During the 19th century, Ernst Chladni and Michael Faraday independently explored the patterns formed by particles of different sizes placed on a vibrating plate. Faraday found that large particles accumulated at nodes of the plate, where there was no vertical vibration, whereas smaller particles moved toward anti-nodes, where air currents caused by the large vibration amplitude lifted them up.
The situation becomes a little different if you submerge the vibrating plate in water. Then large, heavy particles gather at the anti-nodes. Drag keeps the particles on the plate, while acoustic forces and gravity conspire to move the particles horizontally toward the anti-nodes (top). Because anti-node patterns change with frequency, this actually provides a way to manipulate particle’s trajectories. The researchers demonstrated this by steering a particle through a maze (bottom) as well as by manipulating an entire swarm of beads. (Image and research credit: K. Latifi et al.; via Physics World; submitted by Kam-Yung Soh)