Smart and Adaptable, Soft Materials Find Their Place



Alexei Sokolov might be the only physicist you ever encounter who explains his work using haircuts, frozen frogs, and plastic planes as examples. Those may seem like unusual illustrations, but in reality they are vivid, approachable images of the deeper science driving his research. Sokolov, a UTK Governor’s Chair and a Professor of Chemistry and Physics, focuses his efforts on understanding fundamental physics and going a step beyond, into a future rich with complexity.

At the heart of these studies are soft materials, which are generally examples of matter that don’t have a crystalline structure. Liquids, disordered solids (including glasses), and liquid crystals are all examples of soft matter. So too are polymers, which provide an excellent laboratory for experimenting with the structure of materials.

“Polymer  means simply a very, very long molecule,” Sokolov says, “but it doesn’t tell you what the chemistry is. Just by changing the length of the molecule and chemical structure of its building blocks (monomers) we can change properties.

“I can give you a simple example,” he continues. “Proteins are built from small building blocks called amino acids. We have only 20 amino acids. But depending on how you combine them you have millions or even billions of proteins, and that’s an example of how chemistry actually changes the properties of these polymers.”

Understanding polymer dynamics is one avenue that scientists can use in designing materials for specific purposes in areas like energy, or biology and medicine.

 “It depends on what you really want to do,” Sokolov says. “If you want to design, for example, polymers which will replace metals—so they’ll be as tough as metals but they’ll be much lighter—we will try to modify polymers in a way that they will have good mechanical properties.”

An application for such a material would be replacing metal in airplanes with plastic, which would make for lighter aircraft that would be more fuel-efficient.

Another aspect of Sokolov’s studies involves glass transitions, where a liquid is cooled very rapidly to become a glass.

“Glass transition is extremely general,” he says. “Even when you talk about your hair, that’s a glass transition. You wet your hair before you do any kind of haircut, because when you take any kind of biological system and add water, everything starts to move very fast; everything becomes like a liquid. And in that sense, wet hair has a reduced glass transition temperature, so you can do anything you want with your wet hair. But when you dry (it), (it) becomes glassy again.”

Wood frogs, interestingly enough, are another case in point.

“There are, for example, frogs that can be frozen in ice,” Sokolov says. “Dead,” he says, slapping his hand down on his desk to emphasize the point. “No heartbeat. But this frog is alive. When spring comes, the ice melts and it can jump like nothing ever happened. Because this frog, also, undergoes a glass transition.

“That’s why we’re studying this phenomenon; glasses and glass transition,” he explains. “In general, we start with small, simple molecules to understand the basic physics, then we’re trying to apply what we learn to more complex systems, like biological systems.”

Some of his students and post-docs are looking at ways to take protein-based drugs or any protein-based products and find a way to keep them for a decade without a freezer or refrigerator. Technologies that would allow for this kind of vaccines preservation would be a tremendous boon in countries where storing them is difficult due to a shortage of proper cooling facilities.

Something in the Way They Move

In describing the properties of soft materials, the key, Sokolov says, is molecular motion.

“Soft materials, by definition, can change—that’s why we call them soft materials,” he explains. “Any change means the molecules move. So molecular motion is a key to many macroscopic properties. If you talk about polymers for batteries, it will be ionic motion. And ionic motion depends of course on the motion of the polymer itself. If you talk about biological systems, then all of life is all about molecular motion. By learning how molecules move, by developing, for example, ways to suppress motion of biological molecules by using different solids, that’s how we develop technology.”

Motion suppression makes the difference between freezing humans and freezing frogs. As humans, we have too much water to go through the glass transition; if we freeze, the water crystallizes, breaking down tissues and cells. The frog, however, produces glycerol, a kind of natural anti-freeze.

“All metabolic processes inside of this frog will be almost stopped for months. It doesn’t eat, it doesn’t breathe, it does nothing, and still it’s alive. So it’s once again about molecular motion,” Sokolov says.

