Login with your Social Account

Scientists combine light and matter to make particles with new behaviors

Scientists combine light and matter to make particles with new behaviors

Every type of atom in the universe has a unique fingerprint: It only absorbs or emits light at the particular energies that match the allowed orbits of its electrons. That fingerprint enables scientists to identify an atom wherever it is found. A hydrogen atom in outer space absorbs light at the same energies as one on Earth.

While physicists have learned how electric and magnetic fields can manipulate this fingerprint, the number of features that make it up usually remains constant. In work published July 3 in the journal Nature, the University of Chicago researchers challenged this paradigm by shaking electrons with lasers to create “doppelganger” features at new energies—a breakthrough that lets scientists create hybrid particles which are part-atom and part-light, with a wide variety of new behaviors.

The research is part of a greater effort in Assoc. Prof. Jonathan Simon’s lab to break down the walls between matter and light, in order to investigate their fundamental properties. In addition to learning about how materials behave at the quantum level, this work could one day help create more powerful computers or virtually “unhackable” quantum communications.

One step along the way to making matter out of light is to make individual packets of light, called photons, interact with each other like matter does. (Normally photons zip along at the speed of light and don’t react to each other at all.)

“In order to make photons collide with one another, we use atoms as a go-between,” said postdoctoral researcher Logan Clark, who led the research. “But we were running into a problem because the photons only interact with atoms whose electronic orbitals are at very particular energies. So we asked: What if we could make copies of the orbitals at whatever energies we wanted?”

Clark had already developed techniques to manipulate quantum matter by shaking it —called Floquet engineering—as part of his PhD project. The right sort of shaking naturally produces copies of quantum states at multiple energies along the way. “We had always viewed the copies as a side effect rather than the goal,” he said, “but this time, we shook our electrons with the specific intent of making the copies.”

“We asked: What if we could make copies of the orbitals at whatever energies we wanted?”

—Postdoctoral researcher Logan Clark

By varying the intensity of a laser field tuned precisely to an atomic resonance, the team was able to shift the orbitals of an electron. Shaking the orbitals by periodically varying this intensity produced the desired copies.

But these doppelgangers come with an important catch: “While the atomic orbital does appear at multiple distinct energies, it is important to note that these copies are actually bound to the original like puppets,” explained postdoctoral researcher Nathan Schine, a co-author on the study. “When any of the copies shifts, the original and all of the other copies shift with it.”

By allowing photons to interact with these shaken atoms, the team has created what they call “Floquet polaritons”—quasi-particles which are part-light and part-atom, and unlike regular photons, interact with each other quite strongly. These interactions are essential for making matter from light. Making polaritons with shaken atoms can give the polaritons much more flexibility to move around and collide with each other in new ways.

“Floquet polaritons are full of surprises; we’re still continuing to understand them better,” Clark said. “Our next order of business, though, will be to use these colliding photons to make topological ‘fluids’ of light. It is a tremendously exciting time.”

Having copies of an atomic state at multiple energies also offers exciting possibilities for optical frequency conversion—a key tool in creating secure quantum communication methods.

“It turns out shaking things is not only a lot of fun, but can lead to some really fascinating science,” Clark said.

Citation: “Interacting Floquet polaritons.” Clark et al, Nature, July 3, 2019. DOI: 10.1038/s41586-019-1354-5

Materials provided by the University of Chicago

string theory

Physicist one step closer towards solution of the string theory puzzle

A physicist from the University of Colorado, Boulder is very close in solving a puzzle in the string theory which has been unsolved for more than 20 years now. 

Paul Romatschke, an associate physics professor at CU Boulder has figured out a different set of tools for what created the three-quarters dilemma. This mathematical puzzle of string theory has plagued scientists for several years. This has kept them from the realization and proof of the “Theory of everything”. The results of the study have been published in Physical Review Letters.

It may not be applicable for the regular world we see around us but it opens the possibilities for the understanding of high-level physics. The results of the study could change the ways we look at the several important domains of physics such as string theory or quantum field theory. These set of theories describe the field dynamics and the entities which permeate everything. 

