Bill Wagenborg MISEP 2 Capstone Project Summer 2008 Space for Growth: Colloidal Crystallization in Microgravity Overview My capstone project has been an interesting and fulfilling journey. I produced a high quality informative piece on colloids, while discovering what is involved in the formulation of a true scientific content paper. Then I used this knowledge to create a full detailed unit plan around microgravity that would be beneficial to middle school students. Most of all, I was able to take away the experience of working with my content reader, who was a true expert in his field. Through his knowledge I learned about an area in science that in reality I did not know existed before I began this process, but now view it with the utmost respect. Ever since I was a child I have had a fascination with outer space. I have often wondered what it would be like to travel there and to experience the effects of microgravity. This, combined with the fact that my students always had a lot of questions around the subject, helped me to choose in the fall of 2007 my original capstone project topic, The Effects of Space Travel on The Human Body. There was one problem though; a content reader could not be found for this topic. Finally in the spring of 2008, I was notified that Dr. Arjun Yodh, The James M. Skinner Professor of Science in the Physics and Astronomy Department of the University of Pennsylvania, would be interested in working with me. Dr. Yodh has done research and experiments revolving around colloids and their behavior in microgravity. He currently has experiments waiting to be done on future space missions. I could not pass up the opportunity to work with someone so knowledgeable and respected, so I decided to change my topic to Colloidal Crystallization in Microgravity. Colloidal suspensions are not something that I have had a lot of experience with as a student or in my teaching career. In fact, colloids are covered in one paragraph in the
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Bill Wagenborg
MISEP 2
Capstone Project
Summer 2008
Space for Growth: Colloidal Crystallization in Microgravity
Overview My capstone project has been an interesting and fulfilling journey. I produced a
high quality informative piece on colloids, while discovering what is involved in the
formulation of a true scientific content paper. Then I used this knowledge to create a full
detailed unit plan around microgravity that would be beneficial to middle school students.
Most of all, I was able to take away the experience of working with my content reader,
who was a true expert in his field. Through his knowledge I learned about an area in
science that in reality I did not know existed before I began this process, but now view it
with the utmost respect.
Ever since I was a child I have had a fascination with outer space. I have often
wondered what it would be like to travel there and to experience the effects of
microgravity. This, combined with the fact that my students always had a lot of questions
around the subject, helped me to choose in the fall of 2007 my original capstone project
topic, The Effects of Space Travel on The Human Body. There was one problem though;
a content reader could not be found for this topic. Finally in the spring of 2008, I was
notified that Dr. Arjun Yodh, The James M. Skinner Professor of Science in the Physics
and Astronomy Department of the University of Pennsylvania, would be interested in
working with me. Dr. Yodh has done research and experiments revolving around
colloids and their behavior in microgravity. He currently has experiments waiting to be
done on future space missions. I could not pass up the opportunity to work with someone
so knowledgeable and respected, so I decided to change my topic to Colloidal
Crystallization in Microgravity.
Colloidal suspensions are not something that I have had a lot of experience with
as a student or in my teaching career. In fact, colloids are covered in one paragraph in the
Wagenborg 2
textbook I use for teaching matter to my eighth grade students. Through Dr. Yodh’s
guidance, I was able to develop an understanding of colloidal suspensions and gain an
appreciation for microgravity as a science. I am now able to explain the importance of
colloidal science in our world and how it will play an important part in our future.
My content piece was developed in a way that will enable the reader to gain a
solid understanding of colloids and their behavior in microgravity. It is written in two
parts: colloidal behavior in gravity and colloidal behavior in microgravity. I begin by
explaining what a colloidal suspension is and the common examples of them in our
world. I then discuss the history of colloidal studies and why investigation of them is an
important part of science. The interaction and phase change of colloidal particles on Earth
is the next part of my paper. This section is crucial because it will serve as a comparison
when discussing microgravity. This entire first half of my paper has led up to my
discussion of colloidal behavior in microgravity. I begin this second part by explaining
what microgravity is and how it is created. The focus then shifts to recent and current
colloidal exploration in space. I conclude my content piece by discussing the impact of
the research done on colloidal crystallization in microgravity.
