"The
smaller the particle, the faster the interface velocity has to be to incorporate
those particles," like a river sweeping past boulders or sweeping up
pebbles, explained Subhayu Sen of the Universities Space Research Association.November 26, 1997
United States Microgravity Payload-4 - Flight
Day 7
What do stored blood, a roadbed, automobile brakes, and a jet engine have in common?
They all may benefit from a better understanding of how tiny beads are engulfed or pushed out of the way when a liquid freezes in an experiment which made its first runs aboard Space Shuttle Columbia Tuesday evening.
"The
smaller the particle, the faster the interface velocity has to be to incorporate
those particles," like a river sweeping past boulders or sweeping up
pebbles, explained Subhayu Sen of the Universities Space Research Association.
Sen is a co-investigator for the Particle Engulfment and Pushing (PEP) by a Solid/Liquid Interface experiment. Dr. Doru Stefanescu of the University of Alabama is the principal investigator.
While the river is a good analogy, solving the question involves a number of variables, including the speed of the freeze front, the size and mass of the particles, and the viscosity (or thickness) of the liquid.
The objective is to design materials so particles will be incorporated evenly throughout the new solid that is formed, and not pushed out of the way.
"It's a multidisciplinary area," Sen said, that affects the manufacture of composite materials or even roads and the storage of blood.
Most people are familiar with the problem of roadbeds cracking if the ground beneath them becomes cold enough to freeze.
"At first we though that was because water expands when it freezes," Sen explained. "But soon we found that the amount of expansion from water freezing is not enough to account for the cracking." What did cause it was soil particles getting pushed ahead of the ice.
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Freezing whole blood cells for long-term storage poses a similar problem. The ice front pushes the red cells until they are crushed against the walls of the container.
Again, the problem is found in making composite materials such as brake pads or jet engine parts where the designer wants to incorporate a reinforcing agent in the metal. Fiberglass is a composite in which glass fibers are held in place by a plastic resin.
In machine design, the objective is to incorporate tough particles such as silica carbide into metals such as aluminum, magnesium, titanium, and nickel. Like fiberglass, these metal matrix composites - called MMCs - share qualities of both materials without the difficulties of making an exceptional product out of one material or the other.
For example, a metal matrix can give the ductility of aluminum and the cutting edge of silicon carbide.
At present, though, such MMCs are limited to particles no smaller than 20 microns that will stay in place like the boulder in a flood.
"But if you want very high strength,"
Sen continued, "you have to go to a very small particle size, less
than 1 or 2 microns." And that's like the pebble that gets swept away.
"We don't understand the freezing interface interaction with these
particles very well," Sen said. PEP was designed to provide the answer.
Like many other materials experiments, it is a model using transparent stand-ins
for the real thing. Sen and Stefanescu hope the information from it will
provide the data to let them write new equations that describe when particles
will be engulfed or pushed. From that, materials engineers can design new
processes in which the freezing front is controlled to produce the right
compositions.
"What we are trying to do is come up with a model that that can be used for any system so long as you know its properties," he said. The key is the critical velocity , the speed of the freeze front at which the particles are captured or pushed. "If you can predict that velocity, then in some cases you can adjust it to make sure that the particles or captured or rejected."
PEP won't lead directly to roads that don't crack in a deep winter freeze, or to a better way to store blood cells or make high-strength machine parts. But it will give scientists and engineers the tools so they can start those designs.
"This is a fundamental issue, a scientific issue," Stefanescu said. "As with any scientific issue, if we solve it, then it applies to a wide number of problems."
In their model, Stefanescu and Sen are using two sample test cells, each made of two glass slides (like the ones you used in high school biology class) held together by a Teflon gasket just 0.75 mm thick. One cell contains microscopic plastic beads in succinonitrile (an industrial chemical used in making nylon). The other holds glass beads inside biphenyl, an organic acid. The beads range from 1 to 25 microns in diameter.
The experiment is conducted inside the Middeck Glovebox where the astronauts can operate it and provide closeup views through a microscope.
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The entire slide is warmed to melt the liquid inside, then mounted between a heater and a cold block that travel across the slide. As they do, the liquid inside freezes and the astronauts track it with a video camera for two to four hours, depending on the speed of the freeze front.
The apparatus works well on Earth, but is limited because gravity induces convection and particle sedimentation. If the slides are close enough to prevent convection, then the fluid interacts with the walls and, again, the push-or-engulf effect is obscured.
Stefanescu said it is like trying to listen to music while riding on a train: "We want to stop the train and get the noise out."
Thus, the experiment has to be run in space to get the right answer.
A similar experiment was run on the Life and Microgravity Sciences mission in 1995, but provided just one data point since the material used was Aluminum with zirconia particles, which is opaque.
"The reason you use organics is so you can see exactly what's going on," Sen said. "With transparent organics, we hope to get dozens of data points."
In PEP's first run - from Tuesday evening until early Wednesday morning - the glovebox overheated and the experiment shut down. The sample partially solidified, obscuring the view. During restart the sample became translucent and the PEP team could record the positions of particles to determine whether they were being pushed or engulfed.
One unanticipated find was that the clumps of particles (or agglomerates) were easily pushed in space although they are always engulfed on Earth.
"We did find things we did not anticipate last night," Stefanescu said, "and that's one of the beauties of doing this experiment in space."
PEP has another six specimen slides to be processed on this mission.
The team also is looking at the possibility of extended experiments aboard International Space Station. It developing an X-ray microscope that could give them a view inside MMCs as they solidify.
"We are just beginning to see, in metals for the first time, the interface bend around the particle or gas bubble," Sen said.
Check
out the daily
Mission Status Reports prepared by Marshall's Public Affairs Office.
Author: Dave
Dooling
Curator: Linda Porter
NASA Official: Gregory S.
Wilson
