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Binary Colloidal Alloy Test - 3 and 4: Critical Point (BCAT-3-4-CP)


Overview | Description | Applications | Operations | Results | Publications | Imagery

Experiment Overview

This content was provided by David A. Weitz, Ph.D., Peter J. Lu, Ph.D., and is maintained in a database by the ISS Program Science Office.

Brief Summary

Depending on their relative distances and energies with respect one another, atoms and molecules organize themselves to form gases, liquids, and solids. Binary Colloidal Alloy Test - 3 and 4: Critical Point (BCAT-3-4-CP) studies the critical point of these systems, which is defined where gases and liquids no longer exist as separate entities and a new state of matter forms which is known as the critical point. The application of this experiment in the near term is to enhance the shelf life of everyday products and in the longer term, the development of revolutionary materials for electronics and medicine.

Principal Investigator(s)

  • David A. Weitz, Ph.D., Harvard University, Cambridge, MA, United States
  • Peter J. Lu, Ph.D., Harvard University, Cambridge, MA, United States
  • Co-Investigator(s)/Collaborator(s)

    Information Pending


    ZIN Technologies Incorporated, Cleveland, OH, United States

    Sponsoring Space Agency

    National Aeronautics and Space Administration (NASA)

    Sponsoring Organization

    Human Exploration and Operations Mission Directorate (HEOMD)

    ISS Expedition Duration:

    October 2003 - October 2013

    Expeditions Assigned


    Previous ISS Missions

    The predecessors to BCAT-4; BCAT-3 operated on ISS, and BCAT, operated on Mir in 1997 and 1998.

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    Experiment Description

    Research Overview

    • Binary Colloidal Alloy Test - 3 and 4: Critical Point (BCAT-3-4-CP) uses microscopic spheres (described as colloids) suspended in a liquid to serve as a large scale representation of the atoms and small molecules which constitute typical solids, liquids, and gases so that scientists can visualize what happens at the individual particle level when materials transition from liquids to gases.

    • These systems of microscopic spheres help scientists visualize when specific conditions (i.e. temperature, pressure, and concentration) drive the formation of solids, liquids, and gases in smaller systems based upon atoms and simple, small molecules.

    • Depending on their relative distances and energies with respect one another, atoms and molecules organize themselves to form gases, liquids, or solids. By varying the concentrations of the microscale spheres, this experiment studies the critical point of these systems where gases and liquids no longer exist as separate entities and a new state of matter forms when one reaches a critical point.

    • BCAT-3-4-CP can only be performed in microgravity where gravitational forces do not interfere with the experiment.

    • The application of this experiment in the near term is to enhance the shelf life of everyday products and in the longer term, the development of revolutionary materials for electronics and medicine.


    Detailed Research Description: The Binary Colloidal Alloy Test (BCAT) hardware supports four experiments. The first hardware, BCAT-3, consisted of three separate investigations, Binary Alloy (BCAT-3-BA), Critical Point (BCAT-3-CP) and Surface Crystallization (BCAT-3-SC), which were delivered to the International Space Station (ISS) during expedition 8. The next hardware, BCAT-4, consists of two separate investigations, Critical Point (BCAT-3-4-CP, a continuation of the investigation on BCAT-3) and Polydispersion (BCAT-4-Poly).

    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 the liquid and gas merge. Just above this point is the supercritical regime where there are no longer distinct phases, but rather a homogeneous supercritical fluid. Seven of the BCAT-4 samples will examine critical point and add important data points to the phase diagram explored by the critical point samples in BCAT-3, 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 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 might thus contribute to fundamental understanding that may contribute to the future development of such diverse things as new drugs, cleaner power, and interplanetary transportation.

    The colloid-polymer mixtures are in a glass cuvette, which the crewmembers can illuminate with a flashlight from the rear, at a high angle. The colloidal spheres scatter the light from the flashlight, and appear blue, so the bright blue areas in the photographs are regions with high colloid density. The darker areas, filled with solvent and polymer, don't scatter much light, which is why these areas are darker. The term "phase separation" is clearly visualized in the photographs: the sample has separated into two phases, a bright blue "liquid" phase with a high colloid density, and a darker "gas" with far lower colloid-density. We measure the characteristic width of the bright-blue region, quantifying the size of the liquid regions, as a function of time.

    The BCAT-3-4-CP critical point samples may have a 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, which in the absence of gravity should manifest a boundary between separating phases that looks like a jagged coastline.

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    Space Applications

    BCAT-3-4-CP addresses basic physics questions, but some of the areas may eventually have applications for space exploration. Supercritical fluids, which are one of the applications of the critical point experiment, are of potential application in propulsion systems for future spacecraft design.

    Earth Applications

    Increased knowledge of some of the areas of this basic physical research may have future benefits in the application of the same physical processes on Earth. Supercritical fluids (fluids possessing properties of a gas and a liquid, simultaneously) have numerous applications in a wide variety of fields. An example is supercritical carbon dioxide used to decaffeinate coffee beans. Supercritical fluids can also be used to wash toxic waste from soil and to extract higher concentrations of compounds from plants for use in new drugs. The development and use of newer supercritical fluids is dependent on further understanding of the critical point of those fluids, which BCAT-3-CP and BCAT-4-CP are providing. Additionally, product shelf-life may be dependent upon binodal decomposition; so, a better understanding of this could have an enormous commercial impact.

