ISS Science for Everyone
Science Objectives for Everyone
Small particles suspended in a mixture, known as colloids, can combine to form complex structures and be used in new advanced materials. Colloids are found in a wide range of foods and consumer products, but they can also include particles with unique surface chemistry or electrostatic properties that allow them bind to each other in various ways. The Advanced Colloids Experiment-H-2 (ACE-H-2) investigation studies a technique called nanoparticle haloing, which stabilizes colloidal mixtures and may be important for designing advanced materials for use in medicine, imaging and other fields.
Science Results for Everyone
The following content was provided by Stuart K. Williams, Ph.D., Suzanne Smith, Ph.D., and is maintained in a database by the ISS Program Science Office.
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Stuart K. Williams, Ph.D., The University of Arizona, Tucson, AZ, United States
Suzanne Smith, Ph.D., University of Kentucky, Lexington, KY, United States
Gerold Willing, Ph.D., University of Louisvi, Louisville, KY, United States
Hemali Rathnayake, Ph.D., Western Kentucky University, Bowling Green, KY, United States
NASA Glenn Research Center, Cleveland, OH, United States
ZIN Technologies Incorporated, Cleveland, OH, United States
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
NASA Research Office - Space Life and Physical Sciences (NASA Research-SLPS)
Earth Benefits, Space Exploration
ISS Expedition Duration
September 2015 - March 2016; March 2016
- September 2016
New functional materials can, in principle, be created using microscopic particles (called colloids) that self-assemble into a desired structure by means of a recognition and binding scheme. The Advanced Colloids Experiment-Heated-2 (ACE-H-2) experiments utilize optical microscopy for time- and space-resolved imaging of spherical colloids of various sizes and concentrations. Colloidal assembly and the structure’s stability can be controlled by mediating the particles’ electrostatic charge and surface chemistry. The introduction of smaller particles allows the tuning of the interactions between the larger colloids, and in this way control the structure of the colloidal dispersion. The particle surface interactions are reversible and sensitive to temperature.
In ACE-H-2, fundamental insights are to be gained into the interaction of smaller nanoparticles with larger colloids, i.e. the “nanoparticle haloing” (NPH) phenomenon, as a function of particle concentration. Crystallization behavior of the larger colloids, whose structure is a function of the size and concentration of nanoparticles, is also to be observed. In the microgravity environment of space, it is hoped that unobstructed NPH interactions, which would otherwise be significantly hindered by gravity on earth due to sedimentation issues (high density contrast between particles and fluid), can be observed.
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.
Nanoparticle Haloing (NPH) is a colloidal stabilization technique that has been discovered relatively recently. In NPH, highly charged nanoparticles are used to stabilize much larger (by 2 orders of magnitude or more), non-charged particles. To accomplish this stabilization, the volume fraction of nanoparticles within the suspension need only be on the order of 10-5 up to 10-4 to stabilize at 1 to 2 vol% suspension of the larger particles.
When these suspensions were allowed to settle under gravity, those that were stabilized (while in suspension) formed highly ordered, 3-dimensional colloidal crystal structures. Further experiments using confocal microscopy revealed that the crystals were composed entirely of the larger particle with the nanoparticles remaining in suspension. Based on these measurements, it was concluded that the nanoparticles were not creating a charge layer by adsorbing onto the surface of the larger particle. Instead, it was thought that the nanoparticles created a charge layer by forming cage, or Halo, around the larger particles.
Another hypothesis regarding the NPH stabilization mechanism focuses entirely on nanoparticle adsorption to the surface of the larger particle. Based on the measurement of the interaction forces between two larger colloidal surfaces in a nanoparticle suspension, it was found that the force curves obtained could be fitted using a model based on nanoparticle adsorption. In this case, the van der Waals interaction between the larger colloidal surfaces was shielded by the adsorbed nanoparticle layer. This layer also supplied an electrostatic charge barrier to add an additional repulsive force within the system.
Using Atomic Force Microscopy, the interaction forces between two colloidal surfaces in a suspension of highly charged nanoparticles was recently measured. The nanoparticle volume fraction was varied by 5 orders of magnitude (from 10-6 up to 10-2). Results indicated that the two hypotheses are not mutually exclusive, but actually exist at different ends of the nanoparticle concentration spectrum. At low nanoparticle volume fractions, NPH tends to be the dominant stabilization mechanism. Nanoparticle adsorption tends to dominate at higher nanoparticle volume fractions.
The proposed experiment (i) investigates the nature of three-dimensional colloidal structures formed by NPH under microgravity and (ii) assess the structure’s stability under induced thermal “shocks” using Brownian motion analysis.
NPH stabilizes larger colloids to form crystalline structures. Unfortunately, previous work investigating these structures are heavily influenced by gravity, as a typical particle under investigation (ex: 1 mm silica) sediments relatively quickly on earth. This experiment utilizes the microgravity environment to discover how 3D NPH crystalline structures are formed, and to investigate this characteristic as a function of suspended nanoparticle concentration.
The sample contains two particles. The smaller nanoparticles are non-fluorescent 8 nm zirconia (ZrO2) particles. The larger particles are 600 nm particles (not tagged). Nanoparticle (8 nm) visualization is not required; however, aggregations formed by the larger colloids (600 nm) need to be identified.
