Researchers from Binghamton University, North Carolina State University, Harvard Medical School and Duke University have observed a phenomenon that allows them to create spin in liquid droplets using ultrasound waves, which manipulates solid particles suspended in the liquid.
The discovery will allow researchers to engineer technologies that make use of the technique in fields such as biomedical testing and drug development.
"It's a new physics we found. No one has ever found it," said Assistant Professor Yuyang Gu, a faculty member from Binghamton's Thomas J. Watson College of Engineering and Applied Science.
Gu - who joined Watson College's Department of Mechanical Engineering in fall 2024 - was co-lead author of a recent paper in the journal Science Advances with NCSU Assistant Professor Chuyi Chen. The research continues work that they did together while earning their PhDs at Duke University.
Also contributing to the paper were Duke University Professor Tony Jun Huang; Duke PhD students Joseph Rufo, Jinxin Zhang, Kaichun Yang and Ying Chen; and Harvard Medical School Professor Luke P. Lee.
The research shows that creating ultrasound waves on the surface of a piezoelectric substrate can induce spin in a liquid droplet resting on that substrate. 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 it from spreading out.
"If we put microparticles or nanoparticles inside the droplet, they follow a helical trajectory to come together at a central point," Gu said. "We can use this to manipulate particles in 3D. And no matter how small the particles are, they can be concentrated at the bottom of the droplet. This process also can help sort biological samples, for example, by concentrating cells or vesicles for testing."
Gu and Chen first wrote about the phenomenon in 2021 - but to develop technologies using it, they knew they needed to understand exactly what's driving it.
"After we published the first paper, we realized that even though it's useful, we didn't fully understand the mechanism of how it works. That means we couldn't control the system," Gu said. "If someone asked us: 'So in your system, why do particles move like this? Or what are the separation thresholds?' - we didn't understand that. We needed further analysis to understand the mechanism of this droplet-spinning and the particle trajectory."
One key aspect of these findings is that the movement of particles within the droplet can be influenced by manipulating the surface tension of the liquid, the radius of the droplet and the parameters of the ultrasound waves. That allows multiple ways to fine-tune the rotation of the system and the behavior of the particles.
"We found that particles of different sizes can be concentrated in different timescales," Gu said. "If the particle is 100 nanometers, it will concentrate first through the middle, while the smaller nanoparticles will still be randomly distributed in the droplet. Then, if we increase the speed of the rotation, we can further concentrate the smaller nanoparticles."
In addition to its potential utility in biomedical applications, the new technique also holds promise for exploring a range of questions related to the physics of rotating systems.
"We can use this as an analog to systems in nature or quantum physics," Gu said. "We can see similarities to the spinning of electrons or other microscopic phenomena. The vortex inside is also similar to a tornado."
Now that this latest paper is published, the collaborators are looking ahead at what's next for their droplet technique.
"The first thing is that we want to expand the system," Gu said. "We want to explore larger droplets and smaller droplets. We want to explore how different parameters can affect the system. We also want to expand the application. If we want to use it for some biological sensing function, it's a single droplet. The throughput will be low, and that may not be useful for practical applications. We potentially can build a droplet array with 100 droplets that are spinning with all the nanoparticles concentrated to the middle."