Romatschke said that it would have been really great to actually understand the meaning of three-quarters however this outlook is a step towards the solution, if not the solution, and provides a suggestive picture. 

The string theory has puzzled the scientists since 1960’s. It is a theoretical framework which involves fundamental, one dimensional objects known as strings. These entities constitute the fabric of everything. It was first put forward for addressing a number of questions in fundamental physics. But from there it has been applied for studying several topics such as black holes, nuclear physics to even the origin of the universe. 

One of the biggest achievements of the string theory is the conclusion that black holes and matter are nearly the two faces of a single coin. This duality allows researchers to map several properties of matter such as pressure to the black hole properties as obtained from the general theory of relativity of Einstein. It would create the possibility of a greater mathematical exploration of the string theory. However, physicists have not been able to prove a major aspect of the string theory. 

Since the duality was discovered 20 years ago, researchers have tried to clear the roadblock with equations of increasing complexity. But they arrive at the same result always. The free energy obtained from the strong interaction is nearly three-quarters of the strength of weak coupling. 

Romatschke put the equations for space which has only two dimensions. He used equations from the previous research, as well as modern techniques of quantum field theory and he proved that a relationship exists when the matter is forced to interact from zero to infinite interaction. Calculations revealed the infinite coupling’s pressure is four-fifths of that at zero coupling. This can be a standard approach in solving puzzles of this nature. It also indicates there is a stronger relationship for space with lesser dimensions. 

 

Experiments show dramatic increase in solar cell output

Experiments show dramatic increase in solar cell output

In any conventional silicon-based solar cell, there is an absolute limit on overall efficiency, based partly on the fact that each photon of light can only knock loose a single electron, even if that photon carried twice the energy needed to do so. But now, researchers have demonstrated a method for getting high-energy photons striking silicon to kick out two electrons instead of one, opening the door for a new kind of solar cell with greater efficiency than was thought possible.

While conventional silicon cells have an absolute theoretical maximum efficiency of about 29.1 percent conversion of solar energy, the new approach, developed over the last several years by researchers at MIT and elsewhere, could bust through that limit, potentially adding several percentage points to that maximum output. The results are described today in the journal Nature, in a paper by graduate student Markus Einzinger, professor of chemistry Moungi Bawendi, professor of electrical engineering and computer science Marc Baldo, and eight others at MIT and at Princeton University.

The basic concept behind this new technology has been known for decades, and the first demonstration that the principle could work was carried out by some members of this team six years ago. But actually translating the method into a full, operational silicon solar cell took years of hard work, Baldo says.

That initial demonstration “was a good test platform” to show that the idea could work, explains Daniel Congreve PhD ’15, an alumnus now at the Rowland Institute at Harvard, who was the lead author in that prior report and is a co-author of the new paper. Now, with the new results, “we’ve done what we set out to do” in that project, he says.

The original study demonstrated the production of two electrons from one photon, but it did so in an organic photovoltaic cell, which is less efficient than a silicon solar cell. It turned out that transferring the two electrons from a top collecting layer made of tetracene into the silicon cell “was not straightforward,” Baldo says. Troy Van Voorhis, a professor of chemistry at MIT who was part of that original team, points out that the concept was first proposed back in the 1970s, and says wryly that turning that idea into a practical device “only took 40 years.”

The key to splitting the energy of one photon into two electrons lies in a class of materials that possess “excited states” called excitons, Baldo says: In these excitonic materials, “these packets of energy propagate around like the electrons in a circuit,” but with quite different properties than electrons. “You can use them to change energy — you can cut them in half, you can combine them.” In this case, they were going through a process called singlet exciton fission, which is how the light’s energy gets split into two separate, independently moving packets of energy. The material first absorbs a photon, forming an exciton that rapidly undergoes fission into two excited states, each with half the energy of the original state.

But the tricky part was then coupling that energy over into the silicon, a material that is not excitonic. This coupling had never been accomplished before.

As an intermediate step, the team tried coupling the energy from the excitonic layer into a material called quantum dots. “They’re still excitonic, but they’re inorganic,” Baldo says. “That worked; it worked like a charm,” he says. By understanding the mechanism taking place in that material, he says, “we had no reason to think that silicon wouldn’t work.”

