1. BCAT-C1 Three-Phase Dynamics and Seeded Crystal Growth
Simon Fraser University
Scitech Instruments, Inc.
Barbara Frisken
Art Bailey
New York University
Andrew Hollingsworth, Paul Chaikin
The BCAT-C1 experiment will explore two types of phenomena in colloid samples: the kinetics of phase separation in the presence of three coexisting phases and the effects of adding seed particles to colloid samples which crystalize.
The first part is undertaken as a collaboration between Art Bailey and Barbara Frisken at Simon Fraser University. The seeded crystal growth samples will be studied by a group from NYU including Andy Hollingsworth and Paul Chaikin.

2. BCAT Apparatus SFU – 7 samples NYU – 3 samples
The BCAT Slow Growth Sample Module is 12.7cm x 26.7cm x 3.2cm and contains 10 (2.3cc) sample cuvettes.
SFU – 7 samples
NYU – 3 samples
The BCAT experiments are based on the principle that it is possible to achieve useful science by simply placing a liquid sample in a glass sample cell and taking high resolution photographs of it with a modern DSLR. The BCAT experiments have a long and productive history and BCAT-C1 will be the 7th time that some form of this experiment has flown. The experiment was originally developed to explore the crystallization of alloys using colloids (more about colloids later) and to support preliminary investigations for the more major Physics of Colloids in Space experiment. In reality, the apparatus is useful for any science in a liquid which entails self-assembly into structures that will eventually reach sizes of about 100 microns.
The apparatus contains 10 quartz cuvettes, each holding about 2.3 ml of sample. A camera is mounted on one side of the rack holding the BCAT apparatus and a flash is mounted on the other. The entire set up can be installed in various places on the ISS. In C1, SFU will use 7/10 samples and NYU 3 samples.

3. BCAT-C1 Science Objectives
Simple statements defining BCAT-C1 experiment
SFU
Study the kinetics of phase separation when gas, solid and liquid phases of a colloidal suspension coexist.
NYU
An experiment to explore the effects of seeding the growth of colloidal crystals.
This slide defines in one sentence per science team the high-level goals of the experiments. The remainder of this presentation / document is devoted to creating an understanding of these statements.

4. States of Matter
Everyone learns the states of matter in grade school. On the left are the states of matter for water. Gas has a low density and expands to fill the volume. Liquid has a higher density, doesn’t expand to fill the volume but remains mobile in that it flows. Finally, solids have the highest density and the least mobility – they don’t flow.
We are more interested in the particle level of these states of matter as shown on the right. Gas fills the volume because the particles have lots of freedom to move about using their thermal energy. The effect of the low density is that the locations of the particles are not defined relative to each other – there’s no correlation in the position of the particles. Liquids have a higher density; so, the particles are less free to move about. The result of this is that the particles tend to be touching to some degree. In this way, the beginnings of order are seen – if there’s one particle in one position, there’s likely to be another right next to it. This is referred as nearest-neighbor correlation. In solids, the density is the highest; to achieve this density, the particles tend to order themselves onto a lattice (3d grid) forming a crystal. The particles are much less free to move now and the order extends much further than just to the nearest neighbors. Crystals have “long-range” order.

5. Phase Transitions
Transition from one state to another
Boiling
Freezing
Sublimation
Demixing
Mixtures – vinaigrette, fuels
Alloys – Al 6061, SS 316
Polymer blends
What are the “mechanisms” of these phase transitions?
The BCAT-C1 experiments entail learning more about phase transitions. Phase transitions are the changes from one state of matter to another. Examples using the states of matter just discussed are boiling, freezing and sublimation.
It has also been proven over the past 75 years that demixing of materials falls into the same fundamental physics as phase transitions. For example, on a fundamental level the mechanisms that determine the separation of oil and vinegar and a vinaigrette are the same as those that occur when the temperature of water is changed causing it to boil. Therefore, any knowledge that increases understanding of any of these processes increases our understanding of a wide variety of technologically important processes.
For example, metal alloys are mixtures of metal elements. Al 6061 contains mostly aluminum but is also mixed with silicon, copper, iron, manganese and several other components. All of this is heated up to a high temperature so that it liquefies and is and mixed completely. It is then cooled and usually annealed or tempered which entails various cycles of heating the metal and cooling. Each time the metal is heated, a bit of de-mixing occurs. For example, the black and white picture shows a metal alloy after processing. The grain structure results from crystalline regions with slightly different composition. The point at which the demixing is stopped determines the material processes. The same is true of other metals such as stainless steel and plastics.

