Industrial Physics Forum 2013: Opportunities for mesoscale science

Consider, if you would, conquering the macro–nano divide: What if you could examine any material in the world by peering through two different filters—one a powerful lens that would show you the nanoscale details of this matter and the other an ordinary pane of glass revealing how it behaves in the everyday world?

Materials researchers and other “reductionists” would love the first—to see the atomic arrangements, crystal lattices, and molecular orbitals that make a material what it is. Engineers and “constructionists” would appreciate the second—to see and define a material’s useful properties in the real world, to understand how it combines, conducts, contorts, compacts, or crumbles.

Would looking through those two glasses provide the insight sufficient to build any useful new material needed for future applications in everything from green energy production to new medical devices to consumer electronics?

Perhaps not.

George Crabtree will tell you that there is a lot to be learned about materials by looking at them at a scale between these two extremes—something he calls the mesoscopic world. It’s neither atomic nor macroscopic but instead exists at the interface between the two.

For several years, Crabtree, who is with Argonne National Laboratory and University of Illinois at Chicago, has championed the relevance of materials at the mesoscale world for manufacturing and science. Last month at the Industrial Physics Forum, part of the 60th AVS symposium in Long Beach, California, he presented on this subject.

Defining the mesoscale

The difficulty with the mesoscale world, said Crabtree, is that we cannot always understand it through traditional approaches. Therein lies the challenge—we must examine the mesoscale through a third type of filter.

For decades, researchers have looked deeper and deeper into the micro- and nano-nature of materials, teasing apart molecular and atomic interactions to understand how these reactions contribute to a material’s overall properties. Micro- and nanoscientists study two essential factors: 1) the identity and arrangement of atoms joined together by chemical bonds to make the molecular components of the material and 2) the periodic lattices that define its larger landscape.

“These two motifs account for almost all the huge variety of materials we find,” Crabtree said.

Small changes to a material’s macroscopic properties on paper may fundamentally change the nature of the material. One example of the effect is provided by methane and carbon tetrafluoride. Both are tetrahedral molcules composed of a central carbon atom surrounded by four atoms each bound by a single covalent bond—hydrogen in the former case; fluorine in the latter.

On paper they look remarkably the same, either H or F atoms surrounding a central C atom. But the difference between the two materials is profound. Methane is a highly flammable gas at room temperature, whereas CF₄ is a liquid solvent and not particularly combustible. As building blocks for larger structures, the difference between the two is also apparent. String methane hydrocarbons together into long polymers, and we get fossil fuels and waxes. String fluorocarbons together, and we get Teflon, a substance so unreactive that it is used to coat nonstick cooking vessels.

The same is true for lattices, Crabtree said. Periodic lattices are one of the major design motifs of the nanoworld, but a material’s properties can vary dramatically depending on the exact nature of the lattice. The size of the band gaps in a solid material, for instance, can dictate whether that material is a conductor, a semiconductor, or an insulator. The nature of the lattice—whether it is perfect and pure or filled with discontinuities or impurities—will also shape the material’s properties. For example, adding tiny amounts of carbon to iron creates the much stronger steel.

Adding chromium to steel produces rust-proof stainless steel. The mesoscale challenge, said Crabtree, differs from investigating a “real-world” macroscopic material by digging deeper and deeper into its atomic and molecular structure.

“The mesoscale challenge is to go the other way,” he said. “To take the interactions we are beginning to understand at the atomic and molecular level and control them – and make functionality that you don’t find in nature.”

Manufacturing in Mesoamerica

In one sense, meeting the challenge sounds straightforward: Simply take all the nanoparticles, quantum dots, polymer blocks, and chemical compounds we think we understand at the atomic level and use them as building blocks to make designer materials with new properties useful for manufacturing.

It’s not that easy, Crabtree said. Understanding, predicting, and ultimately controlling materials at the nanoscale does not necessarily give command over the mesoscale because many of a material’s defining properties only begin to emerge at meso-length scales. Fully exploiting the mesoscale world will require fundamental shifts in how we approach questions related to materials science, he predicted.

As a starting point, the US Department of Energy’s Basic Energy Sciences Program established a committee, with Crabtree as its chair. Last year, the committee produced a report on mesoscale science and manufacturing. For his audience in Long Beach, Crabtree described how the report defines the “six hallmarks” of emergent mesoscopic properties:

1. Diminished atomic granularity. If we move a single atom of a single molecule here or there, we may profoundly affect its properties. But the position and presence of a given atom means almost nothing to the properties of a bulk material as we include more and more molecules. “If there’s one atom out of place in a handful of sand, it really doesn’t matter,” Crabtree said.
2. Degree of energy quantization. Quantized energy levels rule a material’s behavior at the nanoscale, but increasing the length scale of that material also increases the density of energy levels and the number and variety of its interactions with its external environment. For a chunk of material with dimension above 1000 nm, the spacing between the energy levels becomes almost negligible, allowing new phenomena to emerge.
3. Developed collective behavior. Collections of atoms in a material may behave quite differently from individual atoms of the same material in the same way that a crowd may manifest emergent behaviors unseen in individuals. The mesoscale is where collective behaviors emerge in materials.
4. Interacting degrees of freedom. In the mesoscale world things get big enough to let degrees of freedom interact, and when they do, they can have a profound impact on the material. Superconductivity is one example of this; another is the piezoelectric effect, in which mechanical and electric degrees of freedom interact to generate an electrical voltage and cause a material to mechanically deform.
5. Defects, fluctuations, and statistical variations. Purity and perfection are easy to find in small molecules but less easy once we consider larger mesoscale materials, where defects like missing atoms or interstitial contaminants are common and can profoundly impact the properties. “In almost every macroscopic crystalline material, it’s the defects that count,” Crabtree said.
6. Heterogeneity in structure and dynamics. Nanoscale materials also tend to be perfect in crystal structure or magnetization, but larger amounts of material produce grain boundaries or magnetic domains. That happens either by accident or—in the case of heterogeneous composite materials that combine multiple different materials into one—by design.

Lessons from nature and biomedical Science

Human organs are a good example of how heterogeneity and emergent properties work in the real world. Organs employ multiple types of specialized cells with separate functions in specific arrangements to achieve some overarching function. The heart, for instance, has thick vessel walls made from multiple types of cells. The vessels carry cell- and protein-rich blood that gets pumped by pulsing heart muscles, which are connected to nerve fibers and coordinated by pacemaker cells. No one type of heart cell alone is itself capable of becoming a beating heart.

That gestalt has profound implications for basic biomedical science. Research aimed at treating human disease cannot rely solely on achieving advances at the reductionist molecular and genetic levels, or on testing potential new drugs on a single type of cell in the test tube. The only way to know if a drug is useful is to test as scaled-up treatments on the macroscopic level, either in animal studies or in human clinical trials.

In the same way, the promise of mesoscale manufacturing may only be achieved by realizing cooperation among disparate parts. But if modern biomedical research can offer anything by way of lesson, it would be this: Such work is, almost by definition, interdisciplinary team science.

In some ways, that may be the greatest hurdle in the mesocscale challenge, Crabtree said. In order for mesoscience to really change manufacturing, the culture of graduate education may have to change. The traditional approach of one mentor and one student may have to give way to a broader sharing of protégés, where young scientists are trained by several professors at once.

Traditional collaborations have always been built upon the connections professors have with each other, but a more effective model may be one in which the students are the glue that holds a project together, Crabtree said.

“There is something to be gained from working together that maybe we haven’t explored fully yet,” he said.

Jason Bardi directs media relations at the American Institute of Physics.