What that work showed, Van Voorhis says, is that the key to these energy transfers lies in the very surface of the material, not in its bulk. “So it was clear that the surface chemistry on silicon was going to be important. That was what was going to determine what kinds of surface states there were.” That focus on the surface chemistry may have been what allowed this team to succeed where others had not, he suggests.

The key was in a thin intermediate layer. “It turns out this tiny, tiny strip of material at the interface between these two systems [the silicon solar cell and the tetracene layer with its excitonic properties] ended up defining everything. It’s why other researchers couldn’t get this process to work, and why we finally did.” It was Einzinger “who finally cracked that nut,” he says, by using a layer of a material called hafnium oxynitride.

The layer is only a few atoms thick, or just 8 angstroms (ten-billionths of a meter), but it acted as a “nice bridge” for the excited states, Baldo says. That finally made it possible for the single high-energy photons to trigger the release of two electrons inside the silicon cell. That produces a doubling of the amount of energy produced by a given amount of sunlight in the blue and green part of the spectrum. Overall, that could produce an increase in the power produced by the solar cell — from a theoretical maximum of 29.1 percent, up to a maximum of about 35 percent.

Actual silicon cells are not yet at their maximum, and neither is the new material, so more development needs to be done, but the crucial step of coupling the two materials efficiently has now been proven. “We still need to optimize the silicon cells for this process,” Baldo says. For one thing, with the new system those cells can be thinner than current versions. Work also needs to be done on stabilizing the materials for durability. Overall, commercial applications are probably still a few years off, the team says.

Other approaches to improving the efficiency of solar cells tend to involve adding another kind of cell, such as a perovskite layer, over the silicon. Baldo says “they’re building one cell on top of another. Fundamentally, we’re making one cell — we’re kind of turbocharging the silicon cell. We’re adding more current into the silicon, as opposed to making two cells.”

The researchers have measured one special property of hafnium oxynitride that helps it transfer the excitonic energy. “We know that hafnium oxynitride generates additional charge at the interface, which reduces losses by a process called electric field passivation. If we can establish better control over this phenomenon, efficiencies may climb even higher.” Einzinger says. So far, no other material they’ve tested can match its properties.

Materials provided by Massachusetts Institute of Technology

Diamond graphite structure

Researchers demonstrate quantum teleportation within a diamond

Researchers from the Yokohama National University have been successful in teleporting quantum information securely within the boundaries of a diamond. The implications of this study are huge in the field of quantum information technology. It defines how sensitive information can be safely shared and stored. The results of the study have been published in the journal Communications Physics

Hideo Kosaka, professor at Yokohama National University, also a co-author of the study said that the phenomenon of quantum teleportation permits the transfer of quantum information into space which cannot be normally accessed. In addition to this, it also permits the information transfer into quantum memory by not revealing or destroying the stored information. 

In the case of a diamond, this inaccessible space consisted of its carbon atoms. Since a diamond is composed of well linked, self-contained carbon atoms it has the most suitable ingredients for quantum teleportation to occur. 

In the nucleus of the carbon atom, there are six protons and six neutrons. This nucleus is surrounded by six spinning electrons. In the bonding of atoms to form a diamond a very strong lattice is formed. Although diamond can also have complex defects. When a nitrogen atom is located in one of the two adjacent vacancies where there should be a carbon atom, the defect is called a nitrogen-vacancy-center. 

When surrounded by carbon atoms, the researchers call the nucleus of a nitrogen atom a nanomagnet

For manipulation of the electron and carbon isotope in the vacancy, researchers attached a wire of the dimensions of a quarter width of a human hair to the diamond’s surface. Then the team applied a radio wave and microwave to the wire for making an oscillating magnetic field around the diamond. The microwave was manipulated for generating the optimal conditions for the transfer of quantum information in the diamond. 

Then Kosaka’s team used nitrogen nanomagnet for anchoring an electron. They forced the electron spin for entangling with a carbon nuclear spin by using radio and microwaves. The break down of the electron spin occurs under the magnetic field which has been created by the nanomagnet, making it susceptible to entanglement. 