My pedagogy section is based on the backwards design model by Wiggins and
McTighe (2005). In this model, the educator looks at what the desired results are of the
unit and then decides how they are going to have the students achieve them. Colloidal
crystallization is a much higher level of content than I would teach to my middle school
students. I wanted to keep the microgravity theme because I teach astronomy and I know
most middle school students enjoy the topic. My unit revolves around the students
understanding how microgravity is created, how it affects the behavior of different
objects and how models allow us to study certain phenomena that are impossible to
recreate. My activities are hands on, follow an inquiry format and are relevant to today’s
world.
Completed during the summer of 2008, I present my capstone project.
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Space for Growth: Colloidal Crystallization in Microgravity
What is a Colloidal Suspension?
Colloid suspensions are composed of small solid particles, dispersed and
suspended in a liquid. (Cheng, et. al, 2001). Thomas Graham coined the term in 1861,
which in Greek means glue, based on his observations of the low diffusion rate of
particles in suspension (Antonietti, 2008). Typically the suspended particles are large
enough to scatter light yet cannot be separated by coarse filtration (Holt, 2004). As shown
in Figures 1 and 2, colloidal particles are about 1/100 the thickness of a human hair
(ranging from 50nm to 5um) and can be found in many common materials such as paints
and inks. Mayonnaise and milk are colloidal materials too; in this case where the particles
are made of another liquid, oil, and are suspended in a liquid. Smoke is also a kind of
colloidal dispersion where solid soot particles are suspended in air. Colloidal suspensions
differ from traditional solutions such as water-sugar mixtures in which sugar molecules
are actually dissolved in water (Pellis & North, 2004).
(1) (2)
Figure 1: The Left Image shows an electron micrograph of an aggregate of several particles.
Figure 2: The Right Image is an optical microscope image of many micron-sized colloidal particles that
form a colloidal glass; the black cylinder is a magnetic probe moves within the colloidal glass.
Wagenborg 4
Colloidal Studies
Colloidal Science crosses over physics, biology, chemistry and other fields in
science (Hiemnz & Rajagopalan, 1997, 2). Investigation and experimentation with
colloids traces its history back to the late nineteenth century. Brownian motion was
discovered around this time. Brownian motion refers to the ability of micron size
particles dispersed in a liquid to be in a constant state of random motion. Back then
people were puzzled by how this could happen, if the particles were not living. This was
also about the time when the existence of molecules was being hotly debated. In
Einstein’s Brownian motion theory paper he noted, “according to the molecular –kinetic
theory of heat, bodies of microscopically size suspended in a liquid will perform
movements of such magnitude that they can be observed in a microscope” (Einstein,
1905, 1). Thus, the independent ‘random’ motion is caused by the molecular nature of
matter and is not substantially affected by composition or density of the particle. Jean
Perrin carried out detailed experiments on Brownian motion confirming Einstein’s
theories and concluded that smaller particles, less viscous fluids and higher temperatures
increase the amplitude of the colloidal particle motions (Russel, 1989, 65).
Although colloids found many useful applications, fundamental interest in colloid
science waned by World War II. This situation, however, changed starting in the decade
of the 1960’s as new problems and experimental techniques emerged. These
developments along with an increased understanding of fluid mechanics and the
availability of diversity of monodisperse model suspensions led to new uses and
experimentation with colloids (Russel, et. al, 1989, xii).
Colloidal Suspensions as a Macroscopic Model of Atoms
All materials are made up of atoms, and therefore it is desirable to understand
material structure at the atomic level. In principle, material properties (e.g. weight, color,
density, mechanical strength, conductivity etc.) depend on how the atoms of the material
are arranged and how they interact. Colloidal suspensions can provide scientists with a
macroscopic model for this atomic behavior, where the particles play the role of the
atoms. By understanding the behavior of colloidal crystals, for example, scientists gain
Wagenborg 5
insight into basic solid state and condensed matter physics and also learn how to create
new materials through “Colloidal Engineering” (Cheng et.al, 2001).
This paper will focus on the model systems. Colloidal particles in suspensions
move around, interact with one another and are acted on by thermal forces in equilibrium.
These thermal forces exerted by the fluid on the particles are responsible for Brownian
motion. Left alone, the suspensions evolve towards their lowest free energy state (van
Blaaderen & Wiltzius, 1997). If the particles interact like hard spheres (i.e. no force acts
on them unless they touch), then in equilibrium they arrange themselves in such a way
that each particle has the maximum amount of space or free volume to move (Freeman-
Hathaway, 2002). By studying colloidal crystals, we can gain knowledge about how and
why the particles self assemble, about the relation of this self organization to the forces
between particles in suspension and about many body effects. The model colloids can be
tracked by video microscopy which is impossible for atomic structures (Velev, et. al,
2000).
