Investigation of precise particle manipulation using zeroth- and first-order acoustic Bessel fields.

Acoustic tweezers enable non-contact manipulation across a wide range of biomedical applications. In this work, an in-plane circular array acoustic tweezer device operating at 2.3 MHz is used to manipulate 90 μm polystyrene particles, which have higher density and stiffness than the surrounding water, under both focal-point (zeroth-order Bessel function shape) and vortex-trap (first-order Bessel function shape) fields. It is shown that when such fields are used for manipulation, significant positioning errors can occur. The vortex-trap field creates a convergent trap at its core, yet position errors of 0.266 mm (0.41λ) were observed. The focal-point field produces a dominant pushing force that is highly sensitive to the angle between the focal-point and the particle as well as the distance, which leads to unstable manipulation. To improve performance of both operating modes, we propose a closed-loop controller that uses optical feedback to localize the particle and update the actuation. With feedback, the vortex-trap achieves an error below 0.02 mm (0.031λ), and the focal-point field an error below 0.1 mm (0.15λ). These results reveal the controllability and operating regimes of the two manipulation modes and establish an experimentally validated route to increase the precision of acoustic particle manipulation.