Chladni plates

In 1787, German physicist and musician Ernst Chladni published a technique to visualize the modes of vibration of a rigid surface.

Chladni’s technique […] consisted of drawing a bow over a piece of metal whose surface was lightly covered with sand. The plate was bowed until it reached resonance, when the vibration causes the sand to move and concentrate along the nodal lines where the surface is still, outlining the nodal lines. The patterns formed by these lines are what are now called Chladni figures.

Chladni was inspired by early experiments by Robert Hooke who discovered the phenomenon in 1680, using flour instead of sand. This technique is still in use today to design acoustic instruments, with the difference that a loudspeaker is generally used instead of a bow. In the 1960s, Hans Jenny, a Swiss physician and natural scientist carried many experiments using sand, dust and fluids, and founded an mystical discipline called Cymatics based on exploring these phenomena.