Acoustofluidic Three-Dimensional Motion of Suspended Cells at Near-Zero Acoustic Contrast in Homogeneous Media

Acoustic separation of suspension cells can be achieved by altering the properties of the cell medium. Cells of different types can then migrate in opposing directions in the sound field due to differences in their relative density and compressibility with respect to the surrounding fluid. For near-zero acoustic contrast the acoustic radiation force on cells becomes negligible, and it is the primary objective of this paper to study the interplay of acoustic radiation, acoustic streaming, and buoyancy in this regime and how it may affect the separation outcome. We study the three-dimensional acoustophoretic motion of suspension cells in homogeneous suspending media and link this to the underlying acoustofluidic mechanisms. Cell trajectories are measured by a defocused-image tracking approach, and we assess the technique's applicability for tracking cells by determining the associated error when measuring the out-of-image-plane component of the tracks. For cells at near-zero acoustic contrast, we observe strong effects of buoyancy and acoustic streaming and that small distributions of cell properties within a population leads to large differences in the cell motion patterns. A neural network was developed to classify experimental cell trajectories according to their acoustic contrast in different suspending media. Further, we compare the experimentally measured trajectories to a numerical model by generating simulated trajectories of cells.