Glass From Space

In melts that are more fluid, like those stirred by gravity, the atoms move rapidly, so they can get into geometric arrangements more quickly. In thicker, more viscous melts, the atoms move more slowly. It's harder for regular patterns to form. It's more likely that the melt will produce a glass.

In microgravity, Day believes, melts may be more viscous than they are on Earth.

While this theory has not yet been confirmed, some experimental results suggest that it is correct. NASA researcher Dennis Tucker worked with fluoride melts on the KC-135, a plane that provides short bursts of near zero-gravity interspersed with periods of high gravity.

more The surfaces of ZBLAN fibers formed in near-weightlessness (upper panel) and in normal Earth-gravity (lower panel).

"He did some glass-melting experiments, trying to pull thin fibers out of melts," recounts Day. "During the low-gravity portion of the plane's flight, when g was almost zero, the fibers came out with no trouble. But during the double-gravity portion of the plane's flight, the fiber that he was pulling totally crystallized."

That result, says Day, could be explained by shear thinning. "A melt in low gravity doesn't experience much shear. But as you increase g, there'll be more and more movement in the melt." Shear stresses increase. The effective viscosity of the melt decreases. Crystallization becomes more likely.

Day is currently planning his next experiment in space--onboard the International Space Station--which he hopes will confirm his ideas. He'll be melting and cooling identical glass samples in the same way on Earth and in microgravity. Then he'll count the number of crystals that appear in each sample. If shear-thinning exists, he says, there will be fewer crystals in the space-melted samples than in the ones produced on Earth.

Eventually, Day hopes to take these lessons learned from space and apply them to glass production on the ground. Metallic glasses. Bioactive glasses. Super-clear fiber optics. The possible applications go on and on.... which makes the value of this research crystal clear.