EXPRESS Physics of Colloids in Space (EXPPCS) - 10.11.17

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ISS Science for Everyone

Science Objectives for Everyone
EXPRESS Physics of Colloids in Space (EXPPCS) studied the kinetics of colloidal (fine particles suspended in a fluid) crystal formation and growth. These experiments provided the critical information necessary to use colloidal precursors to fabricate novel materials in the new field of colloidal engineering. Industries using semiconductors, electro-optics, ceramics and composites may benefit from this investigation.
Science Results for Everyone
Just gellin’ in space. Novel materials could result from a study of the kinetics of crystal formation and growth from colloids, or fine particles suspended in a fluid. Colloids can act like a gas, liquid, or solid, or even glass, depending on relative concentrations of suspended material and solution and the level of gravity. This investigation examined colloid behavior in microgravity, collecting data on binary colloidal crystals, colloid-polymer mixtures, colloid-polymer gels and critical point, fractal gels, and colloidal glass. Several surprising behaviors were observed, including formation of a solid gel from colloid-polymer gels and increasing stiffness over time long after gelation occurs.

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


Principal Investigator(s)
David A. Weitz, Ph.D., Harvard University, Cambridge, MA, United States

Peter N. Pusey, Ph.D., University of Edinburgh, Edinburgh, United Kingdom

Information Pending

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)

Research Benefits
Information Pending

ISS Expedition Duration
March 2001 - June 2002

Expeditions Assigned

Previous Missions
The predecessors to EXPPCS Binary Colloid Assembly Test-1,-2 were performed on Mir. The EXPPCS investigation was performed on ISS Increments 2 - 4.

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Experiment Description

Research Overview

  • The International Space Station provides a long-term laboratory for understanding the behavior of colloidal mixtures in a microgravity environment. Some colloids (a system of fine particles suspended in a fluid) have the ability to act like a gas, liquid, solid, or even glass, depending on the relative concentration between the suspended material and the solution they are suspended in, and/or the presence or absence of gravity.

  • The behavior of a densely packed colloid here on earth mimics glass in the distribution of its particles, while in space, the same density colloid acts more closely like a solid. This results in a highly organized, lattice-like arrangement of particles in the colloid. The crystalline particle arrangement within the colloidal suspension creates the maximum amount of particle spacing, which allows for laser-based measurements of the particle structures. The manipulation of a colloid to alter its physical properties is termed colloidal engineering.

  • As the concentration of uniformly sized hard spheres suspended in a fluid is increased, the particle-fluid mixture changes from a disordered fluid state in which the spheres are moving haphazardly to an ordered crystalline state in which they are arranged periodically. Like atoms, the thermal energy of the spheres causes them to bump into each other until they form ordered arrays, or crystals, which gives each sphere the most room to move around.

  • On earth, at even higher concentrations, these hard sphere systems behave like glass. Their true nature and growth manifests itself in microgravity. This has been pleasantly surprising and will be studied with EXPPCS hardware.

Colloids can be defined as fluids with other particles dispersed in them, particularly particles of sizes approximately between 1 nanometer and 1 micrometer. Since colloids have widespread uses in nature and industry, understanding of the underlying physics that controls their behavior is important. Under the proper conditions, colloidal particles can self-assemble to form ordered arrays, or crystals. On Earth, the ordering of these particles is mostly directed by gravitational effects, sedimentation, and buoyancy. Self-assembly does not occur. Thus, the weightlessness of low Earth orbit is an important element in the study of colloids.

Physics of Colloids in Space (PCS) focused on the growth, dynamics, and basic physical properties of four classes of colloids: binary colloidal crystals, colloid-polymer mixtures, fractal gels, and glass. These were studied using static light scattering (for size or positions of the colloids or structures formed), dynamic light scattering (to measure motions of particles or structures), rheological (flow) measurement, and still imaging.

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Space Applications
The colloidal engineering process will play a fundamental role in the creation of new materials and products in space, such as optical switches and lasers for communications and displays.

Earth Applications
EXPPCS will improve such colloids as paints, food products, drug delivery systems and ceramics by providing a better understanding of colloidal behavior.