Colloidal suspensions differ from atomic structures in three major ways. The first
is that since the solvent fixes the sample volume, crystallization occurs at a fixed volume
rather than a fixed pressure. Secondly, since energy and momentum are exchanged
between the particles and the solvent, “only particle conservation should govern the large
time dynamics of the system” (Cheng et. al, 2001, 4146). Thirdly, any latent heat that is
created is not important because of the small number of particles and the quick energy
exchanges with the solvent (Cheng et. al, 2001).
The Interactions and Phases of Colloidal Particles in Suspension
At the beginning of the twentieth century, Perrin came to the conclusion that the
particles in a dilute colloidal suspension behaved like an ideal gas (Lekkerkerker &
Stroobants, 1998). At higher concentrations of particles, the manner in which the
colloidal particles interact, their stability and their phase behaviors can be changed
through the manipulation of their composition and the composition of the solvent. Some
of the forces that arise between particles in suspension are van der Waals attractions
(dispersion forces), electrostatic (Coulomb) repulsion and attraction, hard sphere
repulsion and entropic forces; other forces acting on the particles can be gravity or
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applied electronic and magnetic fields (Russel, 1989). Colloidal crystals are classified as
a soft condensed matter because of their low elastic constraints; they are easily deformed
by interaction with applied force fields such as shear (van Blaaderen et. al, 1997).
Van der Waals forces, as shown in Figure 3, are created by molecular interactions
due to the permanent polarity in the molecules created by the electric fields of other
molecules. These forces generally cause the particles to attract to one another so that they
will sometimes form clusters and aggregate as a result. These aggregates “retain their
identity but lose their kinetic independence” (Hiemnz& Rajagopalan, 1997, 465).
Aggregation demonstrates an attraction between the particles (Hiemnz & Rajagopalan,
1997).
(3)
Figure 3: Negatively charged colloidal particles in suspensions containing positive counter-ions. The
fluctuating polarity of each particle causes an attraction which is the van der Waals force.
Electrostatic repulsion in a colloidal system is responsible for, among other
things, the long shelf life of latex paint (Russel, 1989, 88). This force can be affected by
the addition or subtraction of ions in the suspension (Russell, 1989, 1). According to the
theory of Derjaguin, Landay, Vervey and Overbeek (DLVO), “isolated particle pairs of
like charged (i.e. both positively or both negatively charged) colloidal spheres in an
electrolyte should interact in a “purely repulsion screened electrostatic (Coulombic)”
force (Bowen and Sharif, 1998, 663). Colloidal stability, according to this theory, is
based on a balance of van der Waals attractive forces and repulsive electrical double layer
Wagenborg 7
forces. Some recent experiments have found that this theory works for low concentrations
of particles but may not work for higher concentrations (Bostrom et.al, 2001).
Sometimes particles interact like hard spheres. Hard spheres have no interaction
energy when they are apart, but have an infinite amount when they are touching. Thus
they behave like marbles or billiard balls at the microscopic scale. These suspensions can
be produced if van der Waals attractions are reduced because of the refractive index
matching solvent and when steric stabilization is caused by a thin layer of polymer
attached to the surface of the spheres (Cheng et. al, 2001).
Even in this very simple system of colloidal hard spheres, as the particle
concentration is increased, the particles will arrange themselves into different structures.
At lowest concentrations, gases or fluids of particles form. At higher concentrations, a
fluid coexists with a crystal, and when concentrations are at a volume fraction well above
50%, a fully crystallized sample or a semi crystallized glassy appearance forms (Pusey,
1986). This data is represented in Figure 4.
In early experiments on Earth, samples that were the most diluted (i.e. volume
content fraction below 0.494) exhibited no signs of change over time. It was theorized
that at this “concentration the particles are spatially arranged like atoms in a dense liquid,
exhibiting considerable short range positional ordering” (Pusey, 1986). Samples with
higher concentration levels (i.e. volume fraction content between 0.494 and 0.545)
produced a coexistence of fluid and crystal. At even higher concentrations (i.e. volume
content fraction above 0.545 or above the volume fraction of melting) crystals form
(Yodh, 2007). The structure of these crystals was determined to be a combination of face
centered cubic and hexagonally close packed planes (Zhu et. al, 1997). Samples produced
on Earth that were the most concentrated (i.e. volume content fraction close to 0.637)
exhibited only partial crystallization. This was still the case after the samples were left
undisturbed for several months. Sometimes the particles were described to be arranged as
in a disordered glass (i.e. colloidal glass). It was believed that the high concentration
caused problems with particle diffusion, which resulted in the particles not crystallizing
on the experimental timescale (Pusey, 1986). There is also competition with equilibrium
processes due to particle sedimentation. When concentration level is at a volume content
Wagenborg 8
fraction of around 0.74 the face cubic centered crystal structure has the lowest free
energy and should form (Zhu et. al, 1997). These results are shown in Figure 5.
