Advanced Colloids Experiment-Microscopy-2 (ACE-M-2) - 10.04.18

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

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Sometimes it's hard to tell a gas from a liquid. Advanced Colloids Experiment-Microscopy-2 (ACE-M-2) observes the microscopic behavior of liquids and gases separating from each other.  The investigation examines the behavior of model (colloid rich) liquids and model (colloid poor) gases near the critical point, or the point at which there is no distinct boundary between the two phases.  ACE-M-2 uses a new microscope to record micro-scale events on short time scales, while previous experiments observed large-scale behavior over many weeks. Liquids and gases of the same material usually have different densities, so they would behave differently under the influence of gravity, making the microgravity environment of the International Space Station ideal for these experiments.
Science Results for Everyone
Information Pending

The following content was provided by David A. Weitz, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details

OpNom: ACE

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

Information Pending

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
NASA Research Office - Space Life and Physical Sciences (NASA Research-SLPS)

Research Benefits
Information Pending

ISS Expedition Duration
March 2014 - September 2014; February 2018 - April 2019

Expeditions Assigned

Previous Missions
Information Pending

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Experiment Description

Research Overview

  • Using a model system, Advanced Colloids Experiment-Microscopy-2 (ACE-M-2) is a fundamental investigation of the behavior of liquids and gases near the critical point, where sedimentation does not play a role. Because liquids (colloid rich sample portions) are almost always denser than gases (colloid poor sample portions) of the same material, earth-bound experiments are almost always affected by sedimentation. By going to ISS, these effects are eliminated, allowing a true picture of phase separation behavior, without the interfering effects of gravity.
  • Observations of a number of samples corresponding to different densities and effective temperatures, around the critical point, is the goal. This allows the testing of a number of important theories in physics and chemistry surrounding the areas of critical phenomena, phase transitions, and statistical mechanics/thermodynamics.
  • Fundamental studies are conducted, so the major impact will be a better understanding of these important processes. This understanding will have impact in a number of practical applications, for instance the development of household products by Procter & Gamble (P&G), worth billions of dollars to the U.S. economy.


Advanced Colloids Experiment-Microscopy-2 (ACE-M-2) investigates the behavior of liquid-gas phase separation near the critical point in a model colloid-polymer system. Colloids are here defined as micron-scale solid particles suspended in a fluid. Particles in this size range are small enough so that thermodynamics drives their behavior (as opposed to larger particles in the granular limit, where they are essentially static over experimental time scales), yet the particles are also large enough to interact with light, and so can be probed with scattering (for example, in the Physics of Colloids in Space (PCS) experiment previously flown in microgravity) and microscopy (the present ACE experiments). As a result, single-particle knowledge can be obtained for a wide variety of physical processes, including crystallization, gelation, phase separation, and dynamical arrest in glasses.
Another major advantage of colloidal systems is the control over which their interactions can be designed, ranging from simple hard-sphere, to repulsive and attractive interactions. These systems are, in general, athermal, where a polymer is added to create a depletion attraction. By varying the concentration and molecular weight of the added polymer, the range and strength of attraction between particles can be precisely tuned. This is fixed by the chemistry, and does not change appreciably over a reasonably wide range of temperatures. Therefore, these experiments can be conducted at very precisely controlled values of colloidal density, attraction strength and range, without precise temperature control. For critical-point experiments, which in molecular systems are exquisitely dependent on precise temperatures, the colloid-polymer systems enable a much more careful measurement under existing conditions aboard ISS.
Therefore, we will create a set of samples and make a 2D grid around the critical point in the phase diagram. This will provide a number of new pieces of information. First, we expect to locate the critical point itself, which has not been found while carefully using the systems where sedimentation is absent. We then intend to look at rates of phase separation in the samples around this point, to test directly the predictions from scaling laws developed as part of a large theoretical effort in physics that lead to a Nobel Prize for Kenneth Wilson for his theoretical work with the renormalization group. Almost all existing experimental data have been performed in systems on earth, where sedimentation is known to play a role. Because liquids are almost invariably denser than gases of the same materials, studying liquid-gas phase separation on earth is essentially fundamentally limited in a way that, by using colloidal mixtures in microgravity, we can overcome. This will hopefully enable some fundamental measurements to be made with our system.
Moreover, we are leveraging a decade of macroscopic experiments onboard ISS, as part of the BCAT series. Here, we observed for months the long-time behavior of phase separation, on length scales of up to centimeters (using particles only a few hundred nanometers in size). By looking at the same, or similar, samples in ACE under the microscope, and with BCAT-style macroscopic experiments, we can connect behavior on the micron-scale and fractional-second timescale, to centimeters and months. This unprecedented range of many orders of magnitude in time and space are a testament to the power of the ISS as a unique laboratory to perform important fundamental physics investigations.

