Cover story
17 February 2017

Science on the Edge

Science on the Edge Illustrations by Caroline McNiel

Across disciplines, exciting stuff happens along the boundaries between things. What makes those realms so rich for research, and how do magnets shed light on them?

By KRISTEN COYNE

Fringe physics. Borderline biology. Crossover chemistry.

No matter the name one might concoct to describe it, the phenomenon is the same from one field to the next: At the dividing line between two things, there’s often no hard line at all. Rather, there’s a system, phenomenon or region rich in diversity or novel behavior — something entirely different from the two things that created it.

In ecology, they call this the ecotone — the transition area between adjacent ecosystems (forest meets meadow, say, or dunes meet ocean). In addition to being home to species from neighboring areas, this zone typically gives rise to unique flora and fauna found nowhere else.

It's referred to as the edge effect in the field, but scientists from other disciplines know it well, too. When things overlap, you often get more than the sum of those parts. Or as one chemist put it, A plus B isn’t necessarily AB: It's probably something a lot more interesting.

In the stories below, we learn about chemists, physicists and biologists who are pushing the boundaries of knowledge by exploring the boundaries between things. Exploiting high magnetic fields, they study how proteins police the walls of cells; how to break down the barrier between oil and water in emulsions; why electrons behave bizarrely near the surface of some materials; and which adjoining ingredients yield a better lithium ion battery.

It's no accident many researchers focus on these threshold regions, said Holly Falk-Krzesinski, an expert on interdisciplinary and team research at the scientific publishing giant Elsevier.

"It turns out that work that's done at the interstices, or the interfaces, tends to be highly innovative, higher-impact research," she said.

Although research that probes interfaces is not necessarily interdisciplinary, it often is. Interdisciplinary research is, after all, a kind of intellectual interface that can be very well suited to discovering fringe phenomena. Interdisciplinary research is on the rise and yielding important results, noted Falk-Krzesinski. In the long term, the more interdisciplinary an article is, the more citations it gets compared to other articles, according to a 2013 article in Science. Funding agencies are pushing researchers toward interdisciplinary approaches for problems that traditional research has failed to solve. And although academic traditions and infrastructures can inhibit interdisciplinary work, universities have found an effective work-around in the growth of interdisciplinary centers and institutes.

"Innovation is about taking what we already have or what we already know and combining it with something new," Falk-Krzesinski said. "And the 'new' doesn't have to be brand new knowledge; the 'new' can be new information, new insight from an existing field."

Read on to learn how four researchers using magnetic fields explore science on the edge.

 

Bursting the Oil/Water Bubble

When two strong personalities clash, they're likened to oil and water. But the two fluids can indeed mix, with droplets of H2O distributed throughout the oil in an emulsion.

For crude oil companies, that can spell trouble.

Whether companies extract oil from deep underground or from the surface, water inevitably ends up in the mix. While some naturally separates out, some can remain trapped in the oil as emulsified droplets, causing costly problems when the oil is processed.

Oil companies wanted to burst those pesky water bubbles and figure out what their walls were made of.

Ryan Rodgers

Ryan Rodgers

photo by: Stephen Bilenky

"If you knew their chemistry, then you could target them like a drug targets a virus, to knock them out," said Ryan Rodgers, an expert in petroleum science at the National High Magnetic Field Laboratory who directs the Future Fuels Institute (FFI) based there. "Then you could free the water, then free the oil."

But before you can determine what's in that wall, you have to isolate it. Rodgers and his team at FFI and the MagLab's Ion Cyclotron Resonance (ICR) Facility came up with an ingenious way to do just that.

They faced a sizeable challenge: Crude oil is the most chemically complex substance on Earth — a single sample can contain millions of different kinds of molecules. Some of these molecules are hydrophilic — so they are drawn to any water that gets mixed in the oil. As a result, they fuse into walls around areas of water, forming water droplets. The scientists needed to somehow identify, capture and remove those hydrophilic molecules.

So they created a clever trap for them designed around a simple grain of sand. They applied layer after layer of water onto silica until they figured out the exact amount needed to attract the problem molecules. (After a lot of trial and error, they discovered that 26 layers generally did the trick.) When the water-encased silica was added to the oil, the hydrophilic molecules in the droplet walls headed straight for them. This isolated the molecules that cause an emulsion.

The silica (encased first by its shell of water, then by a layer of water-loving crude oil molecules) was easily separated out. Scientists then simply peeled the target molecules right off and analyzed them. Using ICR instruments that rely on powerful magnets, the chemists were able to precisely identify the molecules that were acting as surfactants, molecules that lower the surface tension between petroleum and water and allow emulsion droplets to form.

What they saw surprised them.

"We've been able to show that the chemical diversity of these naturally occurring surfactants in petroleum is as complex as petroleum itself," Rodgers said. "It's just spectacular. You're looking at hundreds of thousands of naturally occurring surfactants."

ICR and other analytical tools are now helping scientists learn more about those surfactants. Does that emulsion wall vary under different conditions, such as pH, for example?

