November 22, 1997
United States Microgravity Payload-4 - Flight
Day 3
One
of the neat things about science is that experiments don't always run as
we expect. That has been the case on a number of materials experiments in
space where we thought that metals that are immiscible - refuse to mix,
like oil and water - would form a useful, evenly dispersed structure in
the microgravity of space.
Mother Nature had something else in mind, and the Wetting Characteristics of Immiscibles (WCI) experiment on board the U.S. Microgravity Payload (USMP-4) is starting to shed some light on what really happens.
"On the first flight sample, we could see the materials migrating and hitting the gasket," explained Dr. Barry Andrews of the University of Alabama in Birmingham. While he has a total of 12 experiment runs on the mission, this first test supports one of the theories about mixing or unmixing.
Oil and water don't mix because the surface electrical properties of their molecules are different and repel each other. Thus, they quickly separate. A number of metals do the same as they cool towards their freezing points. Even though the metals want to separate, scientists believe that dispersions of these materials could have desirable properties for use in infrared detectors, superconductors, and magnets.
"The idea was that if you took these metals into space and if you go to a high enough temperature (so they mix) and cool them down," Andrews explained, "you would a uniform dispersion instead of a totally separated structure as seen on the ground."
Instead, the samples often came back with all of one material clumped in the middle, and the rest surrounding it.
Exactly why became the new mystery to solve.
The challenge, as with many materials experiments, is that it is difficult to see what is happening inside a pot of molten metals. So, Andrews has put stand-ins under a microscope. The experiments are run inside the NASA/Marshall-developed Middeck Glovebox where the activity can be isolated from the cabin while letting the crew work "hands on."
The stand-ins are succinonitrile, an industrial chemical used in making nylon, and glycerol, a chemical used in the cosmetics industry. Like oil and water, they separate readily. And both are transparent, so segregation can be readily seen.
"These materials are selected because they behave quite similarly to metallic systems," Andrews explained.
The two are mixed in a small test cell made by two glass slides held separated by a Teflon gasket. Each of the dozen cells has a different mixture of the two chemicals as Andrews works to define their wetting properties.
A slide is put inside a small heater - it's like a toaster oven - and heated to 90 deg. C (194 deg. F) so everything mixes evenly.
Then an astronaut places it on a cooling rack to come down to the ambient temperature of 30 deg. C (86 deg. F; it's warmer inside the glovebox than in the cabin).
As the sample cools, the astronauts watch through a microscope and send video to the ground. In time, small droplets appear as the two liquids segregate. This is where the problems may arise, Andrews explained.
As the molecules bounce around, they find their own kind and form ultrasmall droplets. As fluid moves from one spot, something has to move to fill in. The droplets grow, become visible under the microscope, and eventually you can wind up with one giant droplet within the other fluid. So, even though dense fluids don't sink in microgravity, flow can arise as small drops wet the surface of the container.
As was seen on the first experiment, one liquid can work its way to the side, and then wet the entire side of the container.(check out this animation, courtesy of mission operations lab)
That was just the first of Andrews' experiments. A total of 12 will be run by mission's end, and from these he hopes to develop a better understanding of how to control structures formed in immiscible alloys by controlling the wetting characteristics of the materials.
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