Binodal Colloidal Aggregation Test - 4: Polydispersion (BCAT-4-Poly) is an experiment of two samples containing microscopic spheres suspended in a liquid which are designed to determine how crystals can form from the samples after they have been well mixed. The two samples have the same average sphere size but one of them has a wider range (more polydisperse) of sizes in order to demonstrate the dependence of crystallization on particle size range. Results from these experiments help scientists develop fundamental physics concepts which will enable the development of a wide range of next generation technologies (such as in high speed computers and advanced optical devices).Principal Investigator(s)
ZIN Technologies Incorporated, Cleveland, OH, United States
National Aeronautics and Space Administration (NASA)Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)ISS Expedition Duration:
October 2007 - October 2013
16,17,18,19/20,29/30,31/32,33/34,35/36Previous ISS Missions
The predecessors to BCAT-4; BCAT-3 operated on ISS, and BCAT, operated on Mir in 1997 and 1998.
The Binodal Colloidal Aggregation Test (BCAT) hardware supports four experiments. The first hardware, Binary Colloidal Alloy Test - 3, consisted of three separate investigations, Binary Alloy (BCAT-3-BA), Critical Point (BCAT-3-4-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 (a continuation of the investigation on BCAT-3) and Polydispersion (BCAT-4-Poly).
The BCAT-4-Poly polydispersed (characterizing the variation in particle size in the dispersed phase) and seeded samples consist of polymethyl methacrylate (PMMA) particles in an index matching decalin/tetralin mixture (the same colloid and solvent materials as the critical-point samples, but at a volume fraction of ~0.59). Although these samples are at or above the so-called glass transition point, colloidal crystals are expected to form. The particle size distribution and the addition of spherical seed particles should affect the free energy barrier for crystal nucleation, that is, the rate at which crystals nucleate. Photography will be used to study their evolution, with the hope of seeing white light backlit samples diffract the light so that the color changes with viewing angle. This will help reveal the shape of the nuclei, which provide information about the way the crystals grow in microgravity. The crystallites might grow fast in certain crystallographic directions which could give them a layer like structure. Also their shape will give some hints about the processes that limit the growth. Comparison with analogous ground-based experiments will reveal differences in the growth behavior in microgravity.
BCAT-4-Poly will ultimately impact our understanding of the strength and thermal conductivity of materials by providing insight into the effects of size variation in dense suspensions of particles. For example, the careful selection of crystallization promoters for controlling the crystallite size and size distribution may lead to improvement in materials fabrication processes. The suppression of crystal nucleation in polydisperse colloids has important implications for the morphology of polycrystalline materials.Earth Applications
Generally, colloidal nucleation experiments seek an understanding of the most fundamental liquid/solid transition. Though direct applications of that understanding do not drive the research, growth of ordered colloidal phases has attracted interest in a number of areas, e.g. ceramics, composites, optical filters and photonic bandgap materials. Moreover, there is currently great interest in using fields and gradients to control order in self-assembled systems such as diblock copolymers and microemulsions for advanced materials.
The BCAT-4 hardware consists of ten samples of colloidal particles. The microscopic colloid particles and a polymer (samples 8 - 10) 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 8 - 10 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 sessions 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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Cheng Z, Chaikin PM, Zhu J, Russel WB, Meyer WV. Colloidal hard-sphere crystallization kinetics in microgravity and normal gravity. Applied Optics. 2001; 40(24): 4146-4151. DOI: 10.1364/AO.40.004146.
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