After the entanglement of the pieces which mean that the physical characteristics cannot be described individually as they are very similar, a photon holding the quantum information is applied and is absorbed by the electron. The absorption allows the polarization state of the photon to be transferred into the carbon, which is mediated by the entangled electron, demonstrating a teleportation of information at the quantum level.

This method can take chunks of information, from one node to another in the quantum field. Researchers want to develop this method further to enable large scale quantum computation and meteorology. 

MIT Heat Collection

Getting more heat out of sunlight

A newly developed material that is so perfectly transparent you can barely see it could unlock many new uses for solar heat. It generates much higher temperatures than conventional solar collectors do — enough to be used for home heating or for industrial processes that require heat of more than 200 degrees Celsius (392 degrees Fahrenheit).

The key to the process is a new kind of aerogel, a lightweight material that consists mostly of air, with a structure made of silica (which is also used to make glass). The material lets sunlight pass through easily but blocks solar heat from escaping. The findings are described in the journal ACS Nano, in a paper by Lin Zhao, an MIT graduate student; Evelyn Wang, professor and head of the Department of Mechanical Engineering; Gang Chen, the Carl Richard Soderberg Professor in Power Engineering; and five others.

The key to efficient collection of solar heat, Wang explains, is being able to keep something hot internally while remaining cold on the outside. One way of doing that is using a vacuum between a layer of glass and a dark, heat-absorbing material, which is the method used in many concentrating solar collectors but is relatively expensive to install and maintain. There has been great interest in finding a less expensive, passive system for collecting solar heat at the higher temperature levels needed for space heating, food processing, or many industrial processes.

Aerogels, a kind of foam-like material made of silica particles, have been developed for years as highly efficient and lightweight insulating materials, but they have generally had limited transparency to visible light, with around a 70 percent transmission level. Wang says developing a way of making aerogels that are transparent enough to work for solar heat collection was a long and difficult process involving several researchers for about four years. But the result is an aerogel that lets through over 95 percent of incoming sunlight while maintaining its highly insulating properties.

The key to making it work was in the precise ratios of the different materials used to create the aerogel, which are made by mixing a catalyst with grains of a silica-containing compound in a liquid solution, forming a kind of gel, and then drying it to get all the liquid out, leaving a matrix that is mostly air but retains the original mixture’s strength. Producing a mix that dries out much faster than those in conventional aerogels, they found, produced a gel with smaller pore spaces between its grains, and that therefore scattered the light much less.

In tests on a rooftop on the MIT campus, a passive device consisting of a heat-absorbing dark material covered with a layer of the new aerogel was able to reach and maintain a temperature of 220 C, in the middle of a Cambridge winter when the outside air was below 0 C.

Such high temperatures have previously only been practical by using concentrating systems, with mirrors to focus sunlight onto a central line or point, but this system requires no concentration, making it simpler and less costly. That could potentially make it useful for a wide variety of applications that require higher levels of heat.

For example, simple flat rooftop collectors are often used for domestic hot water, producing temperatures of around 80 C. But the higher temperatures enabled by the aerogel system could make such simple systems usable for home heating as well, and even for powering an air conditioning system. Large-scale versions could be used to provide heat for a wide variety of applications in chemical, food production, and manufacturing processes.

Zhao describes the basic function of the aerogel layer as “like a greenhouse effect. The material we use to increase the temperature acts like the Earth’s atmosphere does to provide insulation, but this is an extreme example of it.”

For most purposes, the passive heat collection system would be connected to pipes containing a liquid that could circulate to transfer the heat to wherever it’s needed. Alternatively, Wang suggests, for some uses the system could be connected to heat pipes, devices that can transfer heat over a distance without requiring pumps or any moving parts.

Because the principle is essentially the same, an aerogel-based solar heat collector could directly replace the vacuum-based collectors used in some existing applications, providing a lower-cost option. The materials used to make the aerogel are all abundant and inexpensive; the only costly part of the process is the drying, which requires a specialized device called a critical point dryer to allow for a very precise drying process that extracts the solvents from the gel while preserving its nanoscale structure.

