Measurement of microscopic spatial distribution of acoustic radiation force using microspheres and electrostatic force

A simple method for measuring the microscopic spatial distribution of the acoustic radiation force by exploiting the competition between the acoustical radiation force and the electrostatic force on charged microspheres is investigated. First, the effective charge of polystyrene spheres is measured by monitoring the mobility of the particles in the 1 Hz, 13.4 V_pp/mm alternating electric field. Next, the shift of the particles away from the sound pressure node with an increase in intensity of the electrostatic force is observed by optical microscopy, and finally the acoustic radiation force is estimated. In the experiment, the spatial distribution of the acoustic radiation force in the 500 kHz, 80 Vpp standing wave was proportional to the sine curve, and had the same tendency as the result obtained from calculation. The intensity was 0.5 pN for 7 μm polystyrene spheres.