Acoustics forces on a solid sphere in focused sound fields and their use for acoustical traps

It is known that stationary sound fields can be used to levitate small objects in air; this phenomenon has potential applications in containerless processing of materials. Recently the use of acoustic forces have been considered for the manipulation of small samples, which offers several advantages in the cases of hazardous substances, processing of materials under pure conditions, handling of fragile or sticky objects, for instance. Several theoretical investigations on the use of focused Gaussian and Bessel acoustic beams have been reported in literature. In those papers, water has been assumed as the medium for the propagation of the acoustic waves. The objective of the work to be presented has been to study the extent to which it is possible to use focused sound fields for the manipulation of a rigid sphere in air. The possibility of developing acoustical tweezers has been the main motivation of the work. A theoretical investigation based on the boundary element method (BEM) is first described, where the acoustical forces on a rigid sphere are analyzed. It is assumed that the focused sound field is generated by means of a piezoelectric transducer with a shape of a section of a sphere, which is a common practice. Two configurations are analyzed in the theoretical investigation: (1) a pure focused acoustic beam, and (2) the sound field between one concave transducer and a reflector. The acoustic forces are studied as a function of the size of the sphere, the size of the transducer, the location of the solid in the sound field, and the wave frequency. The use of the BEM allows the study in both the Rayleigh regime (the diameter of the sphere is much smaller than the wavelength) and the Mie regimen (the diameter is larger than or close to the wavelength). The obtained theoretical results on the feasibility of acoustical tweezers are verified by means of experiments.