(4)
Figure 4: Predicted phase behavior of particles as a function of volume fraction. Notice the
suspension changes from a liquid to a solid with a very small change in particle concentration (i.e.
volume fraction).
(5)
Figure 5: Colloidal Hard Spheres. From left to right as particle concentration increases,
phases change from liquid to a coexistence of liquid and crystal to crystal alone to a glassy
appearance. Bright colors indicate that a crystal has formed; the color reflected depends on the
spacing of the planes of the colloidal crystal.
Wagenborg 9
In the hard sphere model, colloidal crystallization is driven by entropy alone and
constrained by the number of packings possible at high densities (Cheng et. al., 2001).
Entropy is the driving force for disorder in nature (Eldridge, et.al, 1993). According to
the Second Law of Thermodynamics, “any spontaneous change in a closed system results
in an increase of entropy” (Frenkel, 1999, 26). In equilibrium all physical systems will try
to minimize their free energy, F= E (or sometimes called U)-TS, where E (or U) is the
sample internal energy, T is the temperature and S is the sample entropy (Yodh, 2007)
(Frenkel, 1999). Using this formula a system (at constant temperature) can lower its free
energy by increasing the entropy or decreasing internal energy. Stable phases are those
with the lowest free energy (F). When a phase change takes place, at a given particle
concentration and temperature, from a fluid (disordered) to a solid (order), for example,
loss of entropy can be offset by the greater change of internal energy. This type of
transition is internal energy driven and is common in many atomic and molecular
materials (see Figure 6). It may be more beneficial to study the hard sphere systems since
they expose the effects of entropy alone and thus test our understanding of basic
statistical physics.
(6)
Figure 6: Colloidal Hard Spheres. From left to right as particle concentration increases,
phases change from liquid to a coexistence of liquid and crystal to crystal alone to a glassy
appearance. Bright colors indicate that a crystal has formed; the color reflected depends on the
spacing of the planes of the colloidal crystal.
Wagenborg 10
Recently, research has shown convincingly that in the hard sphere colloidal
systems, the phase changes are in fact entropy driven (See Figure 7for hard sphere
interaction potential). If the crystallization occurs at constant density, entropy may be
higher in the solid phase than in the corresponding disordered phase. In this case, the
particles in crystals are packed more efficiently and in turn have more room to move (or
more free volume) (Frenkel, 2006).
(7)
Figure 7: Left panel is interparticle potential for hard spheres (zero when apart, infinite
when touching). In this system the system free energy is dependent on entropy alone
Another example of entropic forces in action arises in suspensions of large and
small diameter hard spheres. In this mixture of different sized particles, “an ordered
arrangement of large spheres can increase the total entropy of the system by increasing
the entropy of the small spheres” (Yodh, 2006). For the smaller spheres entropy is
dependent on the number of positions it can occupy in the mixture (or the free volume per
small particle). The more positions a small sphere can occupy in a container, the more
free volume and the more entropy it will have. Since the smaller spheres cannot penetrate
close to the large spheres, there is a forbidden boundary around each of the hard spheres.
When the large hard spheres move closer to each other, this forbidden boundary overlaps
which creates more free volume for the small spheres in the container (see Figure 8).
Thus, the entropy of the entire colloidal suspension is increased by the ‘ordering’ of the
larger spheres. The entropic interaction between large spheres due to the presence of
small spheres is also known as attractive depletion (Yodh, 2006).
