for continuous atomization and show that the angle control, similar to the jetting case, is possible with atomization. These capabilities could be useful as a thruster for small space-craft. One of the major reasons that acoustic microfluidics has not achieved widespread use is that the equipment required to drive the devices is prohibitively complicated, bulky, and expensive. We produce a hand held system to drive high frequency ultrasound devices and demonstrate it's use for several typical applications. We provide all the necessary information to recreate these systems to promote adoption by other labs and entrepreneurs. Our primary application for this system is in nebulization and atomization of liquid, which could be useful for pulmonary drug delivery, agriculture, or fuel injection. We describe in detail how to fabricate the thickness mode devices that we use to produce atomization at large flow rates in our lab. We then, show how these devices have been integrated with the driving system to produce a truly hand-held high-frequency nebulizer. The fundamental physics of atomization is far more complicated than that of jetting, but we have made contributions here as well. It is currently our hypothesis that acoustic radiation sets up a standing wave pattern in the liquid volume based on its initial shape and then a feedback loop between the two leads to an initial set of capillary waves (work that Shuai Zhang will soon publish). At larger power acoustic streaming may also play a roll, but regardless the capillary waves become turbulent before the onset of atomization. For high-frequency forcing, these capillary waves occur at very short time-scales and very small length-scales so that it has been difficult to study them directly, but we have developed a new and unique experimental capability, high-speed digital holographic microscopy. We describe capillary wave turbulence regimes available in our system and show evidence for finite domain effects and strong nonlinearity. We also show quantitatively how energy transfers between time and length scales with increasing power input.

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