Because that is a batch process rather than a continuous one that could be used in roll-to-roll manufacturing, it could limit the rate of production if the system is scaled up to industrial production levels. “The key to scaleup is how we can reduce the cost of that process,” Wang says. But even now, a preliminary economic analysis shows that the system can be economically viable for some uses, especially in comparison with vacuum-based systems.

The research team included research scientist Bikram Bhatia, postdoc Sungwoo Yang, graduate student Elise Strobach, instructor Lee Weinstein and postdoc Thomas Cooper. The work was primarily funded by the U.S. Department of Energy’s ARPA-E program.

Materials provided by Massachusetts Institute of Technology

light rays

Researchers discover a property of light never predicted before

A team of researchers from different institutions in Spain and the USA notified that they have found a brand new property of light known as self-torque. The team’s research paper explained how they came to know about the property and what are the uses of it and their research paper was published in the journal Science.

Researchers knew about the properties of light such as wavelength before. By the property known as angular momentum scientists in the recent times came to know that lights can also be twisted. Vortex beams are that kind of beams that have highly structured angular momentum. They also have orbital angular momentum (OAM).

Vortex beams seem to be a helix which is surrounding a common center and when the beams strike a surface which is flat, they appear as doughnut-shaped. The scientists came to know about the new property of light while researching on OAM beams and while researching they found it out surprisingly where the lights were behaving in a way they had never noticed before. The scientists conducted a lot of experiments and the experiments were to fire two laser lights at the cloud formed by argon gas and by doing this it forced the beams to overlap. Thus they joined and they were emitted as a single beam from the other side of the argon cloud. The beam which resulted was a type of vortex beam.

The scientists then thought what would have happened if the lasers were a little bit out of synchronization and if the lasers had different orbital angular momentum.  After this experiment emerged a beam which looked like a corkscrew with a twist which was gradually changing. When the beam hit the flat surface it looked similar to a crescent moon.

The researchers also noticed that it can be looked the other way round as a single photon which was at the front of the beam was revolving around its center slower than a photon which was at the back of the beam. The scientists named this new property as self-torque. This is not only a newly discovered property of light, but it is also the one which was never ever predicted. This technique can be used to modulate the orbital angular momentum of light in a way very similar to that of modulating frequencies in communicating tools. This could lead to the development of devices that are used to manipulate tiny materials.

How to bend waves to arrive at the right place

How to bend waves to arrive at the right place

In free space, the light wave of a laser beam propagates on a perfectly straight line. Under certain circumstances, however, the behavior of a wave can be much more complicated. In the presence of a disordered, irregular environment a very strange phenomenon occurs: An incoming wave splits into several paths, it branches in a complicated way, reaching some places with high intensity, while avoiding others almost completely.

This kind of “branched flow” has first been observed in 2001. Scientists at TU Wien (Vienna) have now developed a method to exploit this effect. The core idea of this new approach is to send a wave signal exclusively along one single pre-selected branch, such that the wave is hardly noticeable anywhere else. The results have now been published in the journal PNAS.

From Quantum Particles to Tsunamis

“Originally, this effect was discovered when studying electrons moving as quantum waves through tiny microstructures,” says Prof. Stefan Rotter from the Institute of Theoretical Physics at TU Wien. “Such structures, however, are never perfect and they always come with certain imperfections; and surprisingly, these imperfections cause the electron wave to split up into branches – an effect which is called branched flow.”

Soon it turned out that this wave phenomenon does not only occur in quantum physics. In principle it can occur with all types of waves and on completely different length scales. If, for example, laser beams are sent into the surface of a soap bubble, they split into several partial beams, just like tsunami waves in the ocean: the latter do not spread regularly across the ocean, but instead they travel in a complicated, branched pattern that depends on the random shape of the corrugated ocean sea bed. As a result, it can happen that a distant island is hit very hard by a tsunami, while the neighboring island is only reached by much weaker wave fronts.

“We wanted to know whether these waves can be manipulated in such a way that they only travel along one single selected branch, instead of propagating along a whole branched network of paths in completely different directions”, says Andre Brandstötter (TU Wien), first author of the publication. “And as it turns out, it is indeed possible to target individual branches in a controlled way.”