Wagenborg 11
(8)
Figure 8: This figure shows how the smaller spheres have more free volume when the large
spheres overlap. The blue regions correspond to ‘positions’ where the small particles cannot go. The red
regions correspond to the gain in free volume experienced by the small particles when the spheres touch
each other or the wall. In this case the entropy has increased for the entire suspension
Effects of Gravity
Colloidal particles are much larger than atoms and the bonds between them are
relatively weak. For these reasons gravity can play a prominent role in affecting the
structure and formation of colloidal crystals (Zhu et. al., 1997). The force of gravity can
influence the way colloids interact and lead to colloidal crystallization. For example,
gravity causes sedimentation to occur. This is when the colloid particles settle towards
the bottom of the sample cell and because of the increasing density (the lower boundary
wall triggers layering in the liquid), crystallization and crystal growth can take place. The
results of this are pictured in Figure 9.
(9)
Figure 9: Sedimentation of colloidal particles at the bottom of the container due to gravity
In the sedimentation process in a strong gravitational field, a liquid layer will
originate at the bottom before a crystal. These first two layers of this liquid layer will
Wagenborg 12
“undergo a first order transition with increased gravitational strength” (Hoogenboom et.
al., 2003, 3372). When the sediment has a high Peclet number (the ratio between the
gravitational force and the thermal energy) it resembles a 2-D system and a monolayer of
crystals is formed because the Brownian motion is small. When the Peclet number is low,
the crystals grow epitaxially. The relationship between the sediment and the crystals
formed within the sediment can lead to a determination of whether crystalline sediment
or amorphous sediment will be produced (Hoogenboom et. al., 2003). Gravity has also
been found to accelerate aging in ‘glassy’ systems. This reduces both the time in which
crystal nucleation can take place and the glass transition density Simeonova & Kegal,
2004).
Microgravity
Scientists use the term microgravity to mean very little gravity. “Micro” is a
prefix used in science to mean one millionth and in the space shuttle gravity is reduced by
a factor of 1/1,000,000 that we feel on Earth. On Earth, the acceleration of an object that
is falling to the ground (due to gravity alone) is described as having normal gravity or 1 g
(Zona, 2006). This rate of acceleration is 9.8 m/s 2 .
Sir Isaac Newton first studied these phenomena nearly three hundred years ago
with his ‘falling apple’ dilemma. His work led to Newton’s Law of Universal
Gravitation, which is an understanding that gravity exists between any two objects in the
universe that have mass. The strength of this gravitational force was affected by the mass
of the objects and the distance between them, i.e. F ∝
m m
r
1 2
2 , where F is the gravitational
force between the objects, m stands for mass and r is the distance between the centers of
the two objects. This formula shows that as the distance of the objects increases, the force
between them decreases, but it will never reach zero (Walls, 1997).
Many people mistakenly think that there is no gravity outside the Earth’s
atmosphere, and this is the reason given for why astronauts float onboard the shuttle. The
shuttle and its passengers are acted on by the Earth’s gravitational force (and to some
extent the moon), which helps explain why the ship continues to orbit the planet. The
space shuttle (and other space crafts), however, is about 500 km away from the surface of
the Earth. According to the Law of Universal Gravitation, this increased distance away
Wagenborg 13
from the surface of the Earth will reduce the gravity enormously on the space shuttle (i.e.
to microgravity levels) (Walls, 1997).
Colloidal Experiments in Microgravity
According to NASA, microgravity is the newest science because it was created
with the dawn of the space age. The first microgravity experiments were done almost
forty years ago, the space shuttle has been performing tests for the last thirty years and
the International Space Station has just begun to do the same. Space, because of its small
amount of gravity, is an ideal setting to carry out experiments that could not be performed
anywhere else. In principle, microgravity can be recreated on Earth (NASA’s C-9 low g
flight research aircraft and other zero gravity facilities) but the length of time of this
microgravity is too short for the purposes of the research on equilibrium phenomena.
Objects behave differently in this setting and colloids are no exception (Horack, 1997).
In the 1990’s Paul Chaikin and William Russel, then at Princeton University, set
out to learn how colloidal materials reach a state of equilibrium in microgravity. Their
hope was to be able to eventually influence the process and create objects with
controllable properties. Colloidal suspensions on Earth provide an insight into atomic
structures, but gravity restricts more thorough insights. Weight causes the crystals to
settle at the bottom of the container. This creates more of a concentration at the bottom
than at the top and makes it impossible to observe the sample in equilibrium. They
determined that microgravity would prevent the sedimentation from occurring and also
stop convection (swirling) of the fluid that was a result of the movement of the hard
spheres (Freeman-Hathaway, 2002).
