Scattering and radiation force of helicoidal Bessel beams incident on spheres and the movable rigid sphere case.

Abstract

Understanding the scattering properties and coupling to modes of spheres in beams gives insight into several issues. Beam parameters may be selected to suppress or highlight the coupling to specific modes. When considering the radiation force on spheres by acoustic beams, it is also helpful to remember that while the axial wave-vector component has similarities to propagating waves, the transverse components may have similarities to standing waves. The present work examines the scattering and radiation force for the simplest Bessel beam having an axial null [P. L. Marston, J. Acoust. Soc. Am. 125, 3539–3547 (2009); F. G. Mitri, J. Phys. A: Math. Theor. 42, 245202 (2009)]. This is an acoustic vortex beam having a unit magnitude topological charge m. For some elastic and fluid spheres such beams allow conditions giving negative as well as positive axial forces. Negative forces are found to be correlated with reduced backward hemisphere scattering (as previously reported in the m=0 case). A noteworthy example is the case of a movable rigid sphere since for low ka the scattering properties are similar to metallic objects in water and the elastic modulus only weakly affects the scattering and radiation force. [Work partially supported by ONR.]