The Binary Colloidal Alloy Test-3 is an Exploration Systems' transition flight experiment in the Human System Research and Technology area. BCAT-3 provides a unique opportunity to explore fundamental physics and simultaneously develop important future technology, including computers operating on light, complex biomolecular pharmaceuticals, clean sources of geothermal power, and novel rocket engines for interplanetary travel. These studies depend entirely on the microgravity environment provided by the International Space Station (ISS); in all other locations accessible to science, gravity dominates and precludes investigation of any other effects of interest. The experiment itself is simple and elegant, photographing samples of colloidal particles with a digital camera onboard the ISS. Colloids are tiny nanoscale spheres of plexiglass a thousand times smaller than the width of a human hair (submicron radius) that are suspended in a fluid. They are ubiquitous (e.g., milk, smoke, and paint) and therefore interesting to study directly. Colloids are also small enough that they behave much like atoms and so can be used to model all sorts of phenomena because their size, shape, and interactions can be controlled. The 10 samples in BCAT-3 are made from the same ingredients, each a recipe with different proportions, and are grouped into three experiments: critical point, binary alloy, and surface crystallization.

Critical Point

In an ordinary pot of boiling water, bubbles of water vapor coalesce on the bottom of the pot, growing until they detach and float to the surface where they escape into the atmosphere. At the boiling temperature water exists simultaneously in two distinct phases—liquid and gas—and as the bubbles burst, those two phases are spatially separated. But what should the mixture look like in the absence of gravity, when the vapor no longer floats to the top? Moreover, the behavior changes with increasing pressure: seal the pot, as in a pressure cooker, and the boiling temperature rises. Continuing the pressure increase, the mixture will reach its critical point, a unique pressure and temperature value where the properties of liquid and gas merge. Just above is the supercritical regime where they are no longer distinct phases, but rather a homogeneous supercritical fluid. The BCAT-3 samples of David Weitz and Peter Lu of Harvard University model this behavior in a colloidal system, where the phases analogous to liquid and gas can be seen as two different colors.

Supercritical fluids are technologically important because they uniquely combine the properties of liquids and gases, flowing easily (like gases), yet still having tremendous power to transport dissolved materials and thermal energy (like liquids). Supercritical carbon dioxide is used to decaffeinate coffee beans and to extract complex biomolecules from plants for pharmaceutical research. Supercritical water so efficiently transports heat that it is being explored in Iceland as a potentially superior geothermal power source; it is also used to remove toxic waste from contaminated soil. Additionally, NASA's Jet Propulsion Laboratory is working on using supercritical fluids as unique propellants for future rocket engines. A better understanding of critical behavior as a result of microgravity experiments like BCAT-3 might thus facilitate the development of new drugs, cleaner power, and interplanetary transportation.

The BCAT-3 critical point samples may also have tremendous impact upon fundamental physics. Understanding critical phenomena was an important theoretical advance in physics during the last half century, but ground-based experiments have been limited by gravity, which invariably causes the denser liquid phase to fall to the bottom of any container, precluding direct observation of phase separation alone. While previous colloidal experiments have tantalizingly shown that the boundary between separating phases in the absence of gravity is like a jagged coastline, BCAT-3 is the first to systematically attempt to precisely locate the critical point and visualize the behavior around it.

Colloids are also technologically interesting because they are the rightsize to manipulate light. Natural opal is likely the oldest and best known of the "photonic" crystals that direct light: shine white light on the opal and a rainbow appears, demonstrating how colors of light are split up and sent in different directions. The ability to better control the movement of light is a major technological goal, not only to build computers operating on light instead of electricity, but also to harness the full capabilities of existing fiber-optic networks for improving communications. Crystal structures built from only one building block—such as the arrangement of colloidal silica spheres in an opal—are well under-stood, but their optical properties are limited. More useful photonic crystals can be built from two different types of building blocks mixed together, yielding a binary alloy. The resulting structures and their optical properties are vast, as both the size and proportion of the two building blocks can be varied. How the crystallization is affected by these changes is only beginning to be explored. Theoretical studies suggest that desired optical properties require more complicated crystal structures, but this has not been well explored experimentally. Microgravity is crucial to the binary crystal experiments, allowing the growth of crystals far larger than those created on the surface of the Earth. The BCAT-3 binary alloy sample of Peter Pusey and Andy Schofield of the University of Edinburgh furthers previous investigations on binary growth in space.

Surface Crystallization

Crystal structures are affected not only by constituent building blocks, but also by the geometrical environment where they grow. The long, thin blades of ice on the surface of a freezing puddle are far different from both the solid blocks in a freezer ice cube tray and the six-sided needles in a snowflake. Arjun Yodh and Jian Zhang at the University of Pennsylvania have prepared several samples to study the formation of colloidal crystals confined to a surface, allowing comparison with bulk three-dimensional crystallization, to begin teasing out how geometry affects crystallization itself.

The Next Step

BCAT-3 is a simple experiment with both important technological applications and profound implications for fundamental physics. The phenomena under investigation require an environment where gravity plays no significant role, and this can only be done in space. However, BCAT-3 is limited by the nature of macroscopic photography: the camera cannot resolve individual colloidal particles. The more sophisticated Light Microscopy Module (LMM) scheduled to fly to the ISS in 2006 is a microscope designed to view and locate the millions of particles in these samples one by one, further enabling and deepening our understanding. Understanding supercritical fluids may accelerate present efforts to use them as propellants in advanced rocket engines. Developing more efficient optical communications through better photonic crystals is particularly important to onboard computers in outer space, where electronic circuits are constantly being degraded by high-energy cosmic rays; optical circuits are immune to such wear. Those developments may be critical for long, interplanetary missions.

In summary, BCAT-3 provides a unique opportunity to understand fundamental physics and develop important future technologies— all in a package the size of an ordinary textbook.

Image above: Astronaut Leroy Chiao works on the BCAT-3 experiment on the International Space Station. Credit: NASA