Upcoming experiments planned for the International Space Station will help engineers on Earth learn to handle undercooled fluids.

by Patrick L. Barry

High-performance golf clubs. Ultra-sharp knives. Superior fiber optics for telecommunications. Tough, lightweight materials for future spacecraft.

What do all these things have in common? They can all be made using "undercooled" liquids: molten materials that are cooled below the normal freezing point yet, through special handling, are kept in a liquid state.

By avoiding normal freezing, one can coax the liquid into becoming a very different kind of solid. In normal freezing, the molecules of the liquid settle into an orderly crystalline grid, like soldiers falling in line. This is how ice, normal metals, and indeed most solids are formed. Undercooled liquids solidify in a different way. As they cool, they thicken and eventually stop flowing - like a liquid "on pause." The result is a solid whose molecules remain scrambled in a semi-random, amorphous arrangement. This molecular structure, most commonly found in window glass but possible in metals, too, has special properties. Amorphous metal alloys, for instance, can be twice as strong and three times more elastic than steel.

There's great potential for products made from these liquids, but they are notoriously difficult to handle.

An undercooled liquid is a delicate, unstable state of matter. It desperately "wants" to crystallize into a normal solid. All that's needed is a place for the crystallization to begin - such as the crystalline surface of a container wall or even a speck of dust - and the liquid will suddenly freeze solid. In other words, working with undercooled liquids is a bit like juggling mousetraps: they're prone to suddenly "snap" and ruin the trick.

Image courtesy Liquidmetal Technologies. A few of the things manufacturers can make better using undercooled fluids.

Remarkably, manufacturers on Earth have managed to make some products from these liquids anyway: computer components, golf clubs, tennis racquets. There's even a solar wind collector on board NASA's Genesis spacecraft made of undercooled amorphous metal.

These items are just the beginning. As engineers learn more about the basic physics and properties of undercooled fluids, they'll be able to do more with them. And that's where the International Space Station (ISS) can help. In the weightlessness of Earth orbit, it's possible to study fluids without holding them in containers that might trigger premature crystallization.

Edwin Ethridge, a materials scientist at NASA's Marshall Space Flight Centre, and Prof. William Kaukler of the University of Alabama in Huntsville are working on a way to measure the viscosity of containerless fluids onboard the ISS. Their idea is simple: If two floating drops of a liquid touch each other, they will merge to form a single, larger drop. The speed of this merger is partially controlled by viscosity - water will merge much faster than honey, for example. So watching this speed lets scientists measure the liquid's viscosity.

Good viscosity measurements are critical for working with undercooled fluids, which thicken dramatically as they cool. The friction between molecules in one of these cooling fluids can skyrocket by as much as a quadrillion times (10¹⁵) as it solidifies. Without a graph plotting how this thickening occurs in relation to cooling temperatures, engineers can't easily mould these liquids into useful shapes.

The speed at which droplets merge depends on their viscosity.

To understand why, just imagine what would happen if you designed a mould with lots of complex nooks and crannies so that it works well for undercooled liquids with the thickness of vegetable oil. But as you poured the undercooled liquid into the mould, it cooled slightly, causing an unexpected thousand-fold thickening - rendering the liquid as thick as honey. The object produced is likely to look more like modern art than a saleable product.

Getting the data to make viscosity vs. temperature curves is the ultimate goal of Ethridge and Kaukler's research. Their upcoming experiment, called Fluid Merging Viscosity Measurements (FMVM), is a proof of concept. It will show how viscosity measurements of containerless fluids can be made in the microgravity environment of the ISS.

The physics is hard enough, but the scientists had to tackle another problem as well: Because room for sending research equipment up to the station is limited while the shuttle fleet is grounded, the researchers had to find a way to do their experiment using things that can be tucked inside a Russian Progress supply rocket or found already onboard the station.

"I have selected 8 liquids for testing," says Ethridge. "They've been loaded in syringes that will be launched on a Progress rocket to the space station." One of them is ordinary honey. Although it only crystallizes very slowly, honey is actually an undercooled liquid. It works just fine for proving that this "floating drop" method can accurately measure a liquid's viscosity.

The strength and elasticity of amorphous solids ("glassy alloys") exceed that of many other materials.

The experiment goes like this: Honey (or one of the other liquids) will be squeezed from its syringe and transferred onto thin strings. "Nomex thread and string is available on the space station and can be used to confine and control the liquid drops in orbit. Thin solder wire may also be used to manipulate the drops," notes Ethridge. With a drop clinging to each of two strings, a crew member will bring them slowly together, allowing the drops to gently touch and merge. A video camera kept aboard the station will record what happens as the drops slowly form a peanut shape and eventually a single sphere.

Back on the ground, researchers will examine the footage frame by frame to determine exactly how fast the drops merged. Because the viscosity of the test samples is already known, researchers can compare the measured value with the real value to see if they're on the right track.

The researchers currently plan to conduct the FMVM experiment sometime during Expedition 8, which is scheduled to begin in late October 2003. Their work could result in a new way of knowing the viscosity of undercooled liquids. And after that... no one knows, but golf clubs and kitchenware are probably just the beginning.