In October 1995, Chaikin and Russel sent their first experiments into space
onboard STS-73. Labeled the Colloidal Disorder-Order Transition (CDOT), the objective
was to see what effect microgravity would have on the crystals in equilibrium (Freeman-
Hathaway, 2002). This mission produced many different results.
First, the crystals grown in microgravity showed the random stacking of
hexagonally close-packed planes (r.h.c.p.) only. On Earth, the crystals showed a
combination of r.h.c.p. and face centered cubic (f.c.c.) packing when given time to settle.
This led to a theory that gravity induced stress may be responsible for the f.c.c. packing.
Wagenborg 14
Next, the crystals in space displayed dendrite growth (Figure 10). Dendrites are fragile
snow-flake like structures that form around the crystal. They sometimes occur in metal
alloys (atomic systems) and are very important in technology (Cheng et. al, 2002). It was
theorized that this growth is part of the normal process, but on Earth the stress of gravity
causes them to shear off as they settle on crystals. On Earth as a crystal grows its mass
causes it to sink quickly. When the viscous stress of the fluid becomes greater than the
stress that the crystal can withstand, the crystal breaks. Hence the dendrites are sheered
off. Lastly, samples that had high volume content fractions on Earth failed to crystallize
completely and looked instead like a glassy substance even after a full year. In
microgravity these same samples crystallized within two weeks (Figure 11). This led the
researchers to conclude that gravity “masks or alters” parts of the crystallization process
(Zhu et. al., 1997, 885). When the shuttle re-entered the earth’s atmosphere and landed,
most of the crystals were destroyed due to their fragile state, but those glass samples that
crystallized in space survived the landing due to the fact that they were already at high
concentration s (Freeman-Hathaway, 2002).
(10) (11)
Figure 10: Left: CDOT results: dendritic colloidal crystal growth, not seen on the Earth.
Figure 11: Crystallization of high volume content samples that were ‘glassy’ and failed to crystallize on
Earth.
The CDOT project was a good start but clearer images were needed from the hard
sphere crystals. Chaikin and Russel set about to develop a device that would show the
Wagenborg 15
structure of the crystals and the creation (nucleation) and growth of crystals. Their
objective was to find out how elastic they were at nucleation and how long it took to
grow (Freeman-Hathaway, 2002). This next phase was known as the Physics of Hard
Spheres Experiment. It was a “series of light scattering experiments on colloids”
conducted on STS-83 and STS-94 (Cheng, et.al.2001, 4146).
These experiments produced more information regarding colloidal crystallization.
Contrary to what was theorized in CDOT, the f.c.c. structure was determined to be the
equilibrium-stable structure for hard sphere crystals. In all of the samples (liquid/crystals,
crystals, glass crystals) the researchers observed the growth of the f.c.c. structure and saw
it sooner when the volume content fraction was increased (Cheng, et.al.2001).
Larger crystals, but smaller in number, (compared to those on Earth) were found
to have grown (See Figure 12). The PHaSE experiments also brought to light that the
competition process of larger crystal growth happens earlier than expected. It was
discovered that nucleation takes place at a variety of locations in the fluid. Some crystals
begin to grow before others; hence some are large when others are just starting. These
larger crystals will eat away at the smaller crystals until there is on large crystal.
Colloidal suspensions are not the only area where this takes place. Another example is
found by breathing on glass. When someone breathes on glass, the vapor condenses into
water droplets and the larger ones grow into one large droplet. This process is known as
simultaneous coarsening and growth (Cheng et. al, 2002, 015501-3). This again showed
how gravity changes the nature of crystallization in their growth and coarsening
processes. On Earth sedimentation will not only limit the growth of the crystals but also
affect the future interactions of the crystals that are moved in the liquid through diffusion
(Cheng et. al., 2001).
(12)
Figure 12: PHaSE results: Larger colloidal crystals grew with the f.c.c. structure
Wagenborg 16
Research then shifted to the International Space Station’s Destiny Lab. This lab,
initialized in 2001, contains a pressured space platform that allows for long term
exposure to microgravity. The Physics of Colloids in Space (PCS), headed by Professor
David Weitz, took place from June 2001 through February 2002. The focus was on:
binary colloidal crystal alloys (suspensions of particles of two different sizes), colloid-
polymer mixtures, where a “mono-disperse particle mixed with a mono-disperse polymer
in an index-matching fluid where 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” (Pellis & North, 2004,595), and fractal gels (colloids with repeating structural
patterns and networks) (Doherty&Sankaran,2002).The binary colloidal crystal produce a
power law growth (still under investigation), and showed more peaks in the powder
pattern than had been seen on Earth This showed gravity’s affect on the size and
morphology of the crystals and gels (See Figure 13). The colloid-polymer samples
produced samples showing two regions: one colloid rich, one colloid poor (spinodal
decomposition). The fractal gels grew crystals much larger than those on Earth, as gravity
would have caused them to be crushed (Doherty & Sankaran, 2002) (Weitz, 2002).