The EXPPCS investigation has contributed to Earth-based investigations of cataracts, which are caused by the buildup of damaged proteins within the eye lens and are the single largest cause of blindness. Diagnosis of cataracts is normally carried out by looking for protein buildup via a standard ophthalmological device known as a slit-lamp microscope, which can only detect cataracts once they have formed. Fortunately, a new laser probe originally developed for the U.S. space program to study protein crystal formation on the ISS, has been shown to detect cataracts before they are symptomatic. This new technique uses dynamic light scattering (DLS) to detect small proteins called alpha-crystallins in the eye’s lens, which is a reliable biomarker for cataracts. Laser light is shone into the lens of the eye while a highly sensitive photon detector is used to measure light backscattered at specific wavelengths. If the amount of alpha-crystallin proteins has lessened, this is an indication that cataracts are developing. If cataracts are detected early by this new technique, it may be possible to slow or stop the accumulation of damaged proteins by reducing relevant factors.

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Operational Requirements and Protocols

Crew time is required for experiment transfer and activation, checkout, and periodic monitoring. The experiment requires water cooling from its EXPRESS Rack and will be equipped with the Active Rack Isolation System (ARIS) which reduces vibrations.

The Station crew members will mix colloid samples evenly and allow them to sit for several days. Using the Test section, crew members will perform some analyses but the majority of operation will be ground-based observations utilizing cameras installed in EXPPCS software. The EXPPCS hard drive stores all data for downlinking and post-flight analysis. Remote operation takes place from the Telescience Support Center at Glenn Research Center and at Harvard University.

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Decadal Survey Recommendations

Information Pending

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Results/More Information

The Express Physics of Colloids in Space (EXPPCS) hardware was activated onboard the ISS in May 2001 and remotely operated from ground control centers from early June 2001 until February 2002. The entire experimental setup performed well, and accomplished 2400 hours of science operations. The broader objective is to study physics of colloidal particles dispersed in a fluid. This includes nucleation and growth of colloidal crystals and behavior of binary colloidal crystal alloys. The structure and properties of colloidal particles with attractive interactions (depletion interactions) induced by the addition of a non-adsorbing polymer, behavior of large-scale fractal (repeating geometric pattern over the entire surface) aggregates, gels, and glasses are studied. This research and knowledge can aid in the fabrication of novel materials that may have applications in electronic display technology. The new materials could have unique properties such as photosensitivity and serve as light switches and could control the direction or color of light.
Binary colloidal crystals: These alloy samples are dispersions of two differently sized particles in an index-matching fluid. Two samples were studied: an AB13 crystal structure and an AB6 crystal structure. Due to a hardware failure late in Expedition 4, the AB6 experiment was not completed. Unexpected "power law" growth behavior that is still under investigation was observed in the AB13 crystal structure sample.

Colloid-polymer mixtures: These mixtures induce a weak attractive interaction that allows precise tuning of the phase behavior of the mixtures, and approximate the phase separation below the critical point of a gas-liquid mixture. The phase behavior is controlled by the concentration of the colloid, the concentration of the polymer, and the relative size of the colloid and the polymer. The results from the ISS experiments studied the spinodal decomposition, or phase separation near the critical point, unencumbered by density differences of the phases. The growth of the phase separation was studied using both light scattering and imaging. Without gravity, the phase separation took 30 times longer than on Earth. The sample was mixed, then phase separation began, gradually coarsening until the container walls interacted with the mixture (at 42 hours) and the colloid-rich phase wet the container wall, completely coating it after 60 hours. Because the results follow very similar time evolution as a shallow quench of a binary liquid, they provide insight into the importance of the length scale of colloidal gels; separation depends more on coarsening rates than initial colloid size.

Colloid-polymer gels: This sample was expected to be in a fluid-cluster state, but unexpectedly formed a solid gel. The elastic modulus, which was estimated using the experiment's rheology capabilities, will be compared to ground samples. "Aging" characteristics of this gel were found to be similar to those formed on Earth. Surprisingly, upon gelation, the elastic moduli of the gels increase as a power law in time, independent of initial volume fraction. Moreover, changes in the aggregate network persists long after gelation occurs, and for the duration of the measurement. As a consequence, their elasticity maintains the same volume fraction dependence, independent of time. It is theorized that the stiffening of the network is a result of chemical reactions at the junctions between neighboring particles, which increase local bond strength. Measurements show that changes in bonding at the particle level can significantly affect the overall elasticity of colloidal gels. While the focus here is on silica gels, researchers expect similar behavior for gels of different materials. This is in sharp contrast with the more usual time evolution of elasticity due to changes in the network structure (rather than particle bond strength); this provides an alternate potential means for changing the time-dependent materials properties of colloidal gels .