He and his group use various experimental techniques to figure out how molecules move. With light scattering, they observe changes in the light’s frequency due to molecular motion. With dielectric spectroscopy, they apply oscillating electrical fields and watch as molecules try to orient along the field. But the most advanced technique in their repertoire is neutron scattering, a common technique in condensed matter physics where a neutron bounced off atoms provides critical information about their positions and motions.

“There we can really see the details of molecular motion,” Sokolov says.

His group also collaborates with other scientists who run computer simulations, comparing experimental results with digital models to see how well they agree.

Sokolov’s work ties in nicely with an emerging idea in modern science, that of complexity. As scientists learn more about basic particles and systems and how they interact, the more obvious the interplay becomes among physics, chemistry, biology, and materials sciences.

“We’re conducting fundamental studies,” he says. “We don’t do batteries. We don’t do plastic for airplanes. We’re trying to develop basic principles, basic physics, basic understanding of how the nature is, and based on that, give a suggestion how  we should design materials with unique properties.

“If you understand small molecules, you can apply it to complex systems,” he explains. “So we’re stretching all the way from physics to chemistry to biology.”

While Sokolov holds both master’s and doctoral degrees, as well as a habilitation (doctor of science, or “higher doctorate”) in physics, it’s this complex relationship between fields that intrigued him to go beyond his traditional education and look for new opportunities in research.

“Of course I started like a regular physicist: solid state physics, photonics,” he says. “But then we started to ask more complex questions. We realized that there is great potential in the application of what we know from physics in biomedical research or energy-related fields.”

This is when he began to study soft materials.

“The advantage of soft materials is that they are self-adapting, self-healing. They are smart,” he says, with a smile in voice. “You can change them; that’s why they’re called soft. They respond to any external force or field or even a change in pH . . . that’s what we like about soft materials.”

His expertise in soft matter physics and polymer science is what brought Sokolov to UTK as a Governor’s Chair in 2009. He currently leads soft matter research efforts at both the university and Oak Ridge National Laboratory. Prior to his UTK appointment he was the Thomas A. Knowles Professor of Polymer Science at the University of Akron.

As a Governor’s Chair, Sokolov’s time is split evenly between the university and ORNL. Technically he is a professor of both physics and chemistry, although chemistry is his home department.

“My chemistry background stopped in high school, and I’m a chemistry professor,” he jokes.

Still, most of the students in his research group are from the physics department.

“For chemistry students, we are doing very weird stuff,” he says, “because what we are doing is mostly physics. It’s not yet considered to be classical physics because the complexity is so high, but it’s still physics.”

Soft matter physics and biophysics are becoming more and more prevalent in universities throughout the world, Sokolov says, and he predicts they will become increasingly important in attracting strong graduate students. In the past few years, UTK has begun building a biophysics program on campus, with the addition of Jaewook Joo in theory and a new hire in experimental biophysics, Jaan Mannik, who will join the physics faculty in the fall. (Sokolov has already had discussions with him to determine areas of mutual interest.)

All fields of physics are important, Sokolov says, but “the future of physics is in these complex systems. It’s much more complicated—that’s the beauty of physics.

“I really have this feeling that we’re doing exciting and important stuff,” he continues. “It’s far from applications; even if we design the basic understanding of how to create these polymers, it doesn’t mean that the next day you’ll have a battery. It still will take time. But that’s what we like in all the work that we do, that we do something that nobody knows how to do. And that’s what makes it exciting.”

More Information

Professor Sokolov’s Research Group: http://www.chem.utk.edu/sokolov/index.html

UTK Governor’s Chair Program: http://www.utk.edu/govchairs/

Complex Systems: Science for the 21st Century (DOE Report): http://science.energy.gov/bes/news-and-resources/reports/abstracts/#CS

—Catherine Longmire

Reprinted with permission from the Spring/Summer 2011 Issue of Cross Sections, the newsletter of the University of Tennessee Department of Physics and Astronomy.