Understanding droplet production mechanisms in ultrasonic atomization

Ultrasonic atomization converts a liquid into a fine mist by vibrating it at ultrasonic frequencies, i.e., at frequencies above the upper limit of human hearing. This process is used in applications such as drug delivery, coating, and humidification. It is also promising for producing high-quality metal powders for additive manufacturing. However, the mechanisms responsible for droplet formation during ultrasonic atomization are still not fully understood. Hypotheses attribute droplet production to the growth and breakup of surface waves, the oscillation and collapse of microscopic gas bubbles beneath the free surface, or an interaction between both mechanisms. This project will use state-of-the-art computer simulations to investigate these competing hypotheses. It will determine the relative importance of each mechanism and how they interact. A better understanding of droplet formation will provide a scientific foundation for designing more efficient and tunable ultrasonic atomization technologies. The outcomes will benefit applications ranging from advanced manufacturing to healthcare. The project will also promote open and reproducible science through the public release of software and computational models. It will provide research and training opportunities for undergraduate and graduate students and will foster exchange between academia and industry. This project will determine how droplets are generated during ultrasonic atomization by evaluating three competing hypotheses: (1) the “capillary-wave hypothesis”, in which ultrasonic vibrations drive the growth of unstable standing capillary waves, ultimately leading to their breakup; (2) the “cavitation hypothesis”, in which the acoustic cavitation of gas bubbles beneath the free surface induces high-speed upward liquid jets and/or acoustic emissions that perturb the free surface; and (3) the “conjunction hypothesis”, in which both capillary-wave instability and subsurface cavitation act together to produce aerosol droplets. Leveraging recent advances in compressible multiphase flow modeling, including robust high-fidelity two-phase flow solvers and sharp-interface methods, the project will conduct a comprehensive numerical study of free-surface waves, subsurface cavitation, and their interactions under ultrasonic excitation. Simulations will span a broad range of operating conditions for cases of idealized periodic liquid pools and sessile drops, yielding a computational dataset with unprecedented scope and spatiotemporal resolution. These results will establish the relative importance of capillary-wave instability and cavitation across regimes of droplet production, quantify the strength and nature of their coupling, and provide the foundation for physically interpretable reduced-order models of droplet generation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.