“Everything you see around you, the computer, the table, textiles—the reason they’re useful is because of their properties,” says Fernando A. Escobedo, Chemical and Biomolecular Engineering. Escobedo says these properties—mechanical, thermal, electrical, and others—are often associated with the chemistry of the molecules. But there’s another essential component.
“The way the atoms and building blocks are organized at the microscale is equally important, and perhaps it’s crucial in defining why things behave the way they do,” Escobedo says. He uses the example of graphite and diamond. Both have the same chemical building blocks, carbon atoms, but their different arrangement vastly changes their properties.
The perfectly ordered structure of something like diamond may seem straightforward, but the structure in many materials, both natural and newly designed, falls between two ends of a spectrum. On the one hand is order, like in crystals, and on the other is disorder, like in gases. Many interesting materials fall between these phases, and their more ambiguous structures can give rise to novel properties.
Escobedo’s group develops new methods to model and simulate these complex structures at the microscale, asking how the organization of the building blocks correlates to macroscale properties. The lab seeks to discover desirable properties and the ability to engineer them—which could have wide impact on a variety of applications, from solar cells to improved plastics to therapeutics. Their work complements real-life experiments and also imagines materials and properties no one has yet seen. “In some cases, something has been observed and you ask why,” Escobedo says. “But you can also use your imagination, and ask, why not this?”
Entropy—Order and Disorder
Materials whose microstructure or phase falls between order and disorder pose many challenges, including how to tune and measure the degree of order. “We want to understand—how is it that you can quantify these different extents of order, what kinds of order are there, and even semantically, what do we mean by order? Sometimes you have an idea that something is disordered, but seeing it in a different light, you may see order,” Escobedo says.
One force that plays a crucial role in determining order or disorder is entropy, commonly defined as a measure of disorder in a system. This definition, however, is over-simplified. “In nature, entropy is not so much about your intellectual perception of order or disorder but about microscopic degrees of freedom. These atoms, they move and have different ways of moving and arranging in space,” Escobedo says. “Entropy is in some ways a measure of the ability of these molecules to move with different kinds of motion and over different positions. The more freedom the molecules have, the more entropy.”
With more degrees of freedom, there tend to be more possibilities for forming different structures, which is desirable for engineers seeking materials with new properties. “I see entropy as a creative force in nature,” Escobedo says. “And one of the main themes of my research is to understand entropy better, so we can engineer it.”
“I see entropy as a creative force in nature. And one of the main themes of my research is to understand entropy better, so we can engineer it.”
Escobedo’s focus on entropy is somewhat unique. It’s more difficult to measure and to understand than energy, but it’s an equally fundamental concept. In simulations, Escobedo may take a system of particles and turn off the attractions, so only the interactions between the shapes of particles remain. “In some ways, you’re isolating the entropic interactions,” Escobedo says. “So now you can see what would happen if only entropy were playing a role. If that behavior is useful, it may be possible to change the chemistry to approach this pure entropic behavior, but you use the model just to address the question.”
Playing with Superatoms
The original building blocks for chemical engineers are the elements in the periodic table. “You have this play set to create compounds and substances,” Escobedo says. These blocks can be assembled into larger objects like nanoparticles and colloids. But with certain objects, engineers were limited to the shapes found in nature. Colloids in nature, for example, are made up of building blocks that only come in a handful of shapes.
With new synthesis and fabrication techniques, engineers can now design nano-building blocks of any shape and composition. While these are made up of many atoms, Escobedo says they still amount to a new and novel set of blocks, akin to superatoms. “You basically have this play set of building blocks, different shapes, and you can change their characteristics in multiple ways. What can you do with this play set? How do they assemble? Can they form interesting phases with different types of order? Why not think of these building blocks as a way of creating a new chemistry?”
Escobedo’s group is interested in modeling nanoparticle systems that occupy those in-between phases, or mesophases, between order and disorder. Liquid crystals, widely used in displays, are an example of a mesophase material with wide application. “There are all kinds of mesophases, not unlike liquid crystals, which we still haven’t found uses for, but we are discovering more of them and understanding what interesting properties they have,” Escobedo says.
Many of these simulations don’t yet have experimental counterparts. They are pure exploration. “The most intriguing part is to ask curiosity-driven questions just because you can ask them,” Escobedo says. “If history is any guide, discoveries always end up in some application, but in this work, we just want to understand the fundamental science.”
To Make a Polymer
Escobedo’s work is not always so theoretical—the group also uses their models to solve problems from the labs of collaborators. In a recent example, Peng Chen, Chemistry and Chemical Biology, came to Escobedo’s group with data his group didn’t understand.
The experiment involved the making of a single polymer, in a process whereby monomers bind one-by-one to make a polymer chain. Contrary to previous assumptions, Chen found that the progress of the polymerization was surprisingly not continuous. Instead, they saw jumps in the extension.
Escobedo and his group constructed models to test specific hypotheses. They found that as the polymer chain grows, it tends to accumulate some tension, like a wind-up toy, which is quickly released by twisting the chain. Escobedo compares it to a twisting telephone cord. “This tension is making it twist, and it will tend to entangle but only up to a point. Beyond a certain extension, it will unravel,” he explains. “This entangling and unraveling gives rise to these steps in the growth. That idea was really an insight that simulations confirmed.”
The groundbreaking study, published in Science in 2017, has broad implications for determining polymerization rates and for understanding polymerization in cells.
“So the research goes both ways,” Escobedo says. “Sometimes people come to us, and other times we find an interesting behavior in the models and ask others to explore it.”
A Fascination with Scientific Inquiry
Escobedo’s first significant inroad to science was geometry. As a high school student in Peru, he fell in love with the elegance and logic of Euclidean theorems. “It feels magical when you’re young,” he says. “You assume these five axioms are true, then through different proofs, you build this marvelous architecture. It’s the first time you really see the power of science.”
Escobedo also loved the beauty of shapes and the relations that describe areas and volumes but didn’t see a clear way geometry could be incorporated into a career. Nor did he see a clear path to leverage his fascination for computer programming as a chemical engineer. After graduating from college, he worked for five years as an engineer in industry, where he found that pure scientific inquiry took a backseat to the bottom line.
“When I came to the U.S. and did my PhD, I could have chosen to work on something much more applied, but I think your PhD is this golden opportunity to ask more fundamental questions and pursue your scientific inquiries and not think about profit and products. At least that was true in my case.”
Now, Escobedo says he’s come full circle. “Our research entails extensive computer coding, and understanding microstructure is ultimately understanding what kind of geometries these new building blocks can produce at the microscopic level,” he says. “It’s so fascinating that something so pure and seemingly abstract has become a centerpiece of my work. In fact, I sometimes feel that most of my work is driven by a search of microstructural, geometric beauty.”