6. Phase Separation in 3-Phase Coexistence
2-phase Separation studied for more than 50 years.
3-phase Separation relatively unstudied
Crystals, Liquid and Gas all forming out of a homogenized mixture
Applications
Properties of polyolefin blended plastics
Mechanical properties determined by the separation / crystallization.
Polypropylene and Polyethylene
Fire retardants, packing materials
Protein Crystallization
The study of phase separation in the presence of two phases has been on-going for more than 50 years starting with the study of metal alloys. The understanding of these mechanisms is highly developed.
It is also possible for gas, liquid and crystal to coexist simultaneously and the process of phase separation in systems in 3-phase coexistence is virtually unexplored. This is due, in part, to the 3-phase samples being very difficult to prepare in materials which are easy to understand such as water or other simple molecular substance.
Regardless, materials are processed everyday in which some degree of 3-phase coexistence is present.
For example processing of polyolefin plastics often entails processing under conditions in which low density (gas-like), higher density (liquid-like) and ordered (crystal-like) phases of the material all coexist. As with metal alloys, the final state of these phases when cooled determines the material properties.
As another example, protein crystallization is performed to understand the structure of complex protein molecules. However, it is quite difficult to crystallize proteins because the crystallization process is slow and requires conditions which are very accurately controlled. One method of creating these conditions is to create solutions which phase separate into protein-rich and protein-poor regions; protein-rich crystals grow from the protein-rich phase. In this way, 3-phases coexist. Many protein crystallization experiments are undertaken in microgravity and some of these may use this technique.
Tetragonal Lyzozyme crystals
lysozyme.co.uk/crystallization.php

7. What’s a Colloid?
Colloids – Particles of size 1 nm – 10 microns. Larger than atomic/molecular size, smaller than ‘macroscopic’.
BCAT-C1 studies phase transitions in colloids. Colloids are particles whose size is >1 nm and < 10 microns. Such particles are bigger than atoms and simple molecules but smaller than anything considered macroscopic. For a physicist, these particles are ones where thermal energy resulting from the materials being at a temperature such as room temperature still generates significant motion.
Colloids do not have to be solid particles in solution. For sample the lower left image is an image of toothpaste which is a microgel particle suspended in a liquid. The lower right image is solid soot in motor oil. The upper right image is of machining coolant which is an emulsion. In an emulsion like this the colloids are microscopic drops of oils surrounded by surfactant (soap) and they are suspended in water. Cleaning oil spills often involves a processes of emulsifying the oil to break it up.
There are also colloids in food such as milk which is an emulsion of milk fat in water.

8. Why Colloids? Lots of technological examples
Colloids model atomic and molecular systems but are simpler to study
High tech examples of colloids include quantum dot colloids which may become important for their fluorescent-like properties.
We choose to experiment with colloids because they are simpler to work with. In an atomic liquid, to reach three phase coexistence the temperature and pressure of the sample would have to be controlled very precisely and such control makes measuring or observing the sample far more complex. At the same time, the science from the behaviour of colloidal substances provides key insights into simpler materials.

9. Why Colloids in Space Sedimentation
Particle a h
Ball bearing
1 mm
10-20 m
Air molecule
1 nm
103 m
PMMA spheres in water
2 mm
particle size, a
gravitational height, h
if h<a,gravity is important
Gravity causes colloids and colloid structures to fall to the bottom of the sample cell!
We study colloids in space to eliminate the sedimentation that occurs on earth.
All particles have thermal energy, kT. They also all fall due to gravity. If the amount of motion created by thermal energy exceeds that created by gravity then gravity can be ignored – in other words, the particle will remain suspended. The energy of a particle to fall a distance h is mgh. If the distance that thermal energy can push the particle relative to the gravitational energy is less than the particle size, the sedimentation occurs.
Sedimentation occurs when
h<a, or a > kT / mg
Consider the three examples in the table. Large macroscopic objects have a small h (because m is large), much smaller than the object. Thermal energy doesn’t have much affect but gravity does. On the other end of the scale, molecules have a large h so thermal energy keeps them suspended. Colloids are marginal. On their own, they will stay suspended for some time, hours to weeks. But, if they group together for some reason, such as to form a crystal, now h becomes smaller and sedimentation occurs.
Such sedimentation disrupts the phase separation process and makes it harder to understand experiments

