Campus control of crystal growth in space.

A command typed into a computer last week at the Rensselaer Polytechnic Institute (RPI) started a new round of crystal-growing experiments. The signal also represented the first time that NASA has allowed a team of researchers at a campus site to take control of an experiment aboard an orbiting space shuttle.

Following a circuitous route, the signal traveled from a control room at RPI in Troy, N.Y., to NASA's Lewis Research Center in Cleveland, to the Marshall Space Flight Center in Huntsville, Ala., to the Johnson Space Center in Houston, and finally, to the space shuttle Columbia. Normally, commands for shuttle experiments originate at the Marshall Space Flight Center.

"This was an important operational experiment, as well as a significant science experiment," says RPI's Martin E. Glicksman, principal investigator for the isothermal dendritic growth experiment.

This crystal-growing effort was one of four scientific experiments that made up the U.S. Microgravity Payload, launched into Earth orbit aboard Columbia on Feb. 22 and returned to the ground on Mar. 9. Though overshadowed by the drama of a snapped tether and a lost satellite (SN: 3/2/96, p. 134), these experiments generated high-quality data that may prove useful in elucidating the behavior of materials on Earth.

Glicksman and his coworkers focused on the growth of dendritic crystals, which have a tapered, branched shape resembling that of a spruce tree (SN: 1/13/96, p. 31). Such crystals often form when molten metals solidify.

By remote control, the researchers repeatedly melted and cooled samples of an organic material known as succinonitrile. This compound mimics the crystallization behavior of common metals and alloys but is transparent. Photographs, therefore, can reveal any crystals that form within the liquid (diagram below).

Performing these experiments in an environment in which the apparent force of gravity is only one-millionth as strong as it is on the ground, the researchers could observe how crystals form in the absence of gravity-driven settling and fluid flow. They found a basic pattern of branching similar to that observed on Earth, but the growth rate was dramatically slower. Small fluid flows caused by Columbia's gradual change in orientation and other slow movements of the shuttle did not significantly alter that rate.

By generating three-dimensional images from pairs of photographs taken of dendritic crystal formation in space, the researchers at RPI can now begin probing the effect on solidification of such factors as the sample container's shape and size.