Combined effect of acoustic radiation force and acoustic streaming for focused beams to trap cells in three dimensions

Selectively trapping of a single cell has potential applications such as reproductive cell selection, cell mechanics measurement, in-vivo rheology probes, etc. A widely-used technique called the micropipette aspiration has been used [Mitchison and Swann, J. Exp. Biol.,31, 443 (1954).] However, this contact trapping of a cell suffers the disadvantages of possible biological cross-infection and potential damage under mechanical suction. This could be overcome by the single focused-beam acoustical tweezers, as shown with a first trial in theory [Gong et al., J. Acous. Soc. Am. 154, A263 (2023)]. To selectively trap a cell in three dimensions, the beam's wavelength should approximate the typical size of cells, leading to a high frequency up to a few tens of megahertz. However, the acoustic absorption at such frequencies will be significant, and the acoustic streaming induced drag force [Li et al., Phys. Rev. Fluids (in press)] cannot be ignored. Hence, the 3-D trapping by a focused beam under the acoustic radiation force (ARF) as demonstrated previously may fail since the axial trapping is fragile. Here, we study the ability of 3-D trapping under the resultant force of the ARF and streaming-induced drag force and summarize the prior parameters