Tutorial on Microgravity Research
page 2: Space Industry Overview
What is gravity?
Gravity is one of the fundamental physical forces and is such present in our lives that we rarely think about how it affects all physical, chemical and biological processes on Earth. When we drop an object and watch it fall to the ground, we see the effect of gravity. Gravity generates the attraction, which gives us the sensation of weight and keeps us fixed on the Earth's surface. In fact, gravity is not only present on Earth but is also a property of the whole universe. An example is that of gravity forcing the Moon to fly around the Earth, and keeping the Earth orbiting around the Sun.
The effects of gravity can be undesirable if we want to understand certain physical, chemical or biological phenomena or study the complex interaction of different forces involved in a process, for example, in liquids and gases. Gravity can mask our view on the full set of parameters that influence a physical, chemical or biological process. Gravity can also be undesirable in certain industrial production processes. For example, gravity causes sedimentation, buoyancy and convection in liquids, creates hydrostatic pressure in liquids and modifies the behavior of liquid films on surface. All these gravity-induced phenomena mask capillary effects, multiphase flow, and diffusive transport processes and impede crystal growth with three-dimensional structures, which are closer to real crystals.
What does 'microgravity' mean?
Gravity cannot simply be eliminated, but its effects can be compensated with the help of an appropriate acceleration force. The acceleration force must have exactly the same absolute value as the gravity force and it must point into the opposite direction of the local gravity vector. The resulting equilibrium of forces is called in the normal language: "weightlessness". In practice, however, an exact equilibrium state is difficult to obtain and a very small gravity force is always remaining. Specialists therefore speak of "microgravity" rather than "weightlessness".
Under microgravity conditions, various fundamental physical phenomena are significantly altered or even fully removed, in particular convection, buoyancy, hydrostatic pressure and sedimentation.