What researchers have learned about hard sphere colloidal crystals from experiments conducted in space is leading them to ways to control crystal growth for the creation of novel structures, one particle at a time.

But atoms are small and very difficult to study. What researchers need is a system that models the behavior of atoms on a larger scale. Colloidal suspensions provided Chaikin and Russel with just such a model. Colloids are a category of complex fluids consisting of micron-sized particles suspended in a liquid or gas. Chaikin and Russel's model system consists of hard plastic spheres suspended in a liquid. Like atoms, the suspended solid particles move around, bump into one another, and settle into positions where the forces acting on the particles are in balance or equilibrium.

Right: Like atoms but on a larger scale, small particles in a colloidal solution assemble to form an ordered crystalline structure, such as the 1-micron, opalescent crystalline particles shown here. Paul Chaikin and William Russel study how the particles interact to learn how - and perhaps eventually to influence how - atoms reach a state of equilibrium.

This balancing of forces is what gives a solid material its structure, allowing it to keep its shape even though the atoms of which it is composed are in constant motion. At equilibrium, each particle has positioned itself in a way that provides the maximum elbow room between it and its nearest neighbors. This happens when particles bump into each other and are forced into a sort of pocket of space surrounded by the other particles. Atoms prefer to maintain a certain distance from their neighbors, but due to the presence of other atoms in close proximity, the best they can do, Chaikin explains, is for each one of them to have pockets of exactly the same shape around them. These equally sized pockets, one right after the other, form an ordered, or crystalline, structure.

While studies focusing on the structure of a material in equilibrium are useful to researchers for understanding the properties of a substance, by learning how a material reaches this state, Chaikin and Russel hope to be able to influence the process and eventually engineer useful structures with predictable, controllable properties.

Losing Weight

Although their colloidal model allows free movement of the spheres, Chaikin explains that it has a serious limitation: "Weight is a factor and causes the spheres to settle to the bottom of the container." This results in a greater density of spheres at the bottom of the container than at the top. The concentration gradient makes it impossible to study a uniform sample in equilibrium. The problem was how to study these hard spheres without the interference of gravity. The researchers determined that studying the colloid samples in microgravity would not only eliminate sedimentation, it would also reduce the amount of swirling that takes place in the fluid as the hard spheres move. "This swirl-ing motion, called convection, also puts the system out of equilibrium," says Chaikin. "Microgravity gets rid of that motion too."

Chaikin and Russel first flew their colloid samples in microgravity onboard space shuttle flight STS-73, which launched in October 1995. For this experiment, called the Colloidal Disorder-Order Transition (CDOT), the researchers wanted to see what the crystal structure would be when a colloid sample reached equilibrium in microgravity. "We were expecting to see that the samples would form nice big crystals of a particular type," Chaikin recalls. "What we thought would happen didn't happen, but all sorts of other things did, so it was terrific. It was all unexpected."

Above: "A colloid crystal grown in space develops into a snowflake-like structure called a dendrite, a form that it cannot attain on Earth, since gravity causes the fragile arms of the crystal to break off as it comes into contact with other crystals.

Prior to CDOT, two different theories of the type of crystalline structure that would form from hard sphere colloids at equilibrium held sway. One predicted a sixfold axis of rotation, or a hexagonal crystalline structure; the other predicted a fourfold axis of rotation, like a cube. CDOT yielded crystals large enough for qualitative measurements of their structure to be made, which led to theoretical work showing the fourfold axis of rotation to be the most likely structure. This was later confirmed by the Physics of Hard Spheres Experiment (PHaSE), described below.

Other theories of crystallization were also being put to the test by the results of the CDOT experiment. Chaikin and Russel were able to answer some questions about how crystallization actually begins. "The way that people thought that the crystals would grow is that you would have a little seed that was like a sphere, and once you got the radius of the sphere beyond a certain size, it would just continue to grow. The picture we got down immediately from the space shuttle showed that that wasn't what was going on," says Chaikin.

Instead the researchers discovered snowflake-like structures called dendrites, which hadn't been seen before in colloidal experiments conducted on Earth. With these results in hand, the team was later able to show mathematically that dendrites should in fact grow but that the reason they had not been seen on the ground is because the crystals are very fragile. As the dendrites grow in laboratories on Earth, they settle onto other crystals, and the forces on the arms of the dendrites cause them to break off, leaving a spherical structure.

CDOT yielded another surprise for the team of researchers. Chaikin explains that on Earth, if hard sphere colloid samples are too concentrated, they don't crystallize at all. These colloids are said to be in a glass state, because they solidify before they can form crystals. Samples of colloids that had failed to crystallize after months on Earth were sent to space in the CDOT experiments. "We sent them up and they crystallized very quickly," Chaikin recalls. "We still don't understand why, but we do understand some possibilities. On Earth, gravity may pull the particles together even though they are not dense enough in the fluid to be touching one another on their own, jamming them against one another so that they can no longer move."

When crystalline samples return to Earth from space, most get destroyed by the gravitational forces on landing because they are so fragile. "What was really neat about that experiment," says Chaikin, "is that when the samples came back . . . the glass sample [that had crystallized in space] was very strong because it was made from a very high concentration of hard spheres. It survived landing."

In order to be certain that the unusual results obtained in space were not the result of a mix-up in samples, the team decided to stir half the crystallized sample. The portion that was stirred so that it no longer contained any crystals grown in space has remained a glass sample in the lab at Princeton. The portion that crystallized in space has remained crystalline. "That's how we knew that in fact it wasn't a bad sample or anything that resulted in the unexpected crystallization in space — it was the effect of gravity that kept the glass portion of the sample from recrystallizing."

