Languorous Liquids

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.