Researchers have detailed the physics behind a phenomenon that allows them to create spin in liquid droplets using ultrasound waves, which concentrates solid particles suspended in the liquid. The discovery will allow researchers to engineer technologies that make use of the technique to develop applications in fields such as biomedical testing and drug development.

“By creating ultrasound waves on the surface of a piezoelectric substrate, we can induce spin in a liquid droplet that is resting on that substrate,” explains Chuyi Chen, an assistant professor of mechanical and aerospace engineering at North Carolina State University and co-lead author of a paper on the work. “The oscillation of the ultrasound waves pushes the fluid inside the droplet to stream in a circle, but the surface tension of the droplet prevents the droplet from spreading out into a flat sheet. A combination of forces from the ultrasound waves, the spinning droplet, and the fluid moving within the droplet drives particles inside the droplet to move in a helical pattern, essentially corkscrewing through the droplet to come together at a central point.

“This is a novel way of concentrating solid particles in a liquid solution, which can be extremely useful,” Chen says. “For example, concentrating the contents of a cell could make it easier for sensors to detect relevant materials for biomedical assays.”

But in order to develop technologies that make use of this phenomenon, researchers need to understand exactly what is driving it.

“This paper is a significant advance, because it lays out in detail the physics responsible for controlling particles inside the droplet,” Chen says. “Now that we understand the forces involved, we can make informed decisions and engineer technologies to concentrate particles in a liquid sample in a controlled way.”

One key aspect of these findings is that you can influence the movement of particles within the droplet by manipulating any of several parameters: the surface tension of the liquid, the radius of the droplet, and the amplitude of the ultrasound waves.

“This gives us multiple mechanisms for fine-tuning the rotation of the system and the behavior of the particles,” says Chen.

In addition to its potential utility in biomedical applications, the new technique also holds promise for use in exploring a range of research questions related to the physics of rotating systems.

“For example, we can create tornado-like vortex flows or study Coriolis-driven transport on a very small scale,” says Chen. “It allows us to explore physics questions in a way that is compact, easily observable and relatively inexpensive, as compared to larger-scale techniques.”

The paper, “Acoustofluidic Spin Control for 3D Particle Manipulation in Droplets,” is published in the open access journal Science Advances. Co-lead author of the paper is Yuyang Gu of Binghamton University. Co-corresponding authors of the paper are Tony Jun Huang of Duke University and Luke Lee of Harvard Medical School, the University of California at Berkeley, Sungkyunkwan University and Ewha Womans University. The paper was co-authored by Joseph Rufo, Jinxin Zhang and Kaichun Yang of Duke.

The work was done with support from the National Institutes of Health, under grants R01GM132603 and R01GM141055; and from the National Science Foundation, under grant 2104295.