When a dusty plasma gets dirty

When a dusty plasma gets dirty. In a dusty plasma—whether in a laboratory or in outer space—free electrons, ions, and highly charged microscopic particles coexist in a background of neutral gas. In a laboratory version, the particles typically are uniform spheres that with the proper application of an electric field arrange themselves in a two-dimensional crystal structure. The dusty plasma then serves as a model for various condensed-matter systems. (See the article by Robert Merlino and John Goree in Physics Today, July 2004, page 32 .) But what happens if the spherical dust was not purified and is nonuniform in size? Physicists at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, have now found out. Introducing impurities—in the form of odd-sized particles—into their dusty plasma, the researchers first focused their camera on the dust layer as they normally would and saw the nearly regular structure shown in the left panel. Upon defocusing, however, an entirely new layer of particles was revealed to exist just beneath the main layer (right panel). What’s more, the new particles showed interference fringes (stripes in the inset) that often rotated, presumably as the particles moved. Further experiments showed that the lower layer contained the impurities, which had come together to form two-particle dimers. The layers of uniform spheres and of impurities were distinct and easily distinguished. As the number of impurities increases from zero, the pristine crystal layer gradually transitions to a completely disordered state. The technique should be useful in studying problems like the liquid–gas phase transition and glass jamming. (M. Chaudhuri et al., Appl. Phys. Lett. 100, 264101, 2012.)