(13)
Figure 13: PCS results: binary colloidal crystal growth in microgravity
Wagenborg 17
Future Experiments and Research
Future colloidal research in microgravity will revolve around the use of the Light
Microscopy Module (LMM). This device will allow fluid and biology experiments within
the Fluids and Combustion Facility (FCF) Fluids Integration Rack (FIR) on the ISS. The
three experiments that will utilize the LMM will be The Physics of Hard Spheres-2
(Chaikin), The Physics of Colloids in Space-2 (Weitz) and The Low Volume Fraction
Entropically Driven Colloidal Assembly (Dr. Arjun Yodh of the University of
Pennsylvania). These experiments will focus on the “nucleation, growth, structure and
properties of colloidal crystals in microgravity and the effects of micromanipulation upon
their properties” (Motil & Snead, 2002, 5). The confocal microscopy piece of the LMM
will allow these three experiments to observe the interior of the colloidal structures,
which would result in three dimensional models (Motil & Snead, 2002)
The PHaSE-2 ‘s goals are to “observe the effects of hard spheres parametric
conditions on the equilibrium phase diagram and how colloidal systems respond to
applied fields” (Motil & Snead,2002,6). The LMM will give the researchers the ability to
observe the position of the particles and allow them to make determinations regarding the
behavior of these particles. The microscope will be accompanied by a set of laser
tweezers. These tweezers, which are composed of tightly beams of laser light, will allow
the researchers to draw the particles into the light beam. The particles would be grabbed
and brought together with the intention of building nuclei. The scientists would have
control over the formation of the crystals. They would be able to see the growth process
step by step in order to see the reasons certain crystals form and how they could
manipulate the crystals to grow in various states (i.e. non equilibrium) (Freeman-
Hathaway,2002).
The goals of the PCS-2 are to “carry out further investigation of critical,
fundamental problems in colloid science and to create materials with novel properties
using colloidal properties as precursors” (Motil & Snead, 2002, 7). These experiments
will use binary alloys and mixtures of colloidal particles with polymers. The polymers are
meant to create a controllable force of attraction, i.e. the depletion attraction between the
colloidal particles. This force will stimulate the creation of new structures and initiate
phases in the suspension. All of this will be measured using the LMM. The objects will
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be able to be viewed in real space and the tweezers will be used to manipulate the
structures (Motil & Snead, 2002, 7).
The goals of the Low Volume Fraction Entropically Driven Assembly Experiment
are to “create new colloidal crystalline materials, study the assembly of these materials,
measure their optical properties, and then solidify the resulting structures so they can be
brought back and studied on Earth” (Motil & Snead,2002,7). The samples that will be
used in this experiment include colloidal particles suspended in water and other organic
fluids. The LMM will be used to observe the growth of the crystals. After the images are
acquired they will be stored in order to determine crystal structure and quality.
Sedimentation makes it impossible for these structures to be created on Earth. If the
structures are able to survive re-entry and landing, their optical, magnetic and electrical
properties will be studied more extensively (Motil & Snead, 2002).
Possible Applications of These Experiments
Colloidal Crystallization in microgravity research could impact our world in a
variety of ways. Photonic materials such as ultra low-noise light sources, switches and
computers using light instead of electricity could be produced (Motil & Snead, 2002).
The manipulation of the particles could lead to structures with precise spacing that would
improve the control of light, necessary for better long distance telephone communications
(Freeman-Hathaway, 2002). Knowledge gained also could lead to better ways to use
carbon dioxide for food extractions, more efficient prescription drug processing, and
creating stronger ceramics or even better dry cleaning (Pellis & North, 2004).
Acknowledgement: I would like to thank Dr. Arjun Yodh, the James M. Skinner
Professor of Science at the University of Pennsylvania, for his expertise, his guidance and
most of all his time, which allowed me to complete this content section of my capstone
project.
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References
Antonietti, M. (2008). What are colloids? Max Planck Institute of Colloids and