On Earth, gravity overpowers many of the effects of wetting and surface tension, but in microgravity, scientists can better observe the effect of these forces. Experimenting in microgravity will help scientists to understand fluid behavior on Earth and to predict interfacial phenomena in space, which could lead to useful information such as where the fuel is located in a spacecraft tank while in orbit. This causes them to have unusual properties. Colloids are complex fluids that contain small particles of a solid or liquid. Foams, which are made up of many gas bubbles in a small amount of liquid, are also complex fluids.
Foams are neither solid, liquid, nor gas, but they exhibit properties of all three states.
Detergents, cosmetics, fire extinguishers, and some foods contain foams. Granular systems can flow under certain conditions and are considered a complex fluid. In microgravity, where the effects of gravity-driven convection and sedimentation are reduced, scientists can observe other influences on the properties of complex fluids and learn to better control and predict these properties on Earth. Astronauts floating in the shuttle appear weightless not because they have escaped Earth's gravity but because they are in a state of freefall. For example, a scale taped to an apple falling from a tree would register zero because it is falling with the apple.