Advanced Colloids Experiment-Microscopy-3 (ACE-M-3) - 03.04.14
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The ACE-M-3 experiment involves the design and assembly of complex three-dimensional structures from small particles suspended within a fluid medium. These so-called “self-assembled colloidal structures”, are vital to the design of advanced optical materials. In the microgravity environment, insight will be provided into the relation between particle shape, crystal symmetry, and structure: a fundamental issue in condensed matter science.
Science Results for Everyone
ZIN Technologies Incorporated, Cleveland, OH, United States
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
ISS Expedition Duration
March 2014 - September 2014
Previous ISS Missions
New functional materials can, in principle, be created using small particles (called colloids) that self-assemble into a desired structure by means of a recognition and binding scheme. The ACE-M-3 experiments utilize optical microscopy for time- and space-resolved imaging of a particular non-spherical colloid, whose condensed (solid-like) phase can be controlled by adding ‘depletants’. These smaller particles allow the tuning of the interactions between the colloids, and in this way control the structure of the colloidal dispersion.
In ACE-M-3, the crystallization behavior of micron-sized colloidal cubes is studied by means of a tunable depletion interaction. Under certain conditions, the particles should self-organize into crystals with simple cubic symmetry, which is set by the size of the nano-size depletant. In the microgravity, we hope to observe three-dimensional structures that are impossible to create on earth due to sedimentation issues (high density contrast between particles and fluid).
Ultimately, the ability to design colloidal particles with a variety of well-controlled three-dimensional bonding symmetries opens a wide spectrum of new structures for colloidal self-assembly, beyond particle assemblies whose structures are defined primarily by repulsive interactions and shape. Such materials might include photonic crystals with programmed distributions of defects. Optical technology utilizing such materials may offer intriguing solutions to unavoidable heat generation and bandwidth limitations facing the computer industry.
An outstanding problem in condensed matter science concerns the relation between particle shape, crystal symmetry and structure. The simplest and most symmetric crystal is cubic and is naturally comprised of cube-shaped ‘particles’. In atomic systems, these are cubic lattices of atoms; in our colloidal structures, the constituent particles are, in fact, silica cubes.
One research goal is to produce such a colloidal structure, and study the dynamics of crystal nucleation and growth. Recently, simple cubic colloidal crystals have been observed by optical microscopy (Rossi, et al., 2010). These close-packed structures were self-assembled in aqueous dispersions of novel, hollow silica cubes.
Due to their small gravitational height – related to the grossly mismatched density of the colloid and solvent – the Brownian cubes assembled in a quasi 2D fashion, growing only 2 or 3 particles in height. Larger gravitational heights might be achieved by dispersing the particles in a density matching solvent (e.g., an ethanol / bromoform mixture) to retard sedimentation for the assembly of 3D crystals. However, there are numerous difficulties associated with this type of experiment.
Observing the particle interactions in microgravity eliminates these complicating factors, and increases the gravitational height by many orders of magnitude, thus allowing the possibility of making three dimensional cubic crystals.
In our system, cubic crystal nucleation and growth is driven by anisotropic depletion forces that cannot be generated in conventional sphere fluids. This allows additional control of the condensed phase where, for example, decreasing the size of the depletant locks neighboring sliding planes in registry and produces a true, simple cubic crystal.
Microgravity aids self –assembly and motility on materials that have different densities. The work in ACE-M-3 will lay the foundation for the understanding of this process.
One experiment (sample strip of five wells) per week; duration of 1-4 days (depending upon the number of Z-layers that are needed to determine the crystal properties). Repeat until all fifteen wells are tested (3 to 6 strips, depending upon whether or not any of the 3 back-up sample strips were used).
Switch Sample Modules if air bubbles are too big (after running good wells in existing sample module).
Use the rest of the week to analyze data, re-write scripts, adjust parameters.
# wells per experiment limited by:
1. Data bottlenecks on IPSU and IOP
2. XY position repeatability (need to return to the same particle set or don’t move during experiment => one well position. This takes too long, so find a solution. Images can be registered in post-processing via port or stir bar location, or pattern of particles stuck to bottom of cover slip.
3. Oil availability – too much repeat movement from well to well may lose immersion oil, so limit test points; air objective has no such constraints.
1. Inspect all samples.
2. Ground to choose first sample to test; feedback to crew.
3. Mix all samples in sample wells in sample module to be viewed using BCAT drill magnet for 1 minute.
4. Apply immersion oil in auxiliary fluids container (AFC) and install assembly. It may require more than one drop of oil be applied off to the side when initiating a run of five sample wells on a strip.
5. Define XYZ offsets (assembly alignment per ACE-1 method).
6. Adjust camera parameters using 2.5x objective and B/S cube.
7. Survey well(s) at 2.5x; determine possible primary (and secondary) regions of interest (ROI). If the 2.5x objective is difficult to switch in and out with the 63x oil objective, then find ROI for all five wells before using 63x oil objective. Select test locations away from stir bar or bubble. This will be about (9) 800 x 800 micron areas within the 2.5 mm diameter well.
8. Move to first ROI. Using 63x oil objective, focus on the bottom surface of the particle aggregate (crystallite). This side is closest to the objective.
9. Adjust and record best camera parameters using rhodamine-B isothyocyanate RITC filter (e.g, Texas Red filter).
10. Survey and record best Z-depths at each primary ROI.
11. Experiment on one well using 63x oil objective; each image set at 5 frames per sec, no pixel binning, 8 bits per pixel (highest supported), full frame images.
12. Record at ten Z-depths (e.g., 2, 4, 6, …, 20 microns) each primary (and secondary) ROI. This calculates to 1332 minutes at one well if all 10 depths are needed for resolving crystal structure.
13. Repeat at 60-minute intervals while crystals are still growing. Without in-situ mixing, this experiment is not likely to be able to follow the kinetics. Repeat this step if necessary.
14. Complete upon evaluation by PI (estimate up to a 3-day duration).
Imaging goal is to observe crystallites and resolve particle centroid positions with less than 20% error.
Shutter lamp between image sets to prevent sample bleaching.
Repeat of cycles is desired every 60 minutes, but desired as often as every 20 minutes while moving between samples.