In Earthbound Labs, Researchers Unravel The Low-Gravity Physics Of Outer Space

July 26, 1996

You don't need to rocket into space to find out how chemicals will behave when gravity is gone, two researchers at the Johns Hopkins University say. Both are engaged in microgravity research funded by NASA, yet their experiments are taking place almost entirely within campus labs. Michael Paulaitis, chair of the Department of Chemical Engineering, has found a way to mimic the way chemicals mix in weightless conditions. His work could shape the way new polymers are made aboard a future space station.

Kathleen Stebe an associate professor in the same department, is studying the movement of immiscible fluids--ones that don't dissolve into one another, such as oil and water. Her findings could influence the way fuel is pumped aboard a spaceship--as well as the way ink-jet cartridges and pharmaceuticals are manufactured on Earth.

Yet neither is scrambling to put experiments aboard the space shuttle. To place research work on these vessels, scientists face stiff competition and a long wait. And because shuttle flights are short, and many scientific experiments are scheduled for each flight, astronauts have little time to devote to any one experiment. As a result, studying the effects of low gravity without going into space saves money and often yields more results, says Paulaitis, who recently joined JHU's Whiting School of Engineering faculty. "When you do experiments in the space shuttle, you don't have much latitude for doing what scientists and engineers do best," he says. "They make predictions, they try out their predictions, they make mistakes, and they learn from them. Then they go on and do it better the next time.

"It takes several years to get an experiment onto the space shuttle, and during that time your experiment is carefully planned and meticulously designed. Because of that, you don't play a lot of 'what ifs' once your experiment is in space. You don't have the luxury to try a lot of different experiments to come up with the best science. It's more cost-effective, and it's probably the most efficient use of time to try many experiments like these on Earth first."

Paulaitis' research focuses on polymers, and outer space promises to be a fertile new environment for creating them. Polymers are very large molecules consisting of long, interconnected chains of smaller molecules. These macromolecules are the basis for many important materials, including plastics, synthetic fibers and coatings. They form when the smaller molecules, called monomers, come in contact and react or "polymerize" to form chains. These chains then react with more monomers to form longer chains, and eventually the polymer.

Invariably, the large polymer molecules will not mix with the much smaller monomer molecules, and the polymer separates or precipitates to form another distinct liquid or solid phase. This precipitation or phase separation slows down or completely stops the polymerization process.

But scientists often want the polymerization process to continue, so that a particular polymeric material is produced. To encourage this process, scientists and engineers can prepare a stable suspension in which the polymer phase does not separate from the phase containing the monomer. Yet making such suspensions can be a problem on Earth, where gravity causes the two phases to separate quickly. Think of a can of soda pop when it is opened and the pressure inside is released. The lighter gas bubbles "fizz," or phase-separate, from the heavier liquid soda pop and rise to the top. In the weightlessness of space, this separation would occur very slowly. Meanwhile, the bubbles would remain suspended throughout the container. If the gas bubbles contained monomer molecules, and the liquid soda pop contained the polymer that is being formed from them, this weightless suspension would be a good medium for making polymers because it would keep the two key ingredients close to one another. Paulaitis has found a way to recreate this effect on Earth. By combining water, carbon dioxide and a surfactant (for example, soap) under pressure, he has developed a unique solution that forms two phases with the same density. Because of this, these phases do not readily separate. Instead, they form a stable suspension--a friendly environment for forming polymers.

Over the next four years, Paulaitis plans to study how this suspension forms, how long it remains stable, and how its properties change with time. This is important because the properties of a polymer material--whether it is flexible or stiff, hard or soft, for example--are related to the environment in which it forms. Paulaitis' collaborator, Professor Eric Kaler at the University of Delaware, will try to create new polymers in this mixture.

To confirm their results, the professors plan to ask NASA to recreate some of their experiments aboard a shuttle flight in three or four years. But ultimately, Paulaitis believes their findings on Earth may form a blueprintfor creating a wide variety of new polymers in space. In her own lab at Hopkins, Stebe is using a NASA grant to study the motion of immiscible fluids; for instance, the movement of drops and bubbles suspended in other fluids. Specifically, her research focuses on the dynamics of fluid interfaces and how they are affected by surfactant molecules. A simple example of an interface is the surface of a water drop--the boundary between water and air. When a droplet moves in air, its interface also moves.

Surfactants are molecules that possess one segment that prefers to be in water and another that does not. To keep both pieces "happy," or minimize their energy, the molecules sit at interfaces, so that one part can remain in water and another can stick out. This reduces the surface tension. If the density of surfactant at the interface increases, the surface tension decreases still further. As a result, the motion of the interface depends on the amount of surfactant on it.

Surface tension causes interfaces to resist stretching. That's why droplets and bubbles try to remain spherical--to minimize their surface area. Reducing the surface tension by adding surfactants can improve the ability of droplets to deform and mix with other fluids.

Understanding this process can improve our ability to control immiscible fluid systems. That can produce benefits in space and on Earth. "NASA's interest is because they're constantly moving fluids from one place to another in space," says Stebe. "They are interested in the fundamental issues behind fluid flow in order to use it effectively in microgravity."

On Earth, Stebe's research could lead to advances in the booming surfactant industry, where global sales in 1996 are expected to approach $10 billion, according to one recent estimate. Surfactants play a key role in a wide array of applications that require the mixing of two immiscible fluids. These include the creation of coatings for textiles and paper, formulation of inks and paints, and development of biomedical diagnostic tests. Manufacturers have used surfactants for many years, but they know very little about how their molecular structure influences their properties at interfaces, Stebe says. "It's sort of like wine-making in ages past," she says. "The practice ran way ahead of our fundamental understanding of the science behind it."

These microgravity research projects are being funded entirely by NASA. Paulaitis and Kaler will receive almost $500,000 over four years. Stebe will receive approximately $310,000, also over four years.

Johns Hopkins University

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