A micromachined ultrasonic atomizer based on a liquid horn structure

Abstract

A micromachined ultrasonic droplet generator is developed and demonstrated for liquid atomization. The droplet generator uses a 1 mm thick bulk ceramic piezoelectric transducer for ultrasound generation, a reservoir for the ejection fluid, and a silicon micromachined liquid horn structure as the nozzle. The pyramidal-shaped horn structures are formed using a simple batch microfabrication process involving wet etching of (100) silicon in a potassium hydroxide (KOH) solution, and the nozzle openings are defined by dry etching of silicon in an inductively coupled plasma (ICP) environment. Device operation for various applications has been demonstrated by droplet ejection of water, liquid fuels, and measles vaccine through 5–30 μm orifices at multiple resonant frequencies between 0.5 and 5 MHz. Finite element simulations of the electrical input impedance are in agreement with measurements, and the simulated acoustic fields within the cavity indicate that the device utilizes cavity resonances in conjunction with acoustic wave focusing by the horn shaped nozzles to achieve low power operation. Visualization and scaling of drop-on-demand (DOD) and continuous-jet fluid atomization of water are also presented to elucidate the fluid physics of the ejection process and characterize the modes of operation of the ultrasonic droplet generator. The interactions between focused ultrasonic pressure waves and capillary waves formed at the liquid-air interface located at the nozzle tip are found to govern the ejection dynamics, leading to different ejection modalities ranging from DOD to continuous-jet [1]. A time scale analysis of the ejection process, which involves the period of electrical excitation (process), viscous, capillary, and inertial time scales, is used to explain the observed results of high-resolution stroboscopic optical imaging of the liquid-air interface evolution during acoustic pumping and to gain an understanding of the key fluid mechanical features of the ejection process.