06/01/92
CONTACT: Stanford University News Service (650) 723-2558
STANFORD -- Drinking a cup of your favorite beverage in space can be an educational experience.
You might find that the liquid stays remarkably stable - but inaccessible - on the bottom of the glass. Then again, it just might shoot out the top, leaving the spacecraft filled with little balls of cognac or cabernet.
The behavior of liquids in space is one of the unsolved problems of space travel; NASA, for example, seeks ways to to measure where the fluid would be in the fuel tanks of a spacecraft. And it has numerous ramifications for earthbound engineers as well - in designing storage batteries, bubble memories for computers, protective coatings, oil exploration and even in the design of raincoats.
On the June 17 space shuttle mission, some answers may come from an experiment designed by mathematicians Robert Finn of Stanford University and Paul Concus of the Lawrence Berkeley Laboratory and the University of California, in collaboration with Mark Weislogel of NASA.
Liquids behave in different ways depending on the "contact angle" of the liquid with the wall of the container. According to the classical theory, this angle depends only on the materials, and not on the shape of the vessel, fluid surface or gravity field, Finn said.
Consequently, in space, fluid in a cylindrical glass will just sit on the bottom unmoved unless disturbed. If the vessel has a polygonal cross- section, things get more complicated.
Finn and Concus found that if the contact angle is equal to or above a certain value, the fluid will behave just as if it were in the cylindrical glass. If it's less than that, "the liquid rises in the corners, to infinity or to the top of the container, which ever comes first," Finn said.
The first verification of this behavior came in an experiment performed in a "drop tower" at NASA's Lewis Space Center in Cleveland. The tower is actually a hole bored into the ground from which the air was evacuated. Two containers with fluid and a camera were dropped down the hole on tracks, achieving about five seconds of free fall.
The containers were identical, but the fluids (mixtures of alcohol and water) differed, leading to different contact angles that straddled the critical one. In one container, the liquid remained on the bottom; in the other vessel, it shot up to the top.
New and more sophisticated experiments, to be conducted under the joint auspices of NASA and the European Space Agency in the next several years, have been designed in collaboration with Bruce Fisher, who did his work while part of Stanford's Undergraduate Research Program and is now at Massachusetts Institute of Technology. The new experiments are expected to give new insights into the intrinsic significance of the contact angle, a significance that has often been questioned because of the great difficulty in obtaining repeatable measurements.
The June experiment aboard the U.S. Microgravity Laboratory (USML- 1) will involve a circular cylinder with a small symmetric bulge of precisely determined shape. The cylinder will be closed at top and bottom and filled partly into the bulge with a carefully chosen fluid in such a way that researchers can find an infinite number of exact solutions to the equations, as determined by the contact angle.
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