Vortex-field Acoustic Levitation in Tubes

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.