Acoustic Radiation Force Tailored By Wave Beams And Artificial Structures

As a non-contact manipulating tool, optical radiation forces have been attracted growing interest of researchers for broad applications, In recent years, the focus of optical radiation forces has changed from the traditional particle-trapping to new phenomena of manipulating particles, such as negative radiation forces, particle rotations, and photonic clusters. Acoustic waves can also produce similar effects, i.e., so-called acoustic radiation forces. Due to the low required power, less heat damage, owing the ability to penetrate in optically opaque materials, and evident force effects, acoustic radiation forces have thus aroused much attention. Despite following the investigations of optical radiation forces and complementing the insufficiencies of them, the investigations of acoustic radiation forces have developed separately. However, there are still many problems at the aspect of tailoring acoustic radiation forces at will. In this dissertation, we start with acoustic sources (wave beams) and the propagation (artificial structures). Through numerical simulation by the multiple-scattering theory and the finite element software, we investigate acoustic radiation forces tailored by wave beams and artificial structures. The main works are as follows:1. Negative acoustic radiation forces exerted on a spherical particle by two crossed plane waves.A single plane wave and other ordinary beams can only produce positive (i.e., along the direction of wave propagating) acoustic radiation forces. We prove that negative (i.e., opposite with the direction of wave propagating) acoustic radiation forces can be realized by two crossed plane waves due to interfering effects. Through rigid derivations, we rewrite the acoustic radiation forces in the form of the superposition of the non-interfering contribution from the two plane waves separately and the interfering contribution between the two plane waves. Once the interfering part is negative and dominated over the non-interfering part, the acoustic radiation forces reverse. We extend two crossed plane waves to multiple plane waves, i.e., arbitrary non-diffracting beams, finding that the conclusion is still valid. At last, we investigate the acoustic radiation forces on a sphere with all kinds of material parameters in the condition of low frequencies and find that for larger incident angles, heavier but softer, lighter but harder particles are favor to suffer negative acoustic radiation forces.2. The acoustic-wave-induced mutual forces between multiple particles.Through calculating the acoustic radiation forces of the simplest system composed of two cylinders incident by a plane wave, we find that acoustic wave can induce mutual forces between cylinders due to the multiple-scattering effect. The mutual forces can be repulsive and attractive changing with the frequency of incident waves and the distance between scatterers. This fact means that the scatterers have balanced sites and can form some kind of structure. Similar phenomena can also happen for multiple cylinders or spheres. We use the dynamic matrix for the balanced structures to analyze the stabilities with perturbation. Our results demonstrate that in plane standing waves, multiple spheres can assemble to form stable particle clusters, i.e., artificial lattices, by the wave-induced mutual forces due to the multiple-scattering effect.3. Enhancement of the acoustic radiation force on the rigid wall utilizing the artificial grating structures.We calculate the radiation force on a rigid wall combing with an artificial grating structure composed of a rigid plate with periodic slits. The curve of the radiation force as a function of frequency shows a peak in the low frequency range. From the field distributions, it can be found that there is an evident enhancement of pressure filed in the air-borne background between the grating and the rigid wall. Using the effective medium theory, we analytically prove that there is indeed a resonant peak in the low frequency range viewing the artificial grating structure as a homogenous plate. This resonance enhances the pressure of the air near the rigid wall and thus magnifies the acoustic radiation force. Combing with the situation for oblique incidence, we discuss the effect of structure parameters to resonant frequencies and magnification, and then summarize the adjustability of resonant frequencies and magnification for introducing the artificial grating structure.4. Strong attractive forces between two structured steel plates with periodic bumps induced by acoustic waves.We calculate the acoustic radiation forces between two structured steel plates with periodical bumps induced by a plane wave, and find that there is a strong attractive force between the two plates. From the field distributions, it can be found that the strong attractive force is relative with the nonleaky asymmetrical Lamb wave modes in the two plates. These Lamb wave modes excited in the two plates can be coupled with each other and produce a symmetric coupling mode which excites a dominated transversal velocity (parallel with plates) in the fluid background on the middle plane between the two plates. From simplified formulae of acoustic radiation forces by symmetry, it can be known that large transversal velocities in the fluid between the two steel plates are essential for strong attractive mutual forces. This method of realizing strong attractive mutual forces can be extended to other periodic structures when transversal velocities are dominated for coupled modes of resonances.