A Sharper Image

Although a highly successful experiment, CDOT yielded only very rough images of the structure of the hard sphere crystals, so Chaikin and Russel knew the next step would be to sharpen those images. "A whole new apparatus was designed with state-of-the-art optics and some clever designs to help us determine the crystal structure and also to watch the nucleation [the very beginning stages of crystal growth] and later growth of the crystals," says Chaikin. William Meyer, of Glenn Research Center (then called Lewis Research Center); David Cannell, of the University of California, Santa Barbara; and Anthony Smart, of Titan-Spectron, were instrumental in designing the new apparatus and procedures. This time, the team wanted to know even more about the crystalline samples, like how elastic they are, when nucleation occurs, and how quickly the crystals grow. The Physics of Hard Spheres Experiment, designed to answer these questions and to give researchers a chance to see the crystallization process, flew onboard space shuttle flight STS-94 in July 1997.

It's a Matter of Size

What PHaSE revealed to the research team was again both unexpected and exciting. As it turns out, crystals don't just start growing and then continue on that path until they fill up all available space in the sample container. What's actually going on is a much more competitive process. In a given colloid sample, nucleation begins at what appear to be several random points within the fluid. The crystal nuclei that begin growing the earliest can be quite large by the time other nuclei begin to grow.

"What happens," explains Chaikin, "it that the bigger ones slowly eat away at the little ones until the crystallite gets bigger and bigger. This is a very slow process, and finally, if you waited long enough, you would get a single crystal." Although it was known prior to flight that the bigger nuclei should consume the smaller nuclei, no one expected that this would be happening at such an early stage in the growth of a crystal.

Above: Measuring approximately one centimeter in width, the large single crystal in the foreground is the largest hard sphere colloidal crystal grown to date. Researchers were able to grow this "giant" crystal by separating a single nucleus from the colloidal solution, thereby controlling part of the nucleation and growth process.

In fact, says Chaikin, this process is known to happen in other systems as well. For example, "if you breathe on glass you get a fog," he describes. "Initially in that fog there are tiny water droplets that condense onto the glass from water vapor. Immediately what happens is the larger ones start growing and the smaller ones disappear. The large ones grow until you get really big droplets. The same thing happens in a colloid. It's called coarsening.

"What's cool," he continues, "is that once you know that something is happening, you can figure out ways to prevent it. For instance, the dendritic growth we saw in the early research did bother some of our experiments because we wanted to study other phenomena and because we wanted to grow big crystals and because we didn't want to have this coarsening phenomenon. Since we knew what was going on and we knew what caused it, we could figure out a way around it."

To eliminate the problem of competition between growing crystallites, the team grew a single crystal from one nucleus, resulting in the "world's biggest hard sphere colloidal crystal," according to Chaikin. Measuring approximately a centimeter in width, Chaikin admits that the single crystal wasn't enormous, but it proved the theory that the nucleation and growth processes were within the researchers' control.

Building Steps On Solid Principles

The next step for Chaikin and Russel is to use all that they have learned about the nucleation and growth processes to control the outcome of crystallization. Advanced engineering like this requires equally advanced tools. For forthcoming experiments that will be conducted on the International Space Station, Chaikin and Russel will use microscopy to study the crystalline structure of hard sphere colloidal crystals. Their instrument, called a confocal microscope, allows the researchers to get a look at a crystal as it grows, particle by particle.

"Along with this," says Chaikin, "what is even neater, I think, is we've got a set of laser tweezers." Laser tweezers, he explains, consist of a very tightly focused beam of laser light. If the beam is small enough, particles can be sucked into the focus of the beam. "It's like the force of static electricity that draws objects together," explains Chaikin. "The particles like to be where the electric field is intense."

"We can actually take the particles that we are looking at with the microscope and grab one, grab another, and bring them together. So instead of letting the particles nucleate as they want to, we want to form the nuclei and watch them grow particle by particle. We want to see why the glass forms or doesn't form. We want to see if we can make different kinds of crystal structures, maybe even nonequilibrium crystal structures. We are going to be able to do all sorts of things with this apparatus," says Chaikin excitedly.

One promising avenue of research the team is pursuing involves fabricating plastic structures of micron scale to be used as templates for crystal growth. "One of the things we want to do is have in the microscope not only the ability to grab particles in three dimensions, but also the ability to have on the substrate — on the glass slide that we are looking up through — the pattern on which the colloidal particles will nucleate," says Chaikin. These templates would allow researchers precise control of the placement of the individual particles, yielding precisely engineered crystalline structures. With this capability, it might be possible to engineer a crystal in which the spacing between the particles is comparable to a specific wavelength of light. Such precise spacing of particles would permit improved control over directing light for applications like long-distance telephone connections.

Building on more than a decade of research in microgravity, this team of researchers is poised to make the most of their opportunities for research in space. "The more sophisticated we get," says Chaikin, "the more we will be able to do."

Web Links

Chaikin's Colloids Research -- For more information on Chaikin's colloid research visit his Web site at Princeton University.

PHaSE Project Web site -- The NASA Glenn Research Center Project Web site contains a science, experiment hardware and operations overview.

Research Publications -- Results of research on the growth of hard sphere colloidal crystals were published in Cheng, Z., Russel, W.B., Chaikin, P.M. (1999), Controlled growth of hard-sphere colloidal crystals, Nature, 401, 893-895.