Multi-Frequency Acoustic Levitation and Trapping of Particles in All Degrees of Freedom

Stabilization of the position and orientation of non-spherical, sub-wavelength particles in mid-air is required for using acoustic levitation forces in applications such as automation of micro manufacturing processes, 3-D scanning, and inspection. Acoustic locking has previously been demonstrated by time-multiplexing of different acoustic traps at the same frequency. In this case, the magnitude of the acoustic levitation forces and the stabilizing torque are coupled by the ratio of the durations during which the different traps are applied and cannot be adjusted independently assuming operation at maximum power. This work presents a compact device that uses a method for independently adjusting the vertical trapping forces and the stabilizing torque using two different ultrasonic frequencies. A 40-kHz vertical standing wave is used to generate levitation forces that counteract the gravitational force. Additionally, a 25-kHz horizontal standing wave is used to generate a tunable stabilizing torque. Using this method, objects made from high-density materials across a wide range of geometries can be locked acoustically with increased stability compared with state-of-the-art methods. This is demonstrated by locking tin cuboids with a density of 7.3 g/cm³ and plastic cuboids with average side lengths between 0.9 and 3.5 mm. The experimental results demonstrate torsional spring constants of up to 50 nN · m/rad and an orientation stability of <7.5°.