Stable acoustic levitation based on coaxial confocal dual-frequency focused ultrasound and vortex beams

Acoustic levitation enables the suspension of objects of different materials and scales through the acoustic radiation force (ARF), offering advantages of non-invasive, non-contact, deep penetration, label-free, and biocompatibility. However, achieving stable suspension using focused ultrasound (FU) or focused acoustic vortex (FAV) alone remains challenging due to the absence of both trapping and propulsive forces. This study proposes a stable acoustic levitation scheme that employs coaxial confocal dual-frequency FU and FAV beams, implemented by a focused sector array. Theoretical analyses of force balance for objects with the size much smaller than the wavelength are performed, and ARFs in both axial and radial directions are calculated based on the Gor'kov potential. It is demonstrated that the suspension capability primarily depends on the peak pressure of FU, with the minimum threshold determined by the object's gravity. A longer axial range of upward propulsion, characterized by a lower threshold height and a higher steady-state height, is created by a higher peak pressure of FU. The trapping force is governed by the peak-pressure ratio between FAV and FU, with a constant minimum ratio (0.69) being nearly independent of the density and size of objects. A high-precision dual-frequency holographic direct digital synthesis technology based on phase sampling is developed to design an 8-channel driving system capable of real-time adjustments to frequency, pressure, and phase. Focused fields composed of dual-frequency FU and FAV beams are constructed by an 8-element focused sector array. By independently regulating the peak pressures of FAV and FU, the upward and downward movements and stable suspension of polystyrene particles along the beam axis in water are successfully realized. The proposed scheme significantly enhances the stability and precision of on-axis acoustic levitation, validating its potential for contactless manipulation and container-free processing. Additionally, the dual-frequency holographic technology can improve the regulation flexibility of multiplexed fields, making it adaptable to diverse applications while reducing the driving complexity for source arrays.