Institute of Fluid Dynamics

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Ultrasonic Drop Atomization

Ultrasonic atomization is employed to generate size-controllable droplets for a variety of applications, including food and drug encapsulation, creation of metal powders and pulmonal drug delivery. We use high-speed x-ray imaging to uncover how ultrasound breaks droplets into controlled sprays.

Enlarged view: X-ray phase-contrast images showing the different mechanisms of ultrasonic drop atomization driven with a horn frequency of 20 kHz.
X-ray phase-contrast images showing the different mechanisms of ultrasonic drop atomization driven with a horn frequency of 20 kHz. The snapshots are labeled with their non-dimensional times, normalized by the time period of the driving ultrasonic excitation. (a) Formation of Faraday waves and ejection of droplets from a pendant water drop. (b) Extended ligament formation in a viscoelastic drop with the boxes highlighting entrainment of air and formation of drop-encapsulated bubbles. (c) Trapped bubbles in a viscoelastic drop highlight cavitation-based ejection that is significantly different from the wave-based ejection as seen in (a).

Contributors: Anunay Prasanna, Luc Biasiori-Poulanges

We investigate the physics behind the process by studying the atomization of a single drop pending from an ultrasonic horn using spatiotemporally resolved synchrotron x-ray phase-contrast imaging. This technique provides excellent edge contrast due to the spatial beam coherence of the x-rays, ensuring that all ejected droplets are in focus with no overlapping issues and enabling the measurement of millions of ejected droplets, which is not possible with conventional imaging techniques. The high energy flux provided by the synchrotron source additionally allows to image the process at high frame rates (80000 fps). Furthermore, the ability to capture and resolve interfaces with high accuracy allows us to clearly distinguish entrained air and cavitation activity within the deformed drop. The drop’s interface initially forms Faraday waves, which undergo several transitions before ejecting micrometric droplets. The size distribution is found to be controlled by the fluid properties and the driving frequency. The results also show that the ejection dynamics in viscoelastic drops is dictated by extended ligament formation, entrainment of air, and ejection of drop-encapsulated bubbles. Finally, we elucidate the differences between capillary wave-based and cavitation-based atomization using two different configurations - (i) confined water drops and (ii) trapped air bubbles in pendant drops. The occurrence of cavitation-based events within the drop are identified and found to quicken the onset of daughter droplet ejection while impeding their size control.

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