On earth, gravity and shear settling forces limit cluster growth, thus they can ultimately limit gelation. In the absence of gravity, it is thermal fluctuations which ultimately inhibit fractal growth and set the fundamental limitation to the lowest volume fraction which will gel. In space, clusters grow at a rate in excellent agreement with that expected for diffusion-limited colloid aggregation (DLCA). This cluster growth persists for the full duration of the space experiment; gelation is not observed because the duration is less than the gelation time, 90 days, estimated by extrapolating the observed growth to the cluster size expected at the gel point. By contrast, the time evolution of cluster growth on Earth shows significant deviation from the behavior anticipated for DLCA. The initial growth of the clusters is identical to that in space, however, after a period, the growth deviates from the expected behavior and reaches a plateau, with no further growth observed. Thus, the chosen system will not gel as the maximum cluster size is more than 1 order of magnitude lower than that needed to form a gel. This arrested cluster growth observed on. The fractal nature of the aggregates is essential to enable a small volume fraction of colloids to form a connected solid, thus any deviation from fractal growth severely compromises gelation. On Earth, gravity limits the maximum cluster size, thereby determining the minimum volume fraction which will gel.

Colloid-polymer critical point: Immediately after mixing, the colloid-polymer critical point sample began to separate into two phases, one that resembled a gas and one that resembled a liquid, except that the particles were colloids and not atoms. The colloid-poor regions (the colloidal "gas" phase) grew bigger until, finally, complete phase separation was achieved and there was just one region of each, a colloid-rich phase and a colloid-poor phase. None of this behavior can be observed in the sample on Earth because sedimentation would cause the colloids to fall to the bottom of the cell faster than the de-mixing process could occur. Knowledge gained from these runs was used to develop the BCAT-3 later operated on ISS.

Fractal gels: Fractal gels may form when charged colloids have their electrostatic repulsions screened out by the addition of a salt solution, permitting aggregation. These can be formed at very low volume fractions and form highly tenuous aggregates that exhibit a remarkable scaling property, their structure appears the same on all length scales up to a cluster size, and so can be described as a fractal. It was thought that the samples studied (colloidal polystyrene and silica gel) would, in the absence of sedimentation effects, ultimately form a continuous network of fractal aggregate; the polystyrene fractal sample never fully gelled as expected, however. Initial indications are that the volume fraction tested was too low. Large fractal clusters did nevertheless grow (larger than they do on Earth), allowing measurement of the internal vibration modes of these structures. The silica gel is thought to have gelled, and is currently being evaluated.

Colloidal glass: These samples are still under evaluation. Comparison to samples formed in one-g in the laboratory were needed to understand whether the crystallization observed was due to poor mixing or was a true microgravity phenomena. This research and knowledge can aid in the fabrication of novel materials that may have applications in electronic display technology.  The new materials could have unique properties such as photosensitivity  and serve as light switches and  could control the direction or color of light.


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Results Publications

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Ground Based Results Publications

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ISS Patents

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Related Publications

    Ansari RR, Hovenac EA, Sankaran S, Koudelka JM, Weitz DA, Cipelletti L, Segre PN.  Physics of colloids in space experiment. AIP Conference Proceedings: Space Technology and Applications International Forum, Albuquerque, NM; 1999 January 22 108-113.

    Chaikin PM, Russel WB, Kopacka W, van Blaaderen A, Meyer WV, Doherty MP.  Physics of Colloids in Space Plus (PCS+). Conference and Exhibit on International Space Station Utilization, Cape Canaveral, FL; 2001 42932.

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Related Websites
ISS Research Project-EXPPCS
NASA Fact Sheet

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image This is one of the first images from the EXPRESS Physics of Colloids in Space on the ISS. During Expedition Two, sample AB6 was illuminated with white light to produce the image. The colored regions result from refraction of the white light by the sample and sample cell, splitting it up into its component colors. Image courtesy of Glenn Research Center.
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image This photo was taken with the 1X color camera of the AB6 sample after crystallization had occurred. The different colors are the result of different wavelengths of the white light illumination satisfying the Bragg condition at different angles relative to the lights. Bright sports are large crystallites. Diffuse color occurs are due to small crystallites. This image was taken prior to launch. The circular outline is the 2 cm outside diameter of the sample cell. Image courtesy of Glenn Research Center.
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