Table of Contents

Microgravity

The effect of gravity is such an accepted part of our lives that we rarely think about it. These effects are not always desirable, however, especially during certain phases of materials processing. Gravity limits efforts to study the physical processes found in materials manufacturing and other technologies. For example, gravity is a driving force for convection (stirring) currents between hot and cold regions. These currents obscure other events that scientist wish to study and can affect the quality of the final product by causing it to be mixed improperly. In the space environment, the masking forces of gravity are stripped away, and scientists can pursue research not possible on Earth.

The LMS microgravity experiments investigated many gravitational effects on the production and manipulation of certain materials. Scientists studied interacting fluid layers, stability of liquid bridges, and the behaviour and properties of bubbles and drops of particular materials while they are suspended in a test chamber. They investigated the melting and resolidification of solids to learn more about the effects of gravity on high-temperature processing methods. And they grew approximately 90 protein crystal samples.

Bubble, Drop and Particle Unit -- Advances in materials processing have the potential to produce new high-strength metals and temperature-resistant glasses and ceramics for building everything from better electric power plants to future spacecraft. To advance such material research, scientists need a better understanding of the fluid processes that play a role in the production of most materials. One of the most important of these processes is the role of interfacial tension, the force created at the boundary of two immiscible phases (liquid/liquid, liquid/gas, liquid/solid). On Earth, the role of interfacial tension is often masked by other gravity-driven forces, such as buoyancy and sedimentation.

Four LMS investigators used the test cells to study how bubbles and drops react in liquids with varying temperatures and concentrations, how they interact with the solid/liquid interface during melting and solidification, and how convection is driven by differences in interfacial tension between adjoining liquid layers. The unit was also used to study the evaporation and condensation of bubbles and the effect of strong electric fields on the stability of liquid columns in microgravity.

Advanced Gradient Heating Facility -- The manufacturing of semiconductors, the basis for modern electronics, depends on the precise mixing of components and on a highly ordered structure to produce single crystals. While it is possible to grow near-perfect single crystals of silicon and some other materials, advanced semiconductor materials often have multiple flaws when produced on Earth.

In the microgravity of space, many of these gravity-induced imperfections can be eliminated. LMS investigators used the Advanced Gradient Heating Facility to produce advanced semiconductor materials and alloys using the directional solidification process, which depends on establishing a hot side and a cold side in a sample (a temperature gradient).

Growth rate is an important parameter in the production of many materials, yet the solidification rate often is different from the rate of movement of the sample cartridge through the furnace. A precise understanding of all the factors affecting crystal growth will improve the general knowledge of the physical phenomenon involved in the solidification process, improving materials processing on Earth and future semiconductor and materials processing research in space.

Advanced Protein Crystallization Facility-- Conditions on Earth limit the size and quality of many protein crystals, but the microgravity of space allows the growth of larger, more highly ordered crystals. This facility was the first to use three methods of protein crystal growth: liquid/liquid diffusion, in which a protein solution and a salt solution were separated by a buffer and were allowed to flow together slowly once Columbia was in orbit; dialysis, with protein and salt solutions separated by a membrane; and vapour diffusion, where crystals form inside a drop of protein solution as solvent from the drop diffused to a reservoir.

Scientists were interested particularly in how and why crystals begin formation. Video images were made of the crystals as they form, and studied after the mission to establish the history of crystal development in microgravity. When the crystals returned from space, they were analysed, using precision X-ray beams, sophisticated detectors and data-processing equipment to determine the internal arrangement of their atoms. As X-rays diffract off the atoms of crystals, a computer mapped each atom's position. With these maps, scientists are able to expand our understanding of biological processes at the molecular level, which could lead to applications in medicine and agriculture.

Accelerometers-- Microgravity science investigations require a stable, low-gravity environment to yield the most accurate data. Vibrations caused by on-board activity and by the operation of pumps, thrusters, fans and cameras in the orbiter also can impact the quality of the research.

Three instruments were designed to measure these low-level accelerations and vibrations aboard Columbia: the Microgravity Measurement Assembly (MMA), the Orbital Acceleration Research Experiment (OARE) and the Space Acceleration Measurement System (SAMS). These systems collected data about disturbances in the microgravity environment, providing investigators with insight into conditions that might affect the results of their experiments.