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Vortex-field acoustic levitation (VAL) is a versatile acoustic levitation method that allows simultaneous rotation and movement of the levitated object. However, the levitation capability of VAL in air was previously limited to the levitation of objects with a density equal to or less than that of water. Here, we generate first-order acoustic vortices using an annular array of sources and investigate the characteristics of VAL within a cylindrical tube. It is shown that the number of potential minima on the vortex axis and the levitation capability of VAL can be enhanced by careful choice of the geometry of the array and the tube. As an example, a stainless steel sphere (density about 7.9 g/cm3) and a copper sphere (density about 8.7 g/cm3) have been successfully levitated. [ABSTRACT FROM AUTHOR]

Here we observe the attenuation motion of spherical rotors levitated by near-field acoustic radiation force and analyze the factors that affect the duration time of free rotation. It is found that the rotating speed of freely rotating rotor decreases exponentially with respect to time. The time constant of exponential attenuation motion depends mainly on the levitation height, the mass of rotor, and the depth of concave ultrasound emitter. Large levitation height, large mass of rotor, and small depth of concave emitter are beneficial to increase the time constant and hence extend the duration time of free rotation. [ABSTRACT FROM AUTHOR]