10. Hard Sphere Colloids PMMA colloid with PHSA coating
All the colloids we use (both NYU and SFU) are prepared as what we refer to as Hard Sphere Colloids
These are PMMA (polymethylmethacrylate essentially acrylic) particles with PHSA (polyhydroxystearic acid) as small polymer hair on the outside.
The hair prevents the particles from sticking together.
This system is the closest to nano-billiard balls that has been created.
All particles are suspended in a mixture of decalin/tetralin liquids. This serves to match the index of refraction – suitable for photography and also substantially reduces the van der waals attractive interactions between the particles, making them more billiard ball like
PMMA colloid with PHSA coating

11. ‘Phase diagram’ of colloidal hard spheres (CDOT-1 & CDOT-2: Space Shuttle Columbia STS 73; Discovery STS 95)
f =
Now we look at how these hard sphere colloids will be used in the experiments that NYU is doing.
Consider the phase diagram shown in the bottom left. This is a map of what phases the sample will form when prepared at different colloid concentrations. The colloid concentration is expressed as “volume fraction”, the portion of the entire sample used up by colloid. At low concentrations, the left end of the line, samples form colloidal liquids. The particles are free to move about but they are dense enough to have nearest neighbor correlations. Above some crystallites form and coexistence of liquid and solid occurs. As the concentration is increased further above the sample becomes completely full of crystals - completely solid.
Above 0.58 it forms a glass – a metastable state where the particles are so packed together that they don’t have enough room to order.
An entire series of these samples was flown on CDOT-2 and one of the fascinating results was that samples that were supposed to be glassy, shown on the lower right, actually crystallized slowly.
f =
absence of glass transition

12. BCAT – 4 absence of glass transition, f = 0.60
NYU samples 8 & 9
Colloidal ‘glass’, no seeds
absence of glass transition, f = 0.60
absence of glass transition , f = 0.60
Our first glassy samples since the CDOT-1 (1995) and 2 (1998) experiment.
This was tested again on BCAT-4 and shown to occur again. Notice the very large U shaped crystals which span the sample cell. It appears that the compaction on earth is sufficient to prevent crystallization in 1-g but remove gravity and the phase behaviour is different. Now these very slowly crystallizing samples serve as an excellent test bed for the BCAT-C1 seeded growth experiments.

13. Adding spherical seeds (particular size relative to the primary particles) is expected to greatly increase the rate at which crystals nucleate. Rs s
Rs/s = 0
Rs/s = 5
10 kBT
Rs/s = 6
Energy barrier
Rs/s = 7
Theorists (Frenkel et al.) have predicted that adding a small number of larger spheres to a supersaturated suspension should cause many different crystals to form at once; this is called heterogeneous nucleation. Note that such a process is common in your kitchen when you boil water and the bubbles first appear at a pit in the pan or you open a soda bottle and the bubble first appear at an imperfection in the glass of the bottle. It turns out that the size of the spherical seed relative to that of the bulk particles is important and will have a direct influence on the crystal nucleation rate. Therefore, the selection of size of crystallization promoter is critical. Microgravity allows the possibility to study this mechanism without the complicating effects of particle sedimentation. In order to implement this experiment, we needed to figure out how to produce PMMA spheres with up to a 14:1 size ratio.
Phase transitions can be impacted by impurities. Take for example, boiling water in a pot on the stove. Notice that points where the boiling first occurs is at rough points in the surface. NYU will study a similar phenomena in the crystallization of colloids by adding seed particles. It has been predicted theoretically that the introduction of seed particles, spherical particles which are substantially larger than the crystallizing colloids will reduce the energy barrier to form crystal, shown schematically on the right. As the seed size Rs is made larger relative to the colloid size sigma, the barrier is reduced.
In a slowly crytallizing sample, this should manifest as a dramatic increase in the rate of crystallization.
Size of nucleus
Cacciuto, Auer, Frenkel, Nature (2004)

14. primary particles s ~ 420 nm
BCAT-C1
SEM image
seed ~5.4 mm
primary particles s ~ 420 nm
In this experiment, the samples are identical in terms of the primary particles (fixed size, volume fraction and polydispersity). Only the seed particle size is varied (0, 5.4 or 6.5 microns diameter), corresponding to a “Frenkel number” of Rs/s = 0, and 7.7.
The NYU samples on C1 will contain particles similar to those shown in the electron microscope image. As summarized in the table, One sample will contain no seed particles and act as a control. The other two will contain minute amounts, < 0.1% of different sizes of seed particles. In this way we should be able to see the amount of crystallization speed increase.
Next slide. 8
Cuvette, Slow Growth Module *8
420 nm PMMA particles with PHS stabilization coating
45% decalin /55% tetralin by volume
60%
40% 1
2.65ml
NYU seeded growth 9
Cuvette, Slow Growth Module *9
5400 nm PMMA seeds with PHS stabilization coating
< 0.1% 10
Cuvette, Slow Growth Module
*10
6500 nm PMMA seeds with PHS stabilization coating