Analyze and Adapt

The new procedure takes only two steps: First, the wave is allowed to branch out on all possible paths as usual. At one of the locations that are reached with high intensity, the wave is measured in detail. The method developed at the TU Wien can then be used to calculate how the wave has to be shaped at the origin, so that in the second step it can be sent along one selected path, while avoiding all other paths.

“We used numerical simulations to show how to find a wave that behaves exactly the way we want it to. This approach can be applied using a variety of different methods,” says Stefan Rotter. “You can implement it with light waves that are adjusted with special mirror systems or with sound waves that you generate with a system of coupled loudspeakers. Sonar waves in the ocean would also be a possible field of application. In any case, the necessary technologies are already available.”

With this new method, all these different types of waves could be sent out along a single trajectory pre-selected from a complex network of paths. “This trajectory doesn’t even have to be straight,” explains Andre Brandstötter. “Many of the possible paths are curved – the irregularities of the surroundings act like a set of lenses by which the wave is focused and deflected again and again.”

Even pulsed signals can be sent along these special paths, such that information can be transmitted in a targeted manner. This guarantees that a wave signal arrives exactly where it is supposed to be received; at other locations it can hardly be detected, which makes eavesdropping much more difficult.

Original publication:

A. Brandstötter et al., Shaping the branched flow of light through disordered media, PNAS (2019)

Materials provided by TU Wien

galactic clusters plasma

Scientists use X-rays from faraway galaxy cluster to reveal secrets of plasma

Most visible matter in the universe doesn’t look like our textbook picture of a nucleus surrounded by tethered electrons. Out beyond our borders, inside massive clusters, galaxies swim in a sea of plasma—a form of matter in which electrons and nuclei wander unmoored.

Though it makes up the majority of the visible matter in the universe, this plasma remains poorly understood; scientists do not have a theory that fully describes its behavior, especially at small scales.

However, a University of Chicago astrophysicist led a study that provides a brand-new glimpse of the small-scale physics of such plasma. Using NASA’s Chandra X-ray Observatory, scientists took a detailed look at the plasma in a distant galaxy cluster and discovered the flow of plasma is much less viscous than expected and, therefore, turbulence occurs on relatively small scales—an important finding for our numerical models of the largest objects in the universe.

“High-resolution X-ray observations allowed us to learn some surprising truths about the viscosity of these plasmas,” said Irina Zhuravleva, an assistant professor of astrophysics and first author of the study, published June 17 in Nature Astronomy. “One might expect that variations in density that arise in the plasma are quickly erased by viscosity; however, we saw the opposite—the plasma finds ways to maintain them.”

Scattered around the universe are massive clusters of galaxies, some of them millions of light-years across containing thousands of galaxies. They sit in a type of plasma that we cannot recreate on Earth. It is extremely sparse—on the order of a sextillion times less dense than air on Earth—and has very weak magnetic fields, tens of thousands of times weaker than we experience on the Earth’s surface. To study this plasma, therefore, scientists must rely on cosmic laboratories such as clusters of galaxies.

Scientists used NASA’s Chandra X-ray Observatory to take a detailed look at the plasma in a distant galaxy cluster. (Courtesy of NASA/CXC/SAO)

Zhuravleva and the team chose a relatively nearby galaxy cluster called the Coma Cluster, a gigantic, bright cluster made up of more than 1,000 galaxies. They chose a less dense region away from the cluster center, where they hoped to be able to capture the average distance that particles travel between interactions with NASA’s Chandra X-ray Observatory. In order to build a high-quality map of the plasma, they observed the Coma cluster for almost 12 days—much longer than a typical observing run.

One thing that jumped out was how viscous the plasma was—how easily it’s stirred. “One could expect to see the viscosity resisting chaotic motions of plasma as we zoom in to smaller and smaller scales,” Zhuravleva said. But that didn’t happen; the plasma was clearly turbulent even on such small scales.

“It turned out that plasma behavior is more similar to the swirling motions of milk stirred in a coffee mug than the smoother ones that honey makes,” she said.

