High-frequency single-beam acoustic tweezers: radiation pressure versus drag forces

Single-beam acoustic tweezers use focused ultrasound waves to manipulate objects in three dimensions. However, their efficiency is limited by weak axial restoring forces and drag forces generated by acoustic streaming, especially at high frequencies. Indeed, acoustic streaming, a fluid motion induced by ultrasound waves, counteracts the radiation pressure forces responsible for trapping. This study investigates the feasibility of high-frequency acoustic tweezers by analyzing radiation pressure and drag forces in regimes of strong focusing. Using a streaming model and numerical simulations, we analyze the evolution of streaming as a function of acoustic pressure amplitude, beam focusing, and frequency. In particular, we demonstrate that, unlike plane waves where streaming grows quadratically with frequency, for strongly focused beams, streaming becomes less dependent on frequency and eventually stabilizes. This counterintuitive behavior can be explained by the reduction in the size of the acoustic source, which is proportional to the wavelength. A detailed understanding of the evolution of radiation pressure and drag forces enables the identification of trapping regimes to ensure stable trapping, even at high frequencies. These advances pave the way for more reliable devices for the contactless manipulation of micrometric objects, with potential applications in biomedical research and microtechnology.