Acoustic Levitation and the Particle-Size Effect

Synopsis

The phenomena of matter suspended within an acoustic standing wave has been known since the 19th century, with work from L. V. King and L. P. Gor’kov in calculating the acoustic radiation force in the small particle regime (𝑟≪𝜆). Since then, airborne acoustic levitation has found novel application in containerless material analysis, massive object levitation (~10g), and complex three-dimensional (3D) trajectories via Langevin transducer or phased array ultrasonic emitter configurations. Still, little is known on particle stability when size is comparable to driving wavelength (𝑟~𝜆). Exploring the upper limits of object size has led to recent analysis of the particle-size effect, from generalized Lornz-Mie theory and experimental study. The ultrasonic acoustic force and thus the particle’s behavior, critically depends on size. While small particles are trapped on-axis at pressure nodes, larger spheres find equilibrium off-axis at antinodes. Their results suggest extension to any complex amplitude structure, such as in 3D display technologies and surface-mount circuit board construction.