A Troublesome Theory in Materials Science

The Lifshitz-Slyozov-Wagner (LSW) theory predicts the rate of coarsening in alloys. What's wrong with the theory? Strictly speaking, nothing. It's the way engineers have been using it that's wrong. The equations of LSW describe how fast materials will coarsen if you let them sit for an infinite amount of time. Forever. Most engineers can't wait that long, so they've assumed that the theory also works for shorter times - like hours and days.

more Solid tin particles coarsen within a liquid mixture of tin and lead over a 24-hour period. Snapshots of three different samples were combined to create this time series.

Testing this assumption was one of the goals of the Coarsening in Solid-Liquid Mixtures (CSLM) experiment, which flew onboard the space shuttle in 1997.

"The first shuttle experiments worked just as we'd hoped," recalls principal investigator Peter Voorhees, professor of materials science at Northwestern University near Chicago, Illinois. "But when we looked at the sizes of the grains, they were larger on average than the theory would predict."

Something was amiss.

Scientists had never been able to fully test the predictions of LSW in a liquid mixture because gravity always interfered with the most ideal experiments. To mirror the assumptions of the theory, an experiment would need to have solid, microscopic grains scattered evenly within a liquid. If you try this on the ground, the solid particles will quickly settle out of the liquid and accumulate at the top or bottom of the container, ruining the experiment.

Image courtesy NASA Glenn Research Center Gravity causes the tin particles to quickly sediment to the top of the chamber during ground experiments (right). For the same experiment run in orbit, the particles remain evenly dispersed (left).

"In space, the solid particles stay evenly dispersed for hours or even days, so we can compare the results directly with the theory," Voorhees says.