Bizarre Boiling

Merte and other scientists had performed earlier research on weightless boiling using "drop towers," which could simulate zero-G for a few seconds by simply dropping samples inside a tall tower. These early experiments provided some guidance for designing the shuttle-based experiment, but these brief glimpses don't really compare to the minutes-long observation provided by the shuttle.

One important product of that early research, though, was a method for building a boiling chamber that let scientists look through the heater surface and watch the liquid right where it contacts the heater.

"The action is right at the solid-liquid interface at the heater, and you can't look down from the top because you have the refraction of the fluid's upper surface that interferes," says Merte, who recently retired as Emeritus Professor of Mechanical Engineering at the University of Michigan.

Image courtesy NASA Glenn Research Centre One way to simulate the weightlessness of space is to simply let an experiment freefall inside a "drop tower," such as this one at NASA's Glenn Research Centre in Cleveland, Ohio. Other methods for simulating weightlessness include flying parabolic arcs in aircraft - such as NASA's KC-135 "vomit comet" used to train astronauts - and using sounding rockets.

Merte used quartz to make a smooth, hard, transparent bottom for the boiling chamber. Then he coated that quartz with an ultra-thin layer of gold. Less than 400 angstroms thick (an angstrom is one ten-billionth of a meter), this layer was so thin that it allowed visible light to pass through it, yet it still conducted electricity like bulk gold.

Using this apparatus, Merte and his colleagues made some interesting discoveries. For example, depending on the temperature conditions of the experiment, the giant bubble would sometimes float in the centre of the liquid and sometime remained attached to the heater surface. When the bubble remained attached - which Merte calls a "dry out" - it effectively insulated the liquid from the heater, preventing further boiling and causing the heater temperature to soar.

Knowing precisely the conditions when this occurs is vital for designing spacecraft systems that might rely on boiling.

"If you understand a phenomenon better, then you can design for it closer to its limits for optimisation," Merte says. "If you have an uncertainty, then you're going to design conservatively."

Today's researchers continue to expand on the foundation of knowledge laid by these experiments. With a better understanding of the physics of boiling fluids, engineers will be able to design improved cooling and power systems to serve people in the future - both in space and here on Earth.