Such low viscosity means that microscopic processes in plasma cause small irregularities in the magnetic field, causing particles to collide more frequently and making the plasma less viscous. Alternately, Zhuravleva said, viscosity could be different along and perpendicular to magnetic field lines.

Understanding the physics of such plasmas is essential for improving our models of how galaxies and galaxy clusters form and evolve with time.

“Plasma behavior is more similar to the swirling motions of milk stirred in a coffee mug than the smoother ones that honey makes.”—Asst. Prof. Irina Zhuravleva Click To Tweet

“It is exciting that we were able to use observations of clusters of galaxies to understand fundamental properties of intergalactic plasmas,” said Zhuravleva. “Our observations confirm that clusters are great laboratories that can sharpen theoretical views on plasmas.”

Materials provided by the University of Chicago

Electron-behaving nanoparticles rock current understanding of matter

Electron-behaving nanoparticles rock current understanding of matter

It’s not an electron. But it sure does act like one.

Northwestern University researchers have made a strange and startling discovery that nanoparticles engineered with DNA in colloidal crystals — when extremely small — behave just like electrons. Not only has this finding upended the current, accepted notion of matter, it also opens the door for new possibilities in materials design.

“We have never seen anything like this before,” said Northwestern’s Monica Olvera de la Cruz, who made the initial observation through computational work. “In our simulations, the particles look just like orbiting electrons.”

With this discovery, the researchers introduced a new term called “metallicity,” which refers to the mobility of electrons in a metal. In colloidal crystals, tiny nanoparticles roam similarly to electrons and act as a glue that holds the material together.

“This is going to get people to think about matter in a new way,” said Northwestern’s Chad Mirkin, who led the experimental work. “It’s going to lead to all sorts of materials that have potentially spectacular properties that have never been observed before. Properties that could lead to a variety of new technologies in the fields of optics, electronics and even catalysis.”

The paper will be published on Friday, June 21 in the journal Science.

Olvera de la Cruz is the Lawyer Taylor Professor of Materials Science and Engineering in Northwestern’s McCormick School of Engineering. Mirkin is the George B. Rathmann Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences.

Mirkin’s group previously invented the chemistry for engineering colloidal crystals with DNA, which has forged new possibilities for materials design. In these structures, DNA strands act as a sort of smart glue to link together nanoparticles in a lattice pattern

“Over the past two decades, we have figured out how to make all sorts of crystalline structures where the DNA effectively takes the particles and places them exactly where they are supposed to go in a lattice,” said Mirkin, founding director of the International Institute for Nanotechnology.

In these previous studies, the particles’ diameters are on the tens of nanometers length scale. Particles in these structures are static, fixed in place by DNA. In the current study, however, Mirkin and Olvera de la Cruz shrunk the particles down to 1.4 nanometers in diameter in computational simulations. This is where the magic happened.

“The bigger particles have hundreds of DNA strands linking them together,” said Olvera de la Cruz. “The small ones only have four to eight linkers. When those links break, the particles roll and migrate through the lattice holding together the crystal of bigger particles.”

When Mirkin’s team performed the experiments to image the small particles, they found that Olvera de la Cruz’s team’s computational observations proved true. Because this behaviour is reminiscent of how electrons behave in metals, the researchers call it “metallicity.”

“A sea of electrons migrates throughout metals, acting as a glue, holding everything together,” Mirkin explained. “That’s what these nanoparticles become. The tiny particles become the mobile glue that holds everything together.”

Olvera de la Cruz and Mirkin next plan to explore how to exploit these electron-like particles in order to design new materials with useful properties. Although their research used gold nanoparticles, Olvera de la Cruz said “metallicity” applies to other classes of particles in colloidal crystals.

“In science, it’s really rare to discover a new property, but that’s what happened here,” Mirkin said. “It challenges the whole way we think about building matter. It’s a foundational piece of work that will have a lasting impact.”

Materials provided by Northwestern University

Electric Drop

A droplet walks into an electric field …

When a raindrop falls through a thundercloud, it is subject to strong electric fields that pull and tug on the droplet, like a soap bubble in the wind. If the electric field is strong enough, it can cause the droplet to burst apart, creating a fine, electrified mist.

