Acoustic levitation

Acoustic levitation. A popular 1980s television commercial featured a man being blown backward by the sound coming from his stereo. Though a speaker’s pressure field isn’t nearly that strong, acoustic pressure can suspend millimeter-sized objects against the force of gravity. New work by Pierre Lambert and colleagues at Belgium’s Université libre de Bruxelles compares existing theoretical models, numerically computes the strength and direction of the acoustic forces, and validates those results with experimental measurements on objects suspended in standing waves. The researchers find that for spherical objects with radii bigger than one-sixth of the acoustic wavelength—or for nonspherical objects—analytical models don’t suffice and numerical modeling is required. The team’s calculations and experiments show that at pressure nodes, levitation in an axially symmetric standing wave is stable for both small displacements and angular tilt. Furthermore, when an instrument such as tweezers is inserted laterally into the field, the pressure distortions will push the levitated object to the probe, turning the levitator into a component feeder. Levitation could also find use in a wide variety of other settings; isolated analysis of small samples of proteins and the sorting and separating of items according to their size are just a few examples. (V. Vandaele, A. Delchambre, P. Lambert, J. Appl. Phys. 109, 124901, 201110.1063/1.359424500218979.)