Understanding droplet stability in acoustic levitation

Recent advancements in acoustic levitation of droplets have garnered interest in a lab-in-a-drop approach for various applications, including chemical and biomolecular analyses. A main feature of the technique is for the levitated droplets to serve as contactless test tubes or chemical sensors that enable researchers to avoid undesirable side effects, such as unexpected nucleation or contamination, introduced by the surfaces of containers or substrates.

For the technique to work, scientists must learn how to levitate droplets consistently in mid-air without atomizing, i.e., splitting into smaller droplets. To gain a better understanding of the stability of these droplets, Koji Hasegawa and Kohei Aoki investigated the deformation and atomization process to find out when and why instability occurs.

In their experiments, the researchers used water and ethanol droplets, and compared their experimental results on the droplet stability with theoretical predictions. Under the same temperature and humidity, they used 19.3kHz sound waves with varying power and observed the atomization process of the different droplets.

The daughter droplets of the ethanol were finer than the water ones, caused by the lower surface tension of ethanol compared to water. This confirmed the different atomization behavior due to different fluid properties. In all cases, the aspect ratio, defined by the equatorial-to-polar diameter ratio, increased immediately before the atomization. They also demonstrated that the capillary wave, which is dependent on the input frequency of the sound wave, is responsible for triggering the atomization process.

Directly measuring the pressure field in the droplet under the unsteady atomization process was a challenge that the researchers plan to address in future research.

Source: “Acoustically induced breakup of levitated droplets,” by K. Hasegawa and K. Aoki, AIP Advances (2020). The article can be accessed at https://doi.org/10.1063/1.5143395 .