QUESTION:
How does space affect the properties of liquids and gases, are liquids any more compressible while in space, and can gases be expelled from a cavity at high speeds?

ANSWER from Guy Etheridge on June 25, 1997: The microgravity conditions encountered during spaceflight do not have any significant effect on the compressibility of liquids or gases. Of course, compressibility is a key factor in the dynamics of gases being expelled through a small cavity, so that too would not be any different in space or earth.

One effect that microgravity will have on liquids and gases is that free convective forces cease to occur. Buoyancy forces no longer have an effect on diffusion rates of molecules.

ANSWER from Chuck Davis on June 25, 1997:
We should first review definitions of liquid and gas. A liquid takes the shape of the container it resides in and - very importantly - has a "free surface" and "seeks its own level" (tries to spread itself evenly across a surface). A gas also takes the shape of its container but has no free surface - it will tend to equally spread itself throughout the container. For example, add some water to a balloon and inflate it. The liquid has a free surface and conforms to the shape of the balloon's surface with which it is in contact (with gravity, the lower parts). In a gas-filled balloon, there is no free surface and the gas molecules are equally distributed within the balloon. Also, take note of how a free-floating bubble takes the shape of a sphere. This is the shape which requires the least amount of energy to maintain. Only a liquid can have surface tension. Another important parameter is viscosity - it's resistance to flowing, how thick or gooey it is (syrup has a higher viscosity than water). Viscosity will determine how much time or effort is needed for a fluid to take the shape of its container or how much effort is required to remove it (like catsup).

Now, on to the questions: "How does space affect the properties of liquids and gases, are liquids any more compressible while in space, can gases be expelled from a cavity at high speeds?"

The "properties" of the fluid are intrinsic to the fluid and being in space does not change them. However, without gravity, surface tension and viscosity predominate the behavior of a liquid which has been extensively studied. Spacecraft liquid-systems have special design features incorporated to take advantage of surface tension effects - mainly wick-like structures. A good example is the propellant acquisition screens used in the propellant storage tanks of many satellites and the Space Shuttle's OMS/RCS. The propellant in the storage tank is retained on the screens which in fun ensure only liquid propellant gets into the piping feeding the engines.

How fast a gas in expelled from a container is primarily a function of pressure difference, orifice difference in space versus on earth is the lack of atmospheric back-pressure (14.7 psig at sea level) and temperature (can be very! cold). Therefore, a given system, gas released into space would have 14.7 psig less pressure to work against, and therefore, would be blow down faster than the same mass on earth.

One other note on gases regarding temperature. If a gas is confined in a rigid container and is cooled below its boiling point (radiant heat loss into space), what happens?