"The goal is to manipulate the chemistry of those walls so we can use it when it's needed and get rid of it when we don't," Rodgers said.

What scientists learn will also have applications in renewable fuels, whether extracted from algae, pine pellets, sugarcane or other biomass.

"All of those materials have an enormous water content," said Rodgers — water that needs to be removed before you can make fuel from it.

Scientists studying fuels are interested in other problematic interfaces, too, such as fine particles like rust in crude oil.

When two things abut, it's never straightforward, requiring scientists to sort it all out.

"It's an issue because A plus B doesn't always equal AB," Rodgers said. "A plus B can sometimes be equal to Z."

 

Surprise Under the Surface

In her forays into physics, University of Cambridge physicist Suchitra Sebastian heads for the hinterlands, hoping to be the first to leave footprints.

"I like to explore unknown territory," she said.

Which is not to say she's wandering blindly into the science haystack. She has a definite strategy about where the coolest physics needles are hiding.

Sebastian studies correlated electron systems — materials in which electrons can team up and behave radically differently from individual electrons. Recently, Sebastian has set her sites on samarium hexaboride (SmB6). Some physicists who have studied that system have proposed that it belongs to a class of materials called topological insulators.

Suchitra Sebastian

Suchitra Sebastian

photo by: Phillipp Ammon/Quanta Magazine

Discovered in 2007, topological insulators are the split personalities of materials: They conduct electricity in some areas, but don't in others. In topological insulators that are two-dimensional (only one atomic layer thick), current runs only along the edges, like a racecar circling a track while avoiding the infield, which acts as an insulator. In 3-D topological insulators, current can run anywhere on the surface, but not through the interior (called the "bulk" by physicists). Picture that racecar speeding along any road on Earth, but never penetrating the planet.

(If the term "topological" rings a bell, it might be because the 2016 Nobel Prize in Physics went to a trio of scientists who applied concepts of topology, a branch of mathematics, to better explain and predict so-called "exotic" states and transitions of matter.)

Sebastian set out to try and establish the topological character of SmB6 by looking at how electrons travel through its conducting surface. As electrons propagate through a conductor, their orbits forge a rather zigzagging path. Because SmB6 was a supposed topological insulator, Sebastian expected that those orbits would be restricted to the very surface of the material — that their path would be strictly two-dimensional.

But what Sebastian and her group found during experiments in 2015 was far stranger. When they studied SmB6 using a very high magnetic field, they found what appeared to be large, three-dimensional electron orbits. That would mean that the electron orbits were moving through the interior of the material, which is an insulator.

"My first reaction was, 'Wow, something strange is happening,'" said Sebastian, who was conducting the experiment at the National MagLab at the time. "How could we be seeing large electron orbits in the bulk? The electrons in the bulk of SmB6 shouldn't be moving at all — the bulk is insulating!"

She phoned a colleague at Cambridge for his thoughts. "You do realize," he insisted, "this is impossible."

Apparently not.

Sebastian has since been trying to figure out this unusual effect — a material that appears to behave simultaneously as an insulator and a conductor. Is it a never before observed quantum phase of matter? Could it be that electrons behaving collectively (known as quasiparticles) in this strange quantum phase act completely unlike individual electrons?

"Perhaps the quasiparticles we see orbiting are starkly different from electrons or electron-like objects," Sebastian suggested. "We begin to consider the radical possibility that perhaps SmB6 defies conventional understanding, and comprise neutral quasiparticles — ones that, unlike electrons, carry no charge!"

In her search for answers, Sebastian sticks to the fringe. She focuses on critical phase transition regions — the thresholds between states of matter that occur at specific pressures or temperatures, like water boiling as it turns into steam under a critical pressure.

"We are interested in the quantum version of this critical regime, where a growth of quantum fluctuations means that the interactions between electrons grow extremely large, mediating new and exotic quantum phases," she said. "This region is therefore an incredibly exciting region that is ripe for exploration."

 

Border Patrol

"No man is an island," English poet John Donne wrote almost 500 years ago. Neither, to put a modern, scientific twist on that famous line, are the things man is made of: cells.

Cells are surrounded by a protective wall called the lipid bilayer. But they still, at that interface, need to interact with the outside world.

Guido Pintacuda

Guido Pintacuda

photo by: Loren Andreas and Michael J. Knight

That's the job of membrane proteins, the bouncers of the cellular world. They police what comes in and what leaves while keeping an eye out for trouble.

"They're key players in allowing cells to adapt to new living conditions or to protect the cell from certain events happening outside," explained Guido Pintacuda, research director at the Center for High Field Nuclear Magnetic Resonance (NMR) in Lyon, France. "For example, they are able to communicate to the outside world a change which is happening inside the cell. This immediately makes them important drug targets."

In fact, about 40 percent of all FDA-approved drugs are designed to manipulate membrane proteins in some way.

Yet they remain largely a mystery. Despite their numbers (they make up about 20 to 30 percent of all living proteins) and the pivotal roles they play in our bodies, less than 2 percent of all the proteins scientists have characterized to date are membrane proteins.

