Researchers have uncovered the physics behind a phenomenon that allows them to induce spin in liquid droplets using ultrasound waves, concentrating solid particles suspended within the liquid. This discovery could lead to new technologies 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,” said Chuyi Chen, an assistant professor at North Carolina State University and co-lead author of the study. The oscillation of these waves causes fluid inside the droplet to stream in a circle, with surface tension preventing it from spreading out. Forces from the ultrasound waves and spinning droplet drive particles inside to move helically, concentrating them at a central point.
“This is a novel way of concentrating solid particles in a liquid solution, which can be extremely useful,” Chen noted. “For example, concentrating the contents of a cell could make it easier for sensors to detect relevant materials for biomedical assays.”
Understanding this phenomenon’s driving forces is crucial for developing related technologies. “This paper is a significant advance because it lays out in detail the physics responsible for controlling particles inside the droplet,” Chen stated. Researchers can now make informed decisions to engineer technologies that concentrate particles in controlled ways.
Key findings show that manipulating parameters like surface tension, droplet radius, and ultrasound wave amplitude can influence particle movement within droplets. “This gives us multiple mechanisms for fine-tuning the rotation of the system and the behavior of the particles,” said Chen.
The technique also holds promise beyond biomedical applications, potentially aiding research into rotating systems’ physics. “For example, we can create tornado-like vortex flows or study Coriolis-driven transport on a very small scale,” Chen added.
The study titled “Acoustofluidic Spin Control for 3D Particle Manipulation in Droplets” was published in Science Advances. Co-lead author Yuyang Gu is affiliated with Binghamton University. Co-corresponding authors include Tony Jun Huang from Duke University and Luke Lee from Harvard Medical School among others.
The research received support from grants by the National Institutes of Health and National Science Foundation. Huang has cofounded a start-up company focused on commercializing acoustofluidics technology.
