Measurement of distribution of acoustic radiation force perpendicular to sound beam axis

To clarify the range of force acting on a particle in the direction perpendicular to the sound beam axis of an ultrasonic standing wave for particle manipulation, the radiation force on a polystyrene sphere of submillimeter size was measured through observation of its movement in a 1.75 MH7, ultrasonic standing wave. The radiation force was estimated at some points by balancing with the force from viscous drag proportional to the measured velocity of a particle and the gravitational force, and its distribution was determined. The measured distribution of the radiation force agreed qualitatively with the theoretical distribution derived from Nyborg's formula. The influence of gravitational force on a stable position was also investigated. It was shown that the shift from the stable position is marked for particles with large radius. This effect should be considered in particle manipulation.