Quantification of Acoustic Radiation Forces on Solid Objects in Fluid

Theoretical models allow design of acoustic traps to manipulate objects with radiation force. A model of the acoustic radiation force by an arbitrary beam on a solid object is validated against measurement. The lateral force in water of different acoustic beams is measured and calculated for spheres of different diameters (2-6 wavelengths. in water) and compositions. 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 are synthesized in water by a 1.5-MHz, 256-element focused array. Spherical targets (glass, brass, ceramic, 2-6 mm dia.) are placed on an acoustically transparent plastic plate that is normal to the acoustic beam axis and rigidly attached to the array. Each sphere is trapped in the beam as the array with the attached plate is rotated until the sphere falls from the acoustic trap because of gravity. Calculated and measured maximum obtained angles agree on average to within 22%. The maximum lateral force occurs when the target diameter equals the beam width; however, objects up to 40% larger than the beam width are trapped. The lateral force is 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 are used to trap and electronically steer the spheres along a two-dimensional path.