Princeton Weekly Bulletin November 16, 1998

Colloids in space

Astronauts collaborate on Princeton professors' experiment aboard shuttle
 

By Maria LoBiondo

While the nation focused on astronaut John Glenn's return to space, physics professor Paul Chaikin and chemical engineering professor William Russel monitored their own version of a space encore.

	Photo taken on board the shuttle shows colloid sample that "remained in a glassy state on the ground, but fully crystallized in two days in microgravity," according to Princeton researcher Zhengdong Cheng.

An experiment of theirs with colloids -- fine particles dispersed in a fluid -- was performed on a 1997 shuttle voyage; a similar experiment was conducted during the flight of the space shuttle Discovery from October 29 to November 7.

Though colloids are currently used in the technology of materials ranging from paints to drugs to foods, many aspects of their behavior are unknown. Better understanding of the structure of colloids may allow scientists to manipulate their physical properties to develop new materials or improve products already in use.

Nagging mystery

Just what happens when a liquid turns into a solid? It sounds like a simple question, but for Chaikin and Russel, the specifics of how atoms interact in an orderly way, forming rigid patterns to make a solid, or in a disorderly way, flowing in a liquid state, is a nagging mystery. Colloids, perfect spheres all the same size, are larger than atoms and are used to model atomic behavior.

In the recent space experiment, eight test colloid samples in various densities were made with what is commercially called plexiglass. The spheres were about one-tenth the thickness of a human hair. The astronauts recorded what the samples did when gravity was not a variable in their interaction.

The experiment tested whether the colloids would form a crystal (an ordered solid) or a glass (which has a rigid structure but is not ordered). The entire experiment was performed in a "glovebox," a small container where an astronaut reached in to handle the materials with attached gloves rather than holding the samples directly.

Crystal or glass

As Chaikin and Russel explain it, the most surprising finding of the 1997 experiment was that a sample with a little more than 50 percent density, which does not crystallize when tested on Earth, did so in space. A second sample, with a density of 60 percent, which never crystallized on Earth, formed a rigid structure, like a glass, in space.

"We thought they'd grow more compactly, like crystals that settle to the bottom of a cylinder. But in micro-gravity, they didn't settle. They formed dendrites, looking like snowflakes," Chaikin said.

The new experiments confirmed the 1997 results, according to Zheng-dong Cheng, who monitored the experiments from a NASA control center in Cleveland, Ohio. Cheng received his Princeton doctorate in physics two days before the space mission.

The larger picture of what the experiment means to both professors differs. "For Paul, a condensed matter physicist, the focus is how solids make the transition," says Russel. "For me, the experiments give an understanding of very concentrated colloidal dispersion, which is ubiquitous in the chemical industry and in the materials process."

Chaikin and Russel first collabo-rated on a smaller scale colloidal experiment with NASA in 1995, performed on the space shuttle Columbia. NASA has funded their experiment, Dynamics of Colloidal Disorder-Order Transition (CDOT), and approved the project for two more experiments that may be done on the future space station in 2002.