Astronauts photograph the samples in Binary Colloidal Alloy Test - 3: Surface Crystallization (BCAT-3-SC) to document the formation of crystals from microscopic spheres (known as colloids) suspended in a liquid, both on the surface of the sample container walls and in the bulk of the sample container. Results help scientists develop fundamental physics concepts previously unobserved due to the effects of gravity. Ordered arrays of these micron-sized particles may be ideal for development of next generation optical devices.Principal Investigator(s)
National Aeronautics and Space Administration (NASA)Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)ISS Expedition Duration:
October 2003 - April 2006
8,9,10,12Previous ISS Missions
The predecessor to BCAT-3, BCAT, flew on Mir in 1997 and 1998.
The Binary Colloidal Alloy Test-3 (BCAT-3) hardware supported three investigations in which ISS crews photographed samples of colloidal particles (tiny nanoscale spheres suspended in liquid) to document liquid/gas phase changes, growth of binary crystals, and the formation of colloidal crystals confined to a surface. Colloids are small enough that in a microgravity environment without sedimentation and convection, they behave much as atoms and so can be used to model all sorts of phenomena because their size, shape, and interactions can be controlled.
The BCAT-3 payload consists of ten small samples of colloid alloys in which the microscopic colloid particles are mixed together into a liquid. These ten samples are contained within a small case that is the size of a school textbook. At the start of an experiment run, all ten samples are shaken to completely remix the colloid samples, much in the same way that salad dressing must be shaken to remix oil and vinegar. After the samples are mixed, what remains is periodically photographed using a digital camera until the colloid and liquid components of those samples have separated or the polymers have formed crystals. The samples can be remixed to repeat the experiment.
The ten samples in BCAT-3 were selected as part of three separate experiments examining different physical processes: critical point, binary alloys, and surface crystallization.
Crystal structures are affected not only by constituent building blocks, but also by the geometrical environment in which they grow. The long, thin blades of ice on the surface of a freezing puddle are far different from the solid blocks in a freezer ice cube tray and the six-sided needles in a snowflake. BCAT-3 includes 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 the crystallization itself.
This experiment addresses basic physics questions, but some of the areas may eventually have applications for space exploration. Whether materials in fluids prefer to first crystallize at a surface or in the fluid volume when gravity is removed impacts how fluids should be stored.Earth Applications
There are practical reasons for the resurgence of interest in the control of colloidal crystallization. It is difficult to create patterned nano- and micro-scale materials that are ordered in three-dimensions. Modern lithographic techniques can have nanometer resolution and they often have exquisite versatility, but they are generally limited to defining features at surfaces and interfaces; thus, with few exceptions lithographic fabrication in three-dimensions remains a challenge. Laser holographic writing has recently been demonstrated in three dimensions, but this method also has limitations with respect to host material type and spatial resolution. An alternative route for creation of three-dimensional patterned materials is through self-assembly. Micron and submicron colloidal particles can assemble spontaneously into a wide range of highly ordered phases employing a wide variety of particle species. The ordered arrays of particles have numerous potentially useful attributes.
When sufficiently ordered they often have novel optical properties. For example, they can be used to alter the local photon density of states or to redirect light beams via diffraction. Such optical property control can lead to new classes of optical filters, switches and masks, or to host materials, which act as directional reflectors for embedded waveguides or light sources. In a related vein the ordered structures may eventually be employed as ultra-low emissivity materials, or as novel linear and nonlinear optical based sensors. Particle arrays can also be used indirectly, as templates to create novel macroporous materials. Besides the use of the resulting inverse structures in photonics, precision macroporous materials have a wide range of potential chemical applications. For example, macroporous polymers have been used as catalytic surfaces and supports, separation and adsorbent media, biomaterials, and chromatographic materials. Similarly macroporous ceramics can be employed as lightweight thermal and electrical insulators, as well as for catalysis.
The BCAT-3-SC experiment is part of the BCAT-3 payload which consists of ten small samples of colloidal particles. The microscopic colloid particles and a polymer are all mixed together in a liquid. The ten BCAT-3 samples are contained within a small case the size of a school textbook. The experiment requires a crew member taking digital photographs of the samples at close range. The pictures are then be downlinked to investigators on the ground for analysis.Operational Protocols
At the start of the experiment in Increment 8 all viable samples were mixed by using a magnet to run a magnetic stirbar up and down through the samples. This completely mixes and homogenizes the colloid samples. After the samples are mixed, lights in the U.S. Lab are dimmed and photographs are taken to document the evolutionary changes in the samples, which can include the formation of surface and bulk crystals. The photographs are downlinked to the ground.
Crystal formation was observed in at least one sample. The samples have been remixed for another session to verify the observations.
Manley S, Cipelletti L, Trappe V, Segre PN, Bailey AE, Christianson RJ, Gasser U, Prasad V, Doherty MP, Sankaran S, Jankovsky AL, Shiley B, Bowen JP, Eggers JC, Kurta CE, Lorik T, Weitz DA. Limits to Gelation in Colloidal Aggregation. Physical Review Letters. 2004; 93(10): 108302-1 - 108302-4. DOI: 10.1103/PhysRevLett.93.108302.