Acoustophoresis-driven particle focusing and separation with standard/inverse Chladni patterns

Manipulating objects with acoustics has been developed for hundreds of years since Chladni patterns in gaseous environments were exhibited. In recent decades, acoustic manipulation in microfluidics, known as acoustofluidics, has rapidly thrived and many sophisticated technologies were born. However, the basic background motion of particles under acoustic excitation is usually neglected and the classical Chladni patterns haven't been reproduced in an aqueous environment. In this study, we investigated the basic mechanism and the motion of suspended particles and sinking particles in a plain microchamber under low-frequency excitation (3-5 kHz). The mechanisms were clearly distinguished by comparing the differences among colored fluids, suspended particles, and sinking particles. The suspended particles rotated around the antinode with a speed up to 55.1 μm s^-1 at 100 V_pp by the acoustic streaming and they approached each other by the secondary acoustic radiation force. The sinking particles concentrated at the node with a speed up to 22.3 μm s^-1 at 100 V_pp by bouncing on the vibrating surface and the primary acoustic radiation force. We have reproduced the classical standard/inverse Chladni patterns in an aqueous environment for the first time, and they were leveraged to separate SiO₂ particles with different sizes. The big particles with an average diameter of 9.68 μm were concentrated at the node while the small particles with an average diameter of 2.72 μm were collected at the antinode within 2 min. These results not only provide insightful perspectives of basic mechanisms, but also open up new possibilities for advanced acoustic tweezers.