Sorting droplets with acoustic waves

150 years ago, German physicists demonstrated that matter could be manipulated with sound waves. In the decades since, researchers have extensively studied this phenomenon and have discovered many applications for it, such as medicine, food, and chemical science.

To date, the vast majority of this research has focused on particles smaller than the Rayleigh limit. Thirisangu et al. extended this work by studying the suspension of larger particles that are comparable in size to the wavelength of the acoustic wave.

To study the interplay between the acoustic force and gravity, the researchers placed water droplets in an oil-filled quartz minichannel. A piezoelectric transducer attached to the bottom of the minichannel generated acoustic waves and a high-speed camera recorded the droplets’ resulting behavior.

Small droplets below the Rayleigh limit exhibit uniform behavior when suspended by acoustic waves. However, the researchers discovered that larger droplets have more complex interactions dependent on their size.

“Surprisingly, we found that the critical acoustic power required to suspend the droplet is a function of the droplet size,” said author Karthick Subramani. “This finding excited us since it provides a novel way of sorting the larger droplets based on the critical power, as demonstrated in our work.”

The novel sorting method could be applied for the recovery of droplets from emulsion, which is important in the chemical and petroleum industries, Subramani said. The authors are currently working on theoretical and simulation frameworks to predict, explain, and expand their experimental findings, which could also aid in understanding the fundamentals of suspending larger particles.

Source: “Suspending droplets beyond the Rayleigh limit: The interplay of acoustic and gravity forces,” by Jeyapradhap Thirisangu, E. Hemachandran, and Karthick Subramani, Physics of Fluids (2023). The article can be accessed at https://doi.org/10.1063/5.0171492 .