15. BCAT – 5 Rs/s = 5.8 absence of glass transition
NYU Sample 9, ‘SeededGrowth’
0.33 mm
3.8 mm
Rs/s = 5.8
absence of glass transition
Preliminary results: this colloidal ‘glass’ reveals colorful Bragg reflections. The colloidal crystals were nucleated from micron-sized spherical seeds that were added to the dense suspension of smaller particles. These results will provide insight into the question of how small ‘foreign objects’ influence the rate of crystal nucleation. The first observation is that the crystallites appear much different in size and shape as compared with another NYU sample, a monodisperse glassy colloid in BCAT-4. That one also crystallized in microgravity, but without the seed particles.
Effect of spherical seeds on colloidal particle nucleation.
One such sample with a smaller size ratio (less sped up than C1) was flown on BCAT-5.
This preliminary result showed that crystallization occurred much more quickly than any previously flown glassy sample and notice how different the crystals appear. The entire sample is crystalline and there are many crystals oriented in different directions.
So this exciting preliminary results makes us expect good things from the BCAT-C1 results.

16. Attractive Colloids – Depletion Interaction
Range of attraction:
Attraction strength:
__________
x =
Polymer radius Rg
Colloid radius a
Uo ~ Polymer concentration cp
Adding a non-absorbing polymer induces an attractive force between the particles resulting from polymer entropy
Asakura and Oosawa, 1954; Vrij, 1976 a Rg
SFU’s samples are somewhat more complicated. SFU uses the same hard sphere colloids but adds some polystyrene polymers to the suspension.
The polymer adds substantial entropy because the amount of entropy is related to the number of ways that a particle or molecule can move. A spherical colloid can only move in three dimensions and rotate on three axes. A polymer can do all these things but also change it’s configuration in many, many ways.
To maximize the entropy of the system, the polymer wants as much volume as possible. It’s volume is limited not only by the presence of the particles but also the fact that it can’t approach the particles too closely without reducing its entropy. They can’t move closer than their size, known as the radius of gyration Rg. Therefore there is a forbidden region around each colloid.
If the colloids move close to each other, these forbidden regions overlap and the polymer gains volume which is entropically favorable. Therefore, there is an induced attractive interaction between the colloids.
This is very helpful for two reasons. First, the interaction is completely adjustable. The strength is related to the amount of polymer added and the distance over which it acts is related to the size. By changing the size and concentration of the polymer, the interaction is adjusted
Second, in atomic or molecular systems, gas, liquid and crystal can coexist because the atoms or molecules have some interaction with each other – they are not hard spheres.
In this way, the col-pol system is a better model for these other systems.

17. Colloid Polymer Phase Separation
Polymer concentration
When we add the polymer, we obtain a much more complex phase diagram than the simple line shown for hard spheres. That diagram is shown here.
This is a graph of polymer concentration on the y axis and colloid concentration or volume fraction along the x axis.
The hard sphere phase diagram shown earlier is the x-axis – no polymer present. It is difficult to see in this figure, but the same phase behavour is present there.
Look at the rest of the phase diagram. Starting from the origin, bottom left corner, there are no lines here. A sample prepared here is one phase and it is a fluid – possibly gas-like and possibly-liquid like depending on how much colloid is in the sample. Very similar to hard spheres.
If we increase the concentration of both polymer and colloid, we hit the region marked G+L. A sample prepared in this region will phase separate to reach the concentrations at the ends of the straight diagonal lines to form a colloid poor phase – gas, and colloid rich phase – liquid. Note that this is like water at room temperature and 1 atm – coexistance of liquid and gas.
Increasing the concentration of each further until we reach the next region with lines in it, marked G+C, the same thing happens here with coexistence of gas and crystal (like dry ice at room temperature). So a sample prepared here, after mixing, will separate into colloid-poor gas and dense colloid crystal.
Now consider the region shaded gray. If you make a sample in this region and shake it up, the sample will evolve to a final state that contains some gas, some liquid and some crystal. It evolves to form phases whose composition is given by the corners of the triangle. This is 3phase coexistance and is the realm that is largely unstudied in any system.
Colloid volume fraction

