Determination of Radiation Pressure in Acoustic Levitation by Optical Acoustic-Field Measurement

Pulsed digital holography was used to determine the acoustic radiation force on objects suspended in a single-axis acoustic levitator. Digital holography provided both the object images and the optical phase data required to determine the instantaneous acoustic pressure. The phase data were utilized in tomography calculations, converting the measured line-of-sight variations to a pressure field by assuming that the system was axisymmetric. The pressure field was used to calculate the velocity field and these together were used to calculate the acoustic radiation pressure. This pressure was integrated over the surface of the object to determine the vertical force on the object. Force measurements were completed for five different polymer spheres, of different sizes and densities, and for an evaporating water droplet. For all but the heaviest sphere, the optically measured force agreed with the measured weight of the sphere within experimental uncertainty. For the heaviest sphere, the overall average showed an upward bias but was still only 4.7% high. For the water data, the optically measured force was consistent with the droplet volume as the droplet evaporated.