That's because those boundary-dwellers are often in charge of complex tasks and need a complex structure to execute them. Typically they contain several hundred amino acids arranged in rigid scaffolds alternating with less-ordered portions.

Scientists also face big technical hurdles with the NMR instruments they use to understand what these proteins look like and how they function. But groups such as Pintacuda's are developing new tricks to make solid-state NMR a more powerful and versatile tool for the job.

"Increasingly it's the technique of choice," he said.

Using solid-state NMR, scientists can map out the structure of complex macromolecules by using magnetic fields and radio waves that exploit magnetic properties inherent to all atoms. Depending on the specific technique applied, the result is either a "snapshot" detailing the molecule's structure or a kind of movie that reveals how it behaves in its native environment.

"We can see what happens to each atom of a large molecule over time," Pintacuda said. "We can look at the molecule in three dimensions as it if was at the cinema, and we see how it functions."

One protein much scrutinized by scientists working at the Francis Bitter Magnet Lab at MIT, the National MagLab and other labs is known as the M2 proton channel. Located in the wall of the influenza A virus, it ushers protons in and out of the cell. Tim Cross at the National MagLab discovered how the protein functions, and his and other groups are identifying new drugs to block it, thus preventing or thwarting infection.

At the Center for High Field NMR in Lyon, Pintacuda’s group has helped its collaborators in studying a mutated form of that protein on a drug-resistant version of influenza A, shedding light on how the mutation allows the M2 proton channel to keep working even after exposure to drugs.

Their NMR research would not be possible without high magnetic fields. The stronger their magnets, the better NMR systems can "hear" and separate the tiny signals emitted by different atoms in the molecules they're listening to. One might imagine NMR like an elephant, an animal renowned for excellent hearing, thanks to its big, amplifier-like ears. Dr. Seuss fans might even bring to mind the author’s famous pachyderm, Horton, whose sensitive ears could make out voices of the residents of Whoville, who inhabited a speck of dust.

"We're able to isolate the voice of each individual atom in the molecule," said Pintacuda, "and therefore much better characterize what’s happening at atomic resolution."

 

Safer Lithium Batteries

Yan-Yan Hu, a chemist at the National MagLab and Florida State University, loves composite materials. Thanks to all their interfaces, they are often more useful, and definitely more complex, than the sum of their parts.

"When you add one element to the other," said Hu, "that makes the whole system more complicated to study."

Bone is one such system, owing its amazing properties to its composite nature: strength from mineral, and flexibility from protein fibrils that offset the mineral's brittleness.

Yan-Yan Hu

Yan-Yan Hu

photo by: Stephen Bilenky

"So it's a combination of both that produces these synergistic effects, that provides the biological function and the mechanical function," Hu said.

Hu earned her Ph.D. by solving one of the big bone mysteries. Using nuclear magnetic resonance (NMR), she discovered that citrate molecules at the interface of those two ingredients direct the formation of the composite bone matrix.

Since then, Hu has gone from bones to batteries. In particular, she studies the interfaces between electrodes and the electrolyte of lithium ion batteries.

Lithium has a lot going for it as a battery ingredient: It's the lightest metal and has great electrochemical potential and energy density. But it has problems, too, including a spotty safety record.

Like all batteries, lithium ion batteries have an anode and a cathode; lithium ions travel through the electrolyte between these two electrodes to generate energy.

But even with just three main ingredients, problems pop up when you put them together. Materials accumulate on the electrodes’ surface, for example, lowering the battery's capacity. And if the accretions of those metallic dendritic structures eventually reach the opposite electrode, they create a bridge that shorts the battery and causes it to overheat.

Although most lithium batteries today use liquid electrolytes, scientists such as Hu are looking for a solid that could be safer (blocking the formation of those electronically conductive bridges) and yield more capacity. So they ask: What solid electrolytes will hold up at the interface with the electrodes? How do charges travel from one side to the other?

Hu and her group experimented with a composite electrolyte — lanthanum lithium zirconium oxide-polyethylene oxide (LLZO-PEO) — that leverages useful properties of both ceramics and polymers. They wanted to know exactly how lithium ions would travel through LLZO-PEO: Through the ceramic? Through the polymer? Through the interface?

It was a straightforward question — with a well-hidden answer.

"It's difficult to study the interface because it's buried," Hu explained. "The ceramic and the polymer are mixed together. There's no way you can separate them and look at the interface. If you pull them apart, it's not an interface anymore."

High-field NMR allowed Hu and her team a way to see those ions in action at the atomic level. They built small lithium-ion button cells and, with special equipment, observed where the lithium ions traveled while the cells charged and discharged inside a magnetic field.

To do this, her team devised a trick: Use one isotope (or variation) of lithium in the electrolyte (in this case, lithium-7), and another isotope (lithium-6) in the electrodes. This allowed Hu to identify the ions’ pathway: The lithium-7 ions swapped out for lithium-6 ions wherever they traveled.

That path, as it turned out, went through the ceramic. Knowledge like this can help engineers manipulate interfaces to design safer, more powerful battery materials.


Illustrations by Caroline McNiel

Last modified on 14 August 2017