This micrograph shows interparticle collisions induced by ultrasound between tin and iron particles about 20 microns in size. The velocity of such collisions can be as high as 500 m/s (1100 mph). The elemental composition dot map was produced by scanning Auger electron spectroscopy and show tin in orange and iron in blue. [Courtesy of Kenneth S. Suslick and Stephen J. Doktycz, University of Illinois at Urbana-Champaign, on instrumentation in the Center for Microanalysis of Materials, from work funded by the National Science Foundation]

Doktycz, S. J.; Suslick, K. S. "Interparticle Collisions Driven by Ultrasound," Science, 1990, 247 1067-1069.