A Micromachined Ultrasonic Droplet Generator: Design, Fabrication, Visualization, and Modeling
Meacham, John Marcus
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The focus of this Ph.D. thesis research is a new piezoelectrically driven micromachined ultrasonic atomizer concept that utilizes fluid cavity resonances in the 15 MHz range along with acoustic wave focusing to generate the pressure gradient required for droplet or jet ejection. This ejection technique exhibits low-power operation while addressing the key challenges associated with other atomization technologies including production of sub-5 um diameter droplets, low-temperature operation, the capacity to scale throughput up or down, and simple, low-cost fabrication. This thesis research includes device development and fabrication as well as experimental characterization and theoretical modeling of the acoustics and fluid mechanics underlying device operation. The main goal is to gain an understanding of the fundamental physics of these processes in order to achieve optimal design and controlled operation of the atomizer. Simulations of the acoustic response of the system for various device geometries and different ejection fluid properties predict the resonant frequencies of the device and confirm that pressure field focusing occurs. High-spatial-resolution stroboscopic visualization of fluid ejection under various operating conditions is used to investigate whether the proposed atomizer is capable of operating in either the discrete-droplet or continuous-jet mode. The results of the visualization experiments combined with a scaling analysis provide a basic understanding of the physics governing the ejection process and allow for the establishment of simple scaling laws that prescribe the mode (e.g., discrete-droplet vs. continuous-jet) of ejection. In parallel, a detailed computational fluid dynamics (CFD) analysis of the fluid interface evolution and droplet formation and transport during the ejection process provides in-depth insight into the physics of the ejection process and determines the limits of validity of the scaling laws. These characterization efforts performed in concert with device development lead to the optimal device design. The unique advantages enabled by the developed micromachined ultrasonic atomizer are illustrated for challenging fluid atomization examples from a variety of applications ranging from fuel processing on small scales to ultra-soft electrospray ionization of biomolecules for bioanalytical mass spectrometry.