Phys. Rev. Lett. 101, 114501 (2008)

As a stream of liquid flows down a slope, it often begins to meander. Björn Birnir of the University of California, Santa Barbara, and his colleagues show that if the liquid flow fluctuates this can happen even on a perfectly smooth, clean surface that does not erode.

The researchers ran a stream of a water mixed with glycerine down a plastic sheet. As the flow changed from smooth to pulsed, the rivulet began meandering. The wavy path can be sustained once the pulsing stops, as the stream hits droplets left on the surface during its earlier wanderings.