18. Physics of Colloids in Space
Colloid Polymer Phase Separation
(EXPPCS)
Polymer concentration
Before more 3-phase info, look at the G+L region to understand something about the phase separation there.
This was looked at by Art Bailey working with Dave Weitz and others at Harvard as part of PCS. That apparatus allowed both scattering and imaging.
One sample flown was made to be in the position circled on the phase diagram and some of the imaging results are shown in the series of photos on the right. The sample is contained in a 2 cm diameter cell.
Note the times. After mixing the sample, it was completely uniform. After 4hrs it starts to appear grainy as an interconnected domain network forms. The domains coarsen over time. These are the domains of gas and liquid. The gas is the dark regions which don’t scatter much illumination and the liquid is the lighter regions.
We analyzed this behaviour and were able to demonstrate that this system of colloids and polymers behaves in this region exactly like any other liquid or mixture system with the important difference that the single size that governs the process is set by the colloid size. No other complications due to the partitioning of the polymer were important.
Physics of Colloids in Space
Bailey, Weitz et al
Colloid volume fraction

19. Complexity of Phase Dynamics
Polymer concentration
PCS
Turning to the 3-phase region.
Ground-based experiments by W Poon and his group have shown already that there is a variety of different things that can happen depending on where the sample is located within the phase diagram. This is very complicated and only covered superficially here.
On the right are two sets of photographs taken as a time series, increasing time from left to right after mixing. Total time span in these photos is about 60 hrs.
The top set of photographs shows what happens near the middle of the triangle and the lower is a sample prepared at a higher colloid concentration. In the top one you see
Initially mixed, then looking closely, some thing is forming dark regions and falling, then a meniscus settling and becoming sharp. Then in the second to last image, dark regions form and then a second meniscus forms.
In the lower sample, again left to right, the sample start uniform and initially gets dark, but then light. Look close at the third and there are bright spots – crystals. In the fourth, back to dark and there are two meniscus which over the remaining two time intervals change somewhat.
Clearly these are different. Also, it is clear that much of what we are seeing is gravity driven, meniscus forming crystals falling through the sample. BCAT-C1 we study this in more detail, with substantially better imaging and no gravity.
F. Renth, W.C.K. Poon and R.M.L. Evans, PRE 64, (2001)
Colloid volume fraction

20. Results from BCAT-5
PCS
We had the opportunity to fly three 3-phase samples on BCAT-5 and present some of the results in this slide.
One such sample was prepared in a region close to the upper of the two previous images. The equivalent images are shown along the bottom of the slide.
As with the gas-liquid PCS results, we see the interconnected domain network form within a short time after mixing. By 7 hrs we see the first crystals, and although not evident in these slides, the crystals are moving. The process continues up to about 19 hours until it simply stops. After this point, the network pattern doesn’t change – it arrests. Other images taken over longer periods of time, up to a year later, indicate this is the final state of the sample. This is completely different from what’s seen on earth.
We believe that we are looking at a crystaline network that has formed in the liquid phase and spans the cell. Because a structure spanning a volume is referred to as a gel, we refer to this state as a crystal gel. To our knowledge it has never been seen before in any material.

21. Phase Separation + Crystal Growth
mg - imaging
This slide addresses the question of what we measure from these results.
We analyze the photos to determine the domain spacing and the size of the crystals. The results from one BCAT-5 sample are shown on the left as size vs time. The arrest of the size is clear.
We then combine this with scattering measurements we can do on earth in the time domain where sedimentation is not so important.
The graph on the right is an example of such a combination. The dashed line is a universal curve seen in many systems of demixing or G+L phase separation. In this region of the phase diagram the 3-phase mechanism appears to be initially the same as other systems, until the sudden arrest and formation of this new material .
1g - Scattering

22. BCAT C1 SFU Samples Polymer concentration PCS Colloid volume fraction
Moving into BCAT-C1 we intend to extend these results and explore a wider variety of possible phase separation mechanisms by looking at different locations across the three-phase triangle.
This is shown schematically in phase diagram by the positions of the red triangles.
BCAT C1 SFU Samples

23. Conclusion
BCAT-C1 will result in exciting new colloid science that cannot be observed without microgravity.
In conclusion, we have already seen fascinating phenomena on ISS in these systems. As we move forward with BCAT-C1 we expect to see some exciting new results which we hope will be just as surprising as our earlier results.