Published online by Cambridge University Press: 05 December 2011
When very light particles are sprinkled on a resonating horizontal plate, inverse Chladni patterns are formed. Instead of going to the nodal lines of the plate, where they would form a standard Chladni pattern, the particles are dragged to the antinodes by the air currents induced by the vibration of the plate. Here we present a detailed picture of the mechanism using numerical simulations involving both the particles and the air. Surprisingly, the time-averaged Eulerian velocity, commonly used in these type of problems, does not explain the motion of the particles: it even has the opposite direction, towards the nodal lines. The key to the inverse Chladni patterning is found in the averaged velocity of a tracer particle moving along with the air: this Lagrangian velocity, averaged over a vibration cycle, is directed toward the antinodes. The Chladni plate thus provides a unique example of a system in which the Eulerian and Lagrangian velocities point in opposite directions.
Classic Chladni pattern: Top view of a flexible plate of dimensions 40 mm × 40 mm, resonating in its 2 × 2 mode and sprinkled with 80,000 heavy particles (density ρ = 20,000 kg/m3, diameter d = 0.075 mm). After a few seconds most particles have collected at the nodal lines, forming a classic Chladni pattern.
Inverse Chladni pattern: Top view of a flexible plate of dimensions 40 mm × 40 mm, resonating in its 2 × 2 mode and sprinkled with 80,000 very light particles (density ρ = 20 kg/m3, diameter d = 0.075 mm). Due to the presence of air, the particles now migrate to the anti-nodes and after about 4 seconds an inverse Chladni pattern has formed.