Acoustic levitation is a technique that traps solids or liquids in the air. Recently, many studies on stably levitating and manipulating objects larger than the midair ultrasound wavelength have been proposed based on approaches such as the design of a spatially complicated sound field according to the shape of the target object or temporal switching of vortex beams. However, real-time midair manipulation of objects larger than the wavelength has not been demonstrated using those methods due to computational costs for output optimization or reduced output efficiency. To overcome these issues, here we propose a different approach based on the use of multiple facing pairs of acoustic vortex beams for levitating objects larger than the wavelength in the air, which can be applied to real-time midair object manipulation with maximum transducer output efficiency. In this paper, we show that a specific combination of topological charge and phase difference in paired beams can maximize the levitation performance. We experimentally demonstrate stable levitation and real-time manipulation of a midair object larger than the wavelength. This study will drastically widen the scope of target objects for real-time midair manipulation by mitigating the size limit, which will contribute to application scenarios in acoustic levitation handling objects that have not been considered feasible.
Momoki et al. (Mon,) studied this question.