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Space Applications
On Earth, gravity plays a big role in how liquids and gases interact: The heavier liquid phase settles beneath a lighter gas phase, and these differences in density enable easy separation. But the flow and behavior of complex fluids and phase separation where there is little or no gravity is not well understood. A greater understanding of liquid-gas interaction in microgravity could benefit a wide range of fluid storage, transport and processing systems for future spacecraft.

Earth Applications
Particle separation and behavior in liquids, gels, and creams is important for developing consumer and household products, which are worth billions of dollars annually to the U.S. economy. Many consumer products are complex fluids, combining microscopic particles in gel or liquid, which are similar to the model colloidal system used in the ACE investigations which give insight into product formulations that could be used to maximize stability and shelf life.

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Operational Requirements and Protocols

One experiment per week; duration of 1-1 ½ days. Each experiment consists of running one strip of 5 samples (due to oil limitations for 100x oil emersion optics). All samples will record 100 frames (at the fastest frame rate, probably 6 fps) at 0, 2, 4, 8, and 16 hours (viewing all samples on a strip sequentially when these time intervals occur). It is understood that the 0 interval may occur and 1 ½ hours or more after mixing when the mixing occurs outside the LMM.
Run spare Sample Modules if air bubbles are too big. There are 3 strips in each sample cell (of 5 samples each) and 3 backup strips in the spare sample cell (module) to replace runs for samples that have large air bubbles or other problems.
Between weekly runs, analyze data, re-write scripts, and adjust parameters.
Number of wells per experiment are 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 – available immersion oil limits runs to one strip at a time; air objective has no such constraints.
Experiment Steps:
  1. Inspect Samples.
  2. Ground to choose first of three sample strips and first Sample to test; feedback to crew.
  3. Mix all samples in strip using drill BCAT magnet for 30 seconds.
  4. Apply oil in (Auxiliary Fluids Container) AFC and install assembly. It may require more than one drop of oil be applied 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 at 2.5x; determine primary (and secondary?) test locations (select locations away from stir bar or bubble).
  8. Adjust and record best camera parameters using 100x oil objective and Texas Red filter.
  9. Survey and record best Z-depth at each primary test location.
  10. Experiment on one well using 100x oil objective; each image set at highest frame-rate (6 to 8 fps), no binning (8 bpp highest supported), full frame images. Store 100 images using Texas Red filter.
  11. Move to next good sample in strip of 5 samples and Repeat until all good samples in strip have been imaged as outlined above.
  12. Repeat measurements for all good samples in strip using a time interval of 0, 2, 4, 8, and 16 hours.  This means that an entire strip of 5 samples can be run in under 1 ½ days.
  13. The goal is to successfully image 15 unique sample wells. Fifteen of the 30 wells are replicates and are flight spares.

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Decadal Survey Recommendations

Applied Physical Science in Space AP5
Fundamental Physical Sciences in Space FP1

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Results/More Information

Information Pending

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Results Publications

    Kodger T, Lu PJ, Wiseman GR, Weitz DA.  Stable, fluorescent PMMA particles for long-term observation of slow colloidal dynamics. Langmuir. 2017 May 31; epub: 8 pp. DOI: 10.1021/acs.langmuir.7b00852. PMID: 28560881.

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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
Space Flight System

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image NASA Image: ISS039E016369 - Close-up view of an ACE-M-2 sample.
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image NASA Image: ISS039E016376 - Close-up view of an ACE-M-2 sample.
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