Advanced Colloids Experiment-Heated-2 (ACE-H-2) - 12.10.14
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Colloidal mixtures, which are small particles suspended in a fluid, are found in everyday household products, fuels and even foods. They can spoil when solid particles clump together or sink inside the liquid and form distinct layers. Advanced Colloids Experiment-Heated-2 (ACE-H-2) examines the Casimir effect, a function of quantum mechanics that creates a small force of attraction, and heat to determine how colloidal mixtures form clumps and solid structures.
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Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
ISS Expedition Duration
September 2014 - March 2015
Previous ISS Missions
- The assembly of colloidal and nanostructures is important for technological applications and for a fundamental understanding of the self-assembly processes that are ubiquitous in nature and in living matter. A better understanding of complex assembly can lead to the ability to design and grow nanostructured materials.
- The self-assembly is studied on precisely engineered composite colloidal particles, as a function of their rich anisotropic interactions. The range and symmetry of the interactions are controlled via the temperature-dependent critical Casimir force. Particles with increasing rotational symmetry (dimers, tetramers, etc.) are used to study the formation of structures of increasing complexity. In these experiments, changes in temperature are used to vary the interaction potential of the particles to study the growth of complex structures from a dilute particle suspension, as a function of the interaction strength. The motion of the individual particles are tracked from microscopic images to follow the growth of the structures both on a single particle and ensemble-averaged level (average structure factor) .
- These experiments provide insight into complex equilibrium and non-equilibrium structures and the dynamics of their growth.
Colloidal and nano-particles are good candidates to become the building blocks of some of tomorrow’s new materials. Applications of nano and micro materials are rapidly growing: this new particulate matter finds increasing applications in all parts of modern life ranging from food and drug industry to coating and painting to electronic device industry. There is increasing demand for ever more complex, specifically designed micro and nano-scale structures for photonic and electric devices; however, control over the assembly of complex 3D structures is highly challenging, and a basic understanding of the assembly process is an important focus of current research. Recent breakthroughs in particle synthesis have yielded a beautiful manifold of particle shapes with exquisite control over their size and surface properties. In particular the advent of anisotropic colloidal particles with anisotropic shapes and surface properties promises the ability of forming superstructures if their assembly can be controlled. However, the chances to use such particles as the building blocks of tomorrow’s materials will depend critically on our ability to finely control their assembly, and to steer the particles successfully into specifically designed structures.
Direct control over the anisotropic interactions via the critical Casimir effect offers new opportunities for the creation of new, complex structures. In this project, we build complex colloidal superstructures by controlling the strength of the anisotropic potential by temperature (via the Critical Casimir effect). This provides exciting new opportunities to control assembly and equilibrium phase formation. By applying the critical Casimir effect to colloidal particles with anisotropic surface properties, we will induce anisotropic interactions that will in turn lead to a wide variety of new superstructures, ranging from micellar clusters and living polymer networks to tetrahedrally coordinated crystals to complex molecular structures.
The first experiments of this kind used spherical particles, and varied the attractive strength to observe the assembly of isotropically attracting particles. These experiments demonstrated the new opportunities that the critical Casimir effect offers for assembling structures: By controlling the quench depth, we produced structures with largely varying fractal dimension, depending on the attractive strength set upon the quench. On ground, in contrast, a constant fractal dimension was observed, independent of the quench depth.
Anisotropic particles have been recently investigated by us on ground. Suspended in the binary solvent, they show anisotropic interaction, the strength of which changes with temperature in the range of 35-40°C. Interesting structures were already observed in these ground experiments; these structures depended on the solvent composition (left or right side of the critical composition), as well as on temperature. However, these structures and the growth process were strongly affected by sedimentation and convection. The goal of the proposed ACE-H-2 experiment is to study the diffusion-limited growth of these complex structures without the disturbing effects of gravity.
For this purpose, the phase separation temperature Tc of the binary solvent (expected to be around 40°C), and the aggregation temperatures must be first determined using reference cells (first sample module). Samples containing the colloidal particles should then be heated to temperatures between Ta and Tc, roughly Ta -0.5, Ta , Ta +0.5, Ta +1, Ta +1.5°C. The precise numbers depend on the particles and can be delivered when delivering the actual samples. In any case, the temperature difference T = T-Tc is the important parameter that controls the attractive strength. Held at the desired temperature, the aggregation of the particles should then be followed with the LMM microscope. The particles are labeled with fluorescent dye to allow fluorescent imaging on the LMM.
The microscopic structure and dynamics of the particles are to be recorded and tracked to characterize the aggregation. Initially, a layer of particles several microns into the bulk of a single three dimensional sample will be imaged in an epi-fluorescent mode of the LMM using a 63x air objective. (If particle centers are not easily distinguishable / resolvable, then a 100x air objective should be used). Once the microscope is properly focused, the sample will be heated to Ta+x°C, where x = -0.5, 0, +0.5, +1 and +1.5; higher temperatures lead to higher attractive strength between the particles. Videos of the particles are then recorded at 10 fps for at least 105min. In order to assure same initial conditions, the temperature jump to the desired final temperature should always start from the fixed reference temperature Ta-3°C, where samples should be mixed for at least 15min. At this temperature, no critical Casimir interactions occur, and the particles re-suspend. This procedure is repeated for each sample with different geometry of composite particles and/or different solvent composition.
