What happens to the surface of a liquid under the virtually gravity-free conditions of a space craft in orbit about the Earth? This is vital information for, among other things, knowing how much fuel remains in the tank of that craft.
Paul Concus, a mathematician with LBL's Physics Division and professor with UC Berkeley, has been grappling with the question of how liquids behave in zero gravity for more than 20 years. He and collaborator Robert Finn of Stanford University have developed a theory that will be tested in an experiment aboard the Space Shuttle. The formal title of this experiment is the Interface Configuration Experiment (ICE).
ICE is one of 31 independent experiments that was carried out in NASA's United States Microgravity Laboratory (USML-1). The term "microgravity" is used because the gravity aboard an orbiting space shuttle is about one millionth that on Earth.
What ICE will be testing is a phenomenon called "capillarity." It is the mechanism by which the free surface or "meniscus" of a liquid is affected when the liquid comes into contact with a solid. On Earth, the stabilizing effect of gravity ensures that liquid remains at the bottom of most containers (there are vessels designed especially to be exceptions), and that the surface remains essentially flat regardless of the container's shape. In the absence of gravity, however, mathematical analysis by Concus and Finn predicts that the liquid might not remain at the bottom of the container and that an unusual variety of surface configurations can form. Depending upon the shape of the container and the angle of contact between the liquid and the container's walls, enough of the liquid can climb the walls to leave portions of the bottom completely exposed.
Drop tower tests using cylindrical containers with various cross sections (circular, hexagonal, etc.,) have verified some of the predictions of Concus and Finn. USML-1 will now enable the mathematicians to much test their theory in so-called "exotic" containers.
These exotic containers have a specific mathematically derived shape that resembles a circular cylinder with a toroidal (doughnut-shaped) bulge in the center.
Says Concus, "The unique design of the bulge permits the formation of an entire continuum of distinct axially symmetric liquid surface configurations for a given volume of fluid and contact angle."
The exotic containers will be tested inside USML-1's Glovebox, an enclosed compartment through which astronauts can manipulate liquids and other materials under controlled conditions. A pair of video cameras attached to the Glovebox will record what takes place during the tests.
"We need an astronaut to tap the containers to encourage the liquids to move into a variety of equilibrium configurations," Concus says. "Since none of the symmetric configurations are stable, applying a force (in the form of a finger tap) perturbs these surfaces and encourages the shape to change to one of the anticipated non-symmetric stable configurations."
There will be four containers in all, three exotic and one with a spherical-shaped bulge for which a complete mathematical analysis of liquid behavior is available. Two of the exotic containers will be partially filled with a special "immersion fluid" that eliminates any optical distortions. The other containers will be partially filled with distilled water. All of the liquids will be dyed for better visibility.
A pair of video cameras attached to the Glovebox will record the various shapes assumed by the liquids. These results will be compared to the predictions of Concus and Finn to determine how closely reality matched expectations.
Says Concus, "There are bound to be some things that we can't predict in advance. It might also be that there are some effects predicted mathematically that cannot be observed experimentally. Either way, the results should increase science's understanding of the phenomena."
Collaborating on ICE with Concus and Finn is engineer Mark Weislogel of NASA's Lewis Research Center in Cleveland, Ohio.