Acoustic manipulation of particles by a resonant one-dimensional grating in air

Abstract

It is well known that acoustic wave carries momentum and energy. An object in a sound field, which absorbs or reflects sound energy, can be subjected to the acoustic radiation force (ARF), and thus can be manipulated in the contactless and noninvasive manners. This effect has potential applications in the fields of environment monitoring, microbiology, food quality control, etc. Obtaining a tunable trapping or pushing ARF should enable the design of an incident beam profile. However, the conventional acoustic manipulation system with plane wave, standing waves or Gaussian beams, which is usually generated directly by acoustic transducer, cannot be redesigned easily, nor can the corresponding ARF be modulated efficiently. Phononic crystals, which are artificial periodic structure materials, exhibit great advantages in modulating the propagation and distribution of acoustic wave compared with conventional materials, and thus have potential applications in tunable particle manipulation. Here, we present a theoretical study of the ARFs exerted on a cylindrical polystyrene foam particle near the surface of a one-dimensional (1D) grating in air. By using the finite element method (FEM) to investigate the transmission spectra and field distribution of the 1D grating and the FEM combined with momentum-flux tensor to obtain the ARF on the particle, we find that there are two resonance modes in the 1D grating, which origin from the coupling between the diffractive waves excited from the export of periodic apertures and the Fabry-Perot resonance mode inside the apertures. In addition, it can be seen from field distribution that in the first resonant mode, the resonance wavelength is approximate to the period of grating, and the enhanced spatial confinement of acoustic wave is located at the surface of the plate besides in the aperture. In the second resonant mode, the corresponding wavelength is more than twice the period of grating, and the enhanced spatial confinement of acoustic wave is mainly located in the aperture. Moreover, due to the gradient field distribution at the surface of slits and plate in these resonance modes, particles at the surface can be under the action of tunable negative ARFs. In the first resonance mode, the particle can be trapped on the surface of grating. While in the second resonance mode, the particle can be trapped in the aperture, and the amplitude of ARF of this mode is far smaller than that of the first mode. Thus, this system in the first resonance mode may have potential applications in air acoustic manipulation, aligning, and sorting micro-particles.