This experiment uses the existing 15 well chamber for ACE-H. The larger particle (600 nm) volume concentration for the all 15 samples is constant at 1.0%. Three (3) different small particle (20 nm) volume concentrations are prepared (0.01%, 0.055%, and 0.1%). These samples are duplicated for a total of five (5) 0.01% samples, five (5) 0.055% samples, and five (5) 0.1% samples. The solution for the suspensions is 0.35M nitric acid.
General Experimental Trials
Aggregate identification and visualization: Experiments focus initially on determining how the three-dimensional NPH structures form in their environment. First, a specific sample well is scanned at 20X magnification to identify NPH clusters. The Principle Investigators (PIs) will identify, within each sample well, a region/aggregation of interest to be imaged at larger magnification (40X). Digital images are analyzed and processed offline. Measurements should be able to clearly assess how NPH forms 3D colloidal crystals, as replicated experiments on earth would be severely hindered on earth, providing an “incomplete picture” of 3D NPH. It is expected that structure formation is a function of nanoparticle concentration.
Thermal shock: The inherent capabilities of the platform are used to induce a heat shock to the system, i.e. uniformly and rapidly heating the sample from ambient to 43 °C at the maximum rate for the ACE-H platform. During thermal ramp, a region of interest (and/or identified NPH crystal or interest) are imaged. The relative position of the colloids is tracked and the integrity of the crystalline structure is assessed during the thermal shock, providing stability dynamics associated with NPH. This should allow the research team to visualize, for the first time, the three-dimensional dynamics of Halo disruption and reformation.
[Note: ‘Thermal shock’ experiments are only performed on those sample cells that created NPH-induced colloidal aggregates. It is not expected that all 15 samples will be a part of this experiment series.]
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To remain stable, colloids require a balance between attraction and repulsion. One method for stabilizing these mixtures is known as nanoparticle haloing, in which charged nanoparticles are mixed with larger colloidal particles. The nanoparticles self-organize and form a halo-like structure around the colloids. ACE-H-2 uses the microgravity environment of the International Space Station to study how changing the concentration of colloidal particles affects the haloing interaction. On Earth, gravity would cause the particles to sink and clump together, interfering with the observations. Results benefit research using colloids to build self-assembling structures.
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Results from the ACE-H-2 investigation provide new insight that would be difficult or impossible to obtain on Earth, where gravity interferes with particle interactions. Understanding the fundamental physics of colloid interactions benefits materials science research on Earth, including research on self-assembling technologies.
Operational Requirements and Protocols
The experiment is conducted over 12 weeks, including several offline periods for data analysis. Experiments are separated into three four-week experiments. The first four-week period will perform “Aggregate identification and visualization” experiments. The second four-week period will perform the “Thermal shock” experiments. The third four-week period will be a repeat of the “Thermal shock” experiments.
General experiment steps:
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Mix all samples in sample module to be viewed using BCAT drill magnet for 1 minute each (15 minutes). After a brief break (5 minutes) 9 cells (chosen by the PIs) are mixed for an additional minute each.
Load samples - inspect all samples.
Ground to choose first sample to test; feedback to crew.
Define XYZ offsets (assembly alignment per established ACE method).
Adjust camera parameters using 2.5x objective and B/S cube.
Survey chamber(s) at 2.5x. Determine bubble locations and location of stir bar.
Switch to 10x objective.
Adjust and record best camera parameters using an appropriate Z-depth.
Aggregate identification and visualization: Methodically scan all samples with a 10X objective. This includes proper x- and y-scans to capture images of the complete chamber. Acquire each image at full frame resolution. It is assumed that a complete sample scan as outlined here will take approximately 20 minutes (i.e. 15 samples will take 5 hours). Each week three samples will be selected for a comprehensive 20X scan. The PIs select a region within this scan for additional image acquisition at 40X magnification; a z-scan routine accompanies the 40X image acquisition. Two movies (Week 2, Day 1; Week 4, Day 1) are acquired on selected samples at 20X, one image per minute, for an hour each.
Thermal shock: Mix all samples as outlined in (3). Follow the “Aggregate identification and visualization” instructions to acquire images of all sample wells at ambient temperature at 10X magnification. Next, a specific sample is chosen by the PI for a comprehensive sample scan at 20X while at ambient temperature. The platform is then to ramp at its maximum rate to 43°C. During this ramp, a two-hour movie at 40X magnification is acquired with one image being acquired every minute. If the stage temperature is interrupted, allow the sample to reach 43°C, and continue image acquisition. At the end of the two hour period, a comprehensive sample scan at 20X is conducted (while at elevated temperature). At the end of this scan the temperature can be deactivated and returned to ambient conditions. This process is repeated for additional samples during the experiment set.
Decadal Survey Recommendations
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2011 ACE Science Concept Review (SCR) presentations
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European Space Agency Astronaut Paolo Nespoli operating the Light Microscopy Module microscope aboard the International Space Station on a previous mission. Image courtesy of NASA.
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The Light Microscopy Module (LMM) features a modified Leica RXA research imaging light microscope with powerful laser-diagnostic hardware and interfaces. The LMM is being used in the Advanced Colloids Experiment (ACE-H-2) investigation. Image courtesy of NASA.
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