An acoustic levitator design for suspending cosmic dust analogues and aerosol particles in light scattering experiments

We present a design of an acoustic levitator composed of 35 ultrasonic transducers operating at 40 kHz configured to form a spherical cavity. The acoustic radiation force measured experimentally in the center of the cavity is Frad≈9.6mN\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\varvec{F}}}_{rad}\approx 9.6 mN$$\end{document}, enough for levitating spheres as well as irregular particles of different materials of up to ~ 50 mg. Levitation tests have been performed with particles of different geometries and compositions, including liquid droplets and minerals relevant in studies of atmospheric aerosol and cosmic dust. This device has been deployed in the center of a polar nephelometer set-up to conduct studies of light scattering by irregular solid particles and liquid droplets. Test experiments have been carried out using a 1.5 mm diameter NBK- 7 glass sphere, for which three elements of the scattering matrix have been measured as functions of the scattering angle using a 647 nm diode laser. Mie theory calculations of the scattering matrix elements at this wavelength agree well with the measurements, demonstrating the functionality of the whole device.