Taming tornadoes: Controlling orbits inside acoustic vortex traps

Abstract

Bessel-shaped acoustic vortices have been theorized to be ideal candidates for tractor beams—that is, acoustic beams that exert forces opposite to the direction of propagation. Recently, it has been shown experimentally that these pulling forces exist and that particles can be trapped inside a focused vortex beam, both in water and air. However, our experiments and simulations reveal that the behavior of particles trapped inside a focused vortex is strongly dependent on the particle size. We trapped expanded polystyrene spheres (ranging from 0.6 mm to 4 mm diameter) 4 cm above a phased array made of 52 transducers (40 kHz and 1 cm diameter each) that generated an airborne acoustic vortex. Depending on its size, a particle remained trapped, oscillated between being trapped and orbiting, orbited or was ejected; only small particles (< 1 mm) were stably trapped. We then devised a new procedure for making vortex traps stable for a wider range of particle sizes; the strategy relies on switching the direction of the vortex at 4 kHz. In addition, by regulating the amount of time that each direction is emitted, it is possible to controllably rotate symmetric particles. This enables new applications in particle manipulation such as centrifugation of cells or actuation of micromachines.