Direct measurement of the radiation force of a focused acoustic beam on a spherical particle in water

The purpose of this paper is to present a method for direct measurement of the acoustic radiation force exerted by an ultrasonic beam on a millimeter-sized spherical particle in a liquid and to compare the measurements with theoretical predictions. A focused ultrasonic beam was generated by a concave piezoceramic transducer with a center frequency of 1.072 MHz, a focal length of 70 mm, and an aperture of 100 mm. The transducer was placed face down in a water tank. A spherical bead was fixed under the source in the center of a plastic ring with the help of stretched fishing lines. The ring was attached to a rigid frame, which rested on an electronic balance without touching the tank. A thin membrane was placed above the bead to dampen the acoustic streaming. A positioning system was used to place the bead at different points on the transducer axis. The radiation force measurements were in good agreement with theoretical predictions. Some discrepancy occurred when the bead was close to the focal point, which was caused by the appearance of standing waves. Proper selection of the electrical impedance of the transducer helped reduce this artifact.