Efficient Atomization Using MHz MEMS-Based Ultrasonic Nozzles
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Efficient Atomization Using MHz MEMS-Based Ultrasonic Nozzles Shirley C. Tsai1, Yu L. Song2,3, Yuan F. Chou4, J.H Cheng4, and Chen S. Tsai3,5 1 Dept. of Chemical Engineering, California State University, Long Beach, CA, USA 2 Dept. of Physics, National Taiwan University, Taipei, Taiwan 3 Dept. of Electrical Eng. & Computer Science, University of California, Irvine, CA, USA 4 Dept. of Mechanical Eng., National Taiwan University, Taipei, Taiwan 5 Institute of Electrooptical Eng., National Taiwan University, Taipei, Taiwan ABSTRACT This paper reports on the preliminary results of atomization using micro-electro-mechanical system (MEMS)-based 3-Fourier horn 0.5 MHz silicon ultrasonic nozzles. The droplets produced are monodisperse with 7.1 µm drop diameter in good agreement with that predicted by pure capillary wave atomization mechanism. This drop diameter is much smaller than that obtainable using conventional ultrasonic nebulizers operating at three times higher frequency. INTRODUCTION Silicon-based ultrasonic nozzles, previously fabricated at 74 kHz [1], possess a number of advantages over conventional metal-based bulk-type ultrasonic nozzles. Silicon possesses a relatively large electro-mechanical coupling coefficient, a high acoustic velocity, and a high potential for mass production of any resonator profile by MEMS-based fabrication technology. These advantages enable the silicon-based ultrasonic nozzles to overcome the 120 kHzfrequency limitation of the conventional bulk-type ultrasonic nozzles [2]. The conventional ultrasonic nozzle at the highest available frequency of 120 kHz yields water sprays with a peak drop diameter of 55µm [3]. With the addition of air, ultrasound-modulated two-fluid atomization is capable of reducing the peak drop diameter to 22µm [3]. Therefore, silicon-based ultrasonic nozzles capable of producing uniform drops smaller than 10µm in diameter for nanoparticles synthesis and bio-dispersion spray must operate at a frequency much higher than 120 kHz because the drop diameter is inversely proportional to the ultrasonic frequency [4]. In this paper, we report for the first time on successful atomization using the 0.5 MHz MEMS-based silicon ultrasonic nozzles. The MEMS-based 0.5 MHz ultrasonic nozzle is made of a piezoelectric drive section and a multiple Fourier horn silicon-resonator with a central channel for liquid flow. As the liquid is pumped into the central channel, a thin film of liquid forms at the nozzle tip that vibrates at the actual resonant frequency of 484.5 kHz and spray of droplets are produced. EXPERIMENTAL DETAILS 0.5 MHz 3-Fourier-horn nozzles, as shown in Fig. 1, were used in the atomization experiments. Each Fourier horn is of half-wavelength design with a vibration amplitude magnification of two. The results of 3-D simulation [5] are shown in Fig. 1. Specifically, a pure longitudinal vibration occurs at the resonant frequency of 495 kHz. At this resonant frequency, the vibration amplitude gain at the nozzle tip is equal to the theoretical value of 23, namely,
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