QUANTIFICATION OF ACOUSTIC RADIATION FORCES ON SOLID OBJECTS IN FLUID

Theoretical models allow design of acoustic traps to manipulate objects with radiation force. Here, a model of the acoustic radiation force by an arbitrary beam on a solid object was validated against measurement. The lateral force in water of different acoustic beams was measured and calculated for spheres of different diameter (2-6 wavelengths λ in water) and composition. This is the first effort to validate a general model, to quantify the lateral force on a range of objects, and to electronically steer large or dense objects with a single-sided transducer. Vortex beams and two other beam shapes having a ring-shaped pressure field in the focal plane were synthesized in water by a 1.5-MHz, 256-element focused array. Spherical targets (glass, brass, ceramic, 2-6 mm dia.) were placed on an acoustically transparent plastic plate that was normal to the acoustic beam axis and rigidly attached to the array. Each sphere was trapped in the beam as the array with the attached plate was rotated until the bead fell from the acoustic trap because of gravity. Calculated and measured maximum obtained angles agreed on average to within 22%. The maximum lateral force occurred when the target diameter equaled the beam width; however, objects up to 40% larger than the beam width were trapped. The lateral force was comparable to the gravitation force on spheres up to 90 mg (0.0009 N) at beam powers on the order of 10 W. As a step toward manipulating objects, the beams were used to trap and electronically steer the spheres along a two-dimensional path.