Why study fluids in space? For starters, understanding the way fluids behave without gravity may help scientists and engineers to better apply fluid phenomena on Earth- affecting applications like engines, power plants, and heating and cooling systems. Fluids are also integral to life support systems on spacecraft and better understanding of their phenomena is vital to further space exploration. As an example of this, let's examine boiling.

Boiling in microgravity (courtesy of NASA)

Boiling on earth (courtesy of NASA)

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Fluids: A fluid is defined as any material that flows in response to an applied force. This means that besides the obvious fluids, water and liquids, air and gases are also fluids. Even the plasma that makes up the sun and stars is a fluid. And, most surprisingly, some solids behave like fluids--both soil and sand easily flow when a force is applied to it.

A pot of boiling water is something most of us can picture in our heads. Think about the role gravity plays in this everyday phenomenon. As the water heats, gravity causes the hotter regions of liquid to rise--a process known as "convection"--which distributes heat throughout the water. Once the water heats to the boiling point, the gravity-induced buoyant force sends the bubbles upwards, producing a "rolling boil." But this familiar picture changes drastically in a microgravity environment. Gravity-driven convection and buoyancy are non-existent, and without their dominating influence, scientists grow closer to understanding the fundamentals of boiling.

Given our dependence on boiling liquids in food and drink preparation, it is easy to imagine why scientists are interested in studying boiling phenomena. But, beyond the kitchen, boiling is used in many engineering systems as an effective means of transferring heat. Excess heat can be dispelled through the vapor bubbles moving through a boiling liquid. Although boiling figures prominently in the mechanics of heating/cooling systems, power plants, and engines, a better understanding of this phenomenon could lead to more efficient and effective applications.

Pool boiling experiments conducted onboard an orbiting space shuttle provided scientists the first glimpse of boiling in space. In gravity, vapor bubbles form on the heater surface of the liquid and, due to buoyancy, quickly rise to the top. In microgravity, without buoyancy, the bubbles remain attached to the heater surface and continue to grow. As these bubbles grow, the liquid is no longer in contact with the heater surface to cool it down and therefore, the liquid is useless as a heat transfer device. This situation can lead to "dryout" which is the suspected cause of the infamous Chernobyl disaster.

The space shuttle boiling experiments and future space station experiments are increasing our knowledge of boiling phenomena. These results will aid in our further exploration of space…. The final frontier.