10/03/95
CONTACT: Stanford University News Service (650) 723-2558
STANFORD -- As anyone who has seen astronauts juggling balls of water knows, liquids act much differently in outer space than they do on earth.
These differences can create major fluid management problems in space. Space engineers, for example, have to know where fuel will settle in a tank as it empties, or how to keep liquid waste from dispersing into tiny droplets that astronauts could inhale.
A small experiment carried by the space shuttle Columbia when it blasts off on its next mission, scheduled for Oct. 5, may provide new information about fluid behavior in zero gravity, by videotaping the way in which a mixture of water and alcohol behaves when released into unusually shaped acrylic chambers.
The experiment was designed by Stanford mathematics Professor Emeritus Robert Finn and Paul Concus of the University of California- Berkeley and Lawrence Berkeley National Laboratory, in collaboration with Mark M. Weislogel, an aerospace engineer at NASA Lewis Research Center in Cleveland.
"One purpose of the test is to determine if we really can use the equations that we have relied on for all these years," Weislogel said.
These equations have a rich history. They were developed to explain phenomena such as the rise of liquid in small "capillary" tubes and the shape that mercury drops assume on glass plates. Such capillarity studies flourished through the 19th century, then fell into neglect in the 20th century. But the area has experienced a major revival in the last 30 years, due in large part to the requirements of the space program.
Because capillarity affects the way that fluids behave in many situations on earth, the information gained in these tests also could prove useful to engineers for designing such items as improved storage batteries or protective coatings. Battery designers try to maximize capillary action to keep fluid in contact with the electrodes. The coatings on raincoat fibers change their surfaces in a way that keeps water drops from seeping between them.
"Capillary action is much the same in space as it is on earth," Finn said. "But without the restraining influence of gravity it can become much wilder."
The key to a liquid's capillary behavior is the angle that it forms with the wall of a container. According to classical theory, the liquid's surface forms the same angle, called the contact angle, everywhere that it touches the walls of a homogeneous container. The theory asserts that this angle depends only on the chemical makeup of the liquid and the wall, not on the shape of the vessel, the surface geometry or the strength of gravity. Contact angles can range from zero (water on glass) to as much as 140 degrees (mercury on glass). Mixtures of alcohol and water in contact with glass or acrylic plastic exhibit a large range of contact angles.
"A number of people are trying to derive theories to predict this angle, but so far no one has come up with anything that can be used in practice," Finn said.
In 1974, Finn and Concus proved that the capillary effect can be discontinuous -- that is, it can change abruptly with small changes in the shape of the container. This can happen both with and without gravity. The researchers realized that this effect could be used to get accurate measurements of contact angles, which has been a major obstacle to practical applications of the theory.
A NASA engineer, Bill Masica, then tested this idea in five seconds of zero-gravity in a drop tower, essentially a hole bored into the ground from which the air is evacuated.
The researchers found that the method works well for measuring fluid systems with large contact angles. But in systems with small contact angles, they ran into practical problems in making these measurements. In joint work with two students who came to Finn through Stanford's Undergraduate Research Opportunities program -- Bruce Fischer (now at Massachusetts Institute of Technology) and Tanya Leise (now at Texas A&M) -- the researchers found a way around this difficulty. They designed specially shaped "noses" that draw up large amounts of liquid, making it possible to measure the contact angle precisely.
Three of the four vessels being flown in space have two such noses each, one designed to be just below the liquid's critical angle and the other just above it. The fourth has a movable wedge, for measuring larger contact angles.
The experiment is part of the shuttle's "glovebox," a facility on the Second United States Microgravity Laboratory that fits in the shuttle bay. The glovebox allows the astronauts to perform a number of small, quick experiments.
To run the capillary experiment, the astronaut puts his or her hands into the glovebox chamber and sets up the apparatus. He or she then opens the valve that lets the liquid into the vessel and observes and photographs what happens. When the astronaut taps the vessel, the researchers expect that the fluid will flow a short distance up one of the noses and stop. In contrast, it should flow much further& up the second nose, possibly all the way to the top.
"The drop tests indicated that the fluid has a strong inclination to do what we predict," said Finn. "The question remains whether or not it will continue to climb all the way to the top."
The answer to that question will give some clue to the extent that frictional resistance affects the fluid's motion. This effect is not included in the classical equations that the mathematicians are using. Thus, the experiment not only should provide a procedure for measuring contact angles more accurately, it also should indicate the extent to which the classical equations can be relied upon to predict actual fluid configurations.
The ideas being tested in this experiment have been carried further by Finn's work with another undergraduate student, Jonathan Marek. Finn said tests of those ideas are planned for a future shuttle mission.
This is the second shuttle experiment for the two mathematicians. In 1992, Finn and Concus had astronauts inject fluid into cylindrical chambers with a symmetric bulge of precisely determined shape. When the fluid first was injected it took on a symmetric shape, one of an infinite number that it could form. The vessel had been designed so that all the symmetrical shapes the liquid could take were physically unstable. So, when the astronaut tapped on the vessel, the liquid reshaped itself into a non-symmetric shape that looked something like a shoehorn. This showed that the physically stable state for liquids in zero gravity can be asymmetric, even when the determining conditions are symmetric, Finn said.
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During the shuttle mission Finn can be reached either at NASA Marshall Space Flight Center (205) 830-2070 or at the La Quinta Inn in Huntsville (205) 830-2070.
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