From macroscale to microscale characterization of droplet breakup regimes using a micromachined ultrasonic droplet generator

Abstract

Controllable, uniform droplet generation is critical to many industrial processes and biomedical applications (e.g., materials synthesis, cell handling, and aerosol drug delivery). We have developed an ultrasonic droplet generator that can enable dynamic control over the droplet size distribution in the 2–75 μm range. The system uses a bulk piezoelectric transducer to generate a standing acoustic pressure field that drives fluid transport through an array of microscopic orifices. Our previous work suggests that certain resonances lead to variability in the pressure gradient at the orifices, affecting the droplet size distribution of the resultant spray. Here, we present a parametric study relating ejection uniformity to orifice size, operating frequency, and drive amplitude. We introduce a new system configuration that allows high-resolution stroboscopic imaging of the liquid-gas interface evolution at each orifice. Although the entire micronozzle array remains acoustically active, we use orifice size to control the flow resistance, restricting ejection to a single preselected orifice. Observed ejection regimes resemble classical descriptions of jet breakup (Rayleigh, wind-induced, and atomization), depending on drive voltage. A regime map is created using 5, 10, and 15 μm orifices for 0.5–2.0 MHz actuation from threshold ejection amplitude to the point of chaotic wind-induced breakup.