After video acquisition, the structure and dynamics of the aggregation is analyzed. Assuming clear enough resolution of individual particles, positions and dynamics are determined using particle-tracking algorithms. If this level of resolution is not possible, Fourier analysis of the images can still provide the average structure factor of the aggregates, though this analysis will be more qualitative. Analyzing the microscopic structure and the dynamics of aggregation provides insight into the self-assembly process and equilibrium/out-of-equilibrium structures formed. Furthermore, different colloidal geometries and solvent compositions provides insight into the relation between anisotropic interaction and resulting structure.
Previous studies of the Casimir effect, a weak attractive force between spherical particles, have shown that gravity dramatically affects how the particles interact and join together. By comparing and contrasting particle structures grown in microgravity with those grown on Earth, scientists can better understand the fundamental physics of colloidal mixtures. This will improve future designs for nanostructured materials that can be developed in space, as well as control and design methods for colloidal materials that can be used in space and on Earth.
Colloidal mixtures are found in a wide range of consumer and industrial products, and improved understanding of their fundamental physical nature leads to new designs and products with longer shelf lives. In addition, understanding how colloidal particles clump together improves methods for self-assembly, including for nanostructured materials. Experiments in the ACE-H-2 investigation provide further insight into the basic scientific knowledge of complex particle interactions.
Duration of first sample module (5 wells) will be 10h pure measurement time. All other sample modules will take 50h measurement time (10 hours per well). There are 5 strips in a sample cell module. Repeat until all 30 wells (2 sample modules) have been tested (15 good wells required to meet success criteria, 30 wells desired - time allowing).
• Skip wells when air bubbles are too big.
• Use rest of week to analyze data, adjust parameters, and perform repetitions of experiments.
• Number of wells per experiment are limited by:
1. Data bottlenecks on IPSU and IOP
Module 1: Reference samples
1. Inspect Samples
2. Use sample module 1 containing the reference samples.
3. Install assembly.
4. Define XYZ offsets (assembly alignment per ACE-1 method).
5. Adjust camera parameters using 2.5x objective and B/S cube.
6. Jump to 34°C and hold while stirring for 10min using drill BCAT magnet
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 63x air objective and optimal filter for fluorescence. (If needed for better image quality, use 100x air objective)
9. Survey and record best Z-depth at each primary test location.
10. Ramp temperature slowly to 40°C during 80min (ramp rate 4.5°C/hour) while recording microscope video
11. Note aggregation temperature Ta,i and phase separation temperature Tc,i of cell i
12. Return to 34°C and perform steps 6-11 for the same cell
7. Repeat steps 6-12 for well 2
8. Repeat steps 6-12 for well 3
9. Repeat steps 6-12 for well 4
10. Repeat steps 6-12 for well 5
11. Repeat steps 6-12 for well 6
Total measurement time of Module 1 is 15h.
Complete upon dynamics evaluation by PI (estimate 1 to 1 1/2-day duration) Imaging goal is to resolve particle centroid positions with less than 20% error. Shutter lamp between image sets to prevent sample bleaching.
Module 1: Measure aggregation
1. Ground to choose the first colloid sample; feedback to crew.
2. Install assembly.
3. Adjust camera parameters using 2.5x objective and B/S cube.
4. Jump to Ta,i - 3°C (should be approx. 35°C) and stir for 15min using drill BCAT magnet
5. Survey well at 2.5x; determine primary (and secondary?) test locations (select locations away from stir bar or bubble).
6. Adjust and record best camera parameters using 63x air objective and optimal filter for fluorescence.
(If needed for better image quality, use 100x air objective)
7. Survey and record best Z-depth at each primary test location.
8. Jump to Ta,i – 0.5°C and record microscope video for 105min
9. Repeat steps 4-7, Repeat step 8 with temperature Ta,i
10. Repeat steps 4-7, Repeat step 8 with temperature Ta,i + 0.5°C
11. Repeat steps 4-7, Repeat step 8 with temperature Ta,i + 1°C
12. Repeat steps 4-7, Repeat step 8 with temperature Ta,i + 1.5°C
13. Repeat steps 4-12 for all other wells, i=2,3,4,5, in the module.
Shutter lamp between image sets to prevent sample bleaching.
Total measurement time per well is 10h (2h at each temperature),
Total measurement time per module is 50h.
Module 2-5: Measure aggregation
Repeat steps 1-13 as done for Module 2 for all other modules.
2011 ACE Science Concept Review (SCR) presentations
European Space Agency astronaut Paolo Nespoli operating the Light Microscopy Module microscope aboard the International Space Station on a previous mission. (Credit: 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. (Credit: NASA)
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