Fine points of melting plasma crystals.

The melting of a solid is such a commonplace occurrence that it may seem surprising that scientists do not yet completely understand the behavior of atoms and molecules during this transition.

The process is so complicated that computer simulations fail to capture its subtleties. Moreover, because relatively few particles make the transition at any given time, it's difficult to detect their movements amid those of all the other particles present in the coexisting liquid and solid states. Now, researchers can view on a microscopic scale how melting takes place. Instead of looking at water molecules in ice or atoms in solid copper, they peer at tiny plastic spheres immersed in a plasma of ionized gas. Under suitable conditions, these spheres spontaneously arrange themselves into orderly patterns resembling arrays of atoms in a crystal (SN: 8/6/94, p. 84). Lowering the gas pressure of the plasma causes this orderliness to disappear, just as raising the temperature causes an ordinary solid to melt. Such plasma crystals "are ideally suited for investigating the processes underlying the solid-to-liquid phase transition," Hubertus M. Thomas and Gregor E. Morfill of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, report in the Feb. 29 Nature.

The researchers use a high voltage to strip electrons from krypton atoms and create a weakly ionized plasma similar to that in a fluorescent light. Micrometer-size plastic spheres sprinkled into the plasma collide with electrons and ions, quickly picking up a negative electric charge. Repelling each other, the spheres space themselves out uniformly across a dozen or more layers. These layers stack up to form a thin, disk-shaped cloud only a few millimeters in diameter.

Using a sheet of laser light to illuminate a layer in the plasma crystal, the researchers can observe what happens to the particles as the gas pressure is lowered. These observations suggest that melting passes through two intermediate stages between the crystalline solid and the liquid state. As melting begins, the array breaks up into islands of crystalline order (somewhat like ice cubes in water) around which flow streams of particles. These small crystalline regions then disintegrate, but the particles settle into a new orderly pattern. The particles also vibrate noticeably about their equilibrium positions (see image).

What isn't clear at this stage is whether this unusual vibrational state is peculiar to plasma crystals or whether it could be a sign of a hitherto unknown intermediate stage of melting in a wide range of materials. "You're drawing an analogy between spheres in a plasma crystal and atoms in a simple solid," comments David G. Grier of the University of Chicago. "A lot more needs to be known about the interaction between these spheres and the electric field [in the plasma]."