Coulomb dust balls

Physicists at the Universities of Kiel and Greifswald in Germany have been able to produce spherical clouds of plastic 3.4-micron balls—“dust” particles—in the middle of a hot plasma. The particles managed to balance their mutual negatively charged repulsions with the plasma’s compressive force—shaped by a glass tube enclosing the particles—and arranged themselves into a void-free spherical cloud. A cloud’s internal form depended on its size. Coulomb balls of up to a few thousand particles showed nested, concentric shells near their surfaces. But no such order was apparent in the largest Coulomb ball (more than 6000 particles and 15 mm in diameter). The vertical slice through the center of that one, shown here, reveals a region of rapid liquid-like flow on the right and a frozen, solid-like region at the bottom. For more on dusty plasmas, see Physics Today, July 2004, page 32 . (O. Arp et al., Phys. Rev. Lett. 93 , 165004, 2004 http://dx.doi.org/10.1103/PhysRevLett.93.165004 .)