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    Operational Requirements

    The BCAT-4 hardware consists of ten samples of colloidal particles. The microscopic colloid particles and a polymer (samples 1 - 7) are all mixed together in a liquid. The BCAT-4 samples are contained within a small case the size of a school textbook. The experiment requires a crew member to set up on the Maintenance Work Area (MWA) or on a handrail/seat track configuration, EarthKAM hardware and software to take digital photographs of samples 1 - 7 at close range using the onboard Kodak 760 camera. The pictures are then downlinked to investigators on the ground for analysis.

    Operational Protocols

    A crewmember sets up all hardware on the Maintenance Work Area (MWA). The crewmember then homogenizes (mixes) the sample(s) and takes the first photographs manually. The crewmember activates the EarthKAM software to automate the rest of the photography session over a 3-day to 3-week period. Crewmembers perform a daily status check to assure proper alignment and focus of the camera. At the completion of the session, a crewmember tears down and stows all hardware.

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    Results/More Information

    The BCAT Critical Point samples comprise of colloids and polymers diffusing in a background solvent; this model system mimics the behavior of molecular liquids and gases in microgravity. The network structure that appears in the phase separation images has a characteristic length scale. How this length changes with time gives insight into the thermodynamics driving the phase separation, and can be quantified using image correlation (an optical method that employs tracking and image registration techniques for accurate two and three-dimension measurements of changes in the length) computer programs. By creating different program coding running in parallel with various computing platforms, many orders of magnitude improvement in analysis speed were achieved over standard "off-the-shelf" programs. The speed increases allow for very rapid analysis of images downlinked from the ISS and quick advantageous feedback to astronauts on orbit in time to make changes while the experiment is still running (Lu et al. 2009).

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    Results Publications

      Lu PJ, Frey CA, Fincke EM, Chiao LN, Meyer WV, Foale CM, Owens JC, McArthur WS, Hoffmann MI, Rogers R, Williams JN, Sicker RJ, Krauss AS, Funk GP, Havenhill MA, Anzalone SM, Weitz DA, Yee H.  Microgravity Phase Separation near the Critical Point in Attractive Colloids. 45th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV; 2007; Reno, NV.
      Lu PJ, Chamitoff GE, Whitson PA, Frey CA, Fincke EM, Meyer WV, Chiao LN, Magnus SH, Foale CM, McArthur WS, Tani DM, Williams JN, Sicker RJ, Au BJ, Weitz DA.  Long-Time Observation of Near-Critical Spinodal Decomposition of Colloid-Polymer Mixtures in Microgravity. 47th Aerospace Sciences Meeting and Exhibit, Orlando, FL; 2009; Orlando, FL.
      Christiansen M, Chamitoff GE, Lu PJ, Oki H, Whitson PA, Frey CA, Schofield AB, Fincke EM, Chiao LN, Meyer WV, Magnus SH, Foale CM, McArthur WS, Tani DM, Williams JN, Sicker RJ, Au BJ, Weitz DA.  Orders-of-magnitude Performance Increases in GPU-accelerated Correlation of Images from the International Space Station. Journal of Real-Time Image Processing. 2010; 5(3): 179-193. DOI: 10.1007/s11554-009-0133-1.

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    Ground Based Results Publications

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    ISS Patents

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    Related Publications

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    Related Websites
  • NIH BioMed-ISS Meeting, 2009—BCAT-3-4-CP
  • NIH BioMed-ISS Meeting Video Presentation, 2009— BCAT-3-4-CP
  • ISS Research Project-BCAT-3-4-CP
  • BCAT-3
  • Photographing Physics: Critical Research in Space
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    image NASA Image: ISS008E20221- BCAT-3 sample holder affixed to a wall inside ISS on Expedition 8. Image courtesy of NASA.
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    image NASA Image: ISS010E06640 - ISS Commander and Science Officer, Leroy Chiao performing BCAT-3 operations on board ISS during Expedition 10.
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    image Critical point fluctuations observed in BCAT-3 onboard the ISS. Image courtesy of Glenn Research Center
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    image NASA Image: ISS012E07685 - Expedition 12 Commander and Science Officer William McArthur photographs BCAT-3 experiment samples.
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    image NASA Image: ISS012E13082_nr - BCAT-3-CP sample 1 taken during ISS Expedition 12 using a new camera setting and flash placement. The new camera flash position allows the camera to see the light from the evolving critical fluid experiment interface. These tests will now enable the EarthKAM computer to take a series of computer controlled photographs, which will track the evolution of this critical fluid experiment after it has just been mixed in the absence of gravity.
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    Information provided by the investigation team to the ISS Program Scientist's Office.
    If updates are needed to the summary please contact JSC-ISS-Program-Science-Group. For other general questions regarding space station research and technology, please feel free to call our help line at 281-244-6187 or e-mail at JSC-ISS-Payloads-Helpline.