All Wet

Barbara Hale
September 01, 1996

We'll never get wet the same again.

Results from a Space Shuttle experiment have disproved Young's equation, the accepted explanation for how liquids wet solids. Liquids form regular "beads" on a solid surface, like rain on a polished car hood, as Young's equation predicts, only when the surface is horizontal to gravity. On any other surface on Earth—or in space—the drops are irregular, like teardrops.

According to Randall M. German, the Brush Chair Professor in Materials at Penn State and principal investigator on the project, disproving Young's equation will have far-reaching effects: The equation underpins basic knowledge in physics and chemistry, and is used extensively in German's own field of materials science.

Wetting, however, wasn't the main focus of the experiment. German and his colleagues had thought that a low-gravity environment would improve liquid sintering. In this alloy production technique—used to make the fillings in your teeth as well as numerous other products, from golf clubs to radioactive shielding—several types of powdered metals are mixed, compacted, and heated. Some of the metal particles melt and surround the ones that don't. On Earth, the metal alloys produced this way sometimes have empty spaces or pores where the liquid melt failed to wet the surfaces of some of the unmelted particles. The solid particles can also clump together, forming grainy areas, and the finished product sometimes assumes an unintended shape.

In space, the researchers reasoned, the powdered metals would not settle or clump without gravity's downward pull, and pores would be tiny or non-existent, based on Young's equation.

The experiment turned theory on its head. The samples processed in space had large irregular pores impossible to predict with Young's equation. They had grainy areas and got just as distorted in space as on Earth.

German says the experiment shows that we probably can't make alloys in space that are very different from the ones on Earth. But the new insight will most likely lead to previously undreamed of applications. For example, since pores persist in metal alloys in space, it will be easier to make foam metal materials in microgravity. Metal foam, which resembles plastic foam board, is a new class of materials that is ultra-lightweight but very strong.

German and his research team are preparing to send another experiment to space in March 1997 to verify the unusual findings of his first space experiment.

Randall M. German, Ph.D., is Brush Chair Professor of Materials, 118 Research Building West, University Park, PA 16802; 814-863-8025. Nearly a dozen graduate and postdoctoral students have worked with him on this experiment. Their results on Young's equation were published in Acta Materialia 44:4.

Last Updated September 01, 1996