Nodes and Antinodes in Two-Color Chladni Figures

Chladni patterns are one of the most fascinating physics demonstrations. They can be produced very easily using commercially available apparatus based on metallic plates attached to a mechanical driver, similar to a loudspeaker without the membrane, and a signal generator. The figures are observed using sand sprinkled on the surface of the plate. The demonstration is even more impressive if the figures are produced by using a violin bow. In the usual demonstration the patterns show the nodal lines of the vibrating plate: the sand moves from the vibrating regions towards the nodes, where it remains at rest. This paper describes how the demonstration can be improved by using two different powders to produce two-color Chladni figures, where nodes and antinodes display different colors.