Scientists began taking notice of how droplets behave in electric fields in the early 1900s, amid concerns over lightning strikes that were damaging newly erected power lines. They soon realized that the power lines’ own electric fields were causing raindrops to burst around them, providing a conductive path for lightning to strike. This revelation led engineers to design thicker coverings around power lines to limit lightning strikes.

Today, scientists understand that the stronger the electric field, the more likely it is that a droplet within it will burst. But, calculating the exact field strength that will burst a particular droplet has always been an involved mathematical task.

Now, MIT researchers have found that the conditions for which a droplet bursts in an electric field all boil down to one simple formula, which the team has derived for the first time.

With this simple new equation, the researchers can predict the exact strength an electric field should be to burst a droplet or keep it stable. The formula applies to three cases previously analyzed separately: a droplet pinned on a surface, sliding on a surface, or free-floating in the air.

Their results, published today in the journal Physical Review Letters, may help engineers tune the electric field or the size of droplets for a range of applications that depend on electrifying droplets. These include technologies for air or water purification, space propulsion, and molecular analysis.

“Before our result, engineers and scientists had to perform computationally intensive simulations to assess the stability of an electrified droplet,” says lead author Justin Beroz, a graduate student in MIT’s departments of Mechanical Engineering and Physics. “With our equation, one can predict this behavior immediately, with a simple paper-and-pencil calculation. This is of great practical benefit to engineers working with, or trying to design, any system that involves liquids and electricity.”

Beroz’ co-authors are A. John Hart, associate professor of mechanical engineering, and John Bush, professor of mathematics.

“Something unexpectedly simple”

Droplets tend to form as perfect little spheres due to surface tension, the cohesive force that binds water molecules at a droplet’s surface and pulls the molecules inward. The droplet may distort from its spherical shape in the presence of other forces, such as the force from an electric field. While surface tension acts to hold a droplet together, the electric field acts as an opposing force, pulling outward on the droplet as charge builds on its surface.

“At some point, if the electric field is strong enough, the droplet can’t find a shape that balances the electrical force, and at that point, it becomes unstable and bursts,” Beroz explains.

He and his team were interested in the moment just before bursting, when the droplet has been distorted to its critically stable shape. The team set up an experiment in which they slowly dispensed water droplets onto a metal plate that was electrified to produce an electric field, and used a high-speed camera to record the distorted shapes of each droplet.

“The experiment is really boring at first — you’re watching the droplet slowly change shape, and then all of a sudden it just bursts,” Beroz says.

After experimenting on droplets of different sizes and under various electric field strengths, Beroz isolated the video frame just before each droplet burst, then outlined its critically stable shape and calculated several parameters such as the droplet’s volume, height, and radius. He plotted the data from each droplet and found, to his surprise, that they all fell along an unmistakably straight line.

“From a theoretical point of view, it was an unexpectedly simple result given the mathematical complexity of the problem,” Beroz says. “It suggested that there might be an overlooked, yet simple, way to calculate the burst criterion for the droplets.”

A water droplet, subject to an electric field of slowly increasing strength, suddenly bursts by emitting a fine, electrified mist from its apex.

Volume above height

Physicists have long known that a liquid droplet in an electric field can be represented by a set of coupled nonlinear differential equations. These equations, however, are incredibly difficult to solve. To find a solution requires determining the configuration of the electric field, the shape of the droplet, and the pressure inside the droplet, simultaneously.

“This is commonly the case in physics: It’s easy to write down the governing equations but very hard to actually solve them,” Beroz says. “But for the droplets, it turns out that if you choose a particular combination of physical parameters to define the problem from the start, a solution can be derived in a few lines. Otherwise, it’s impossible.”

Physicists who attempted to solve these equations in the past did so by factoring in, among other parameters, a droplet’s height — an easy and natural choice for characterizing a droplet’s shape. But Beroz made a different choice, reframing the equations in terms of a droplet’s volume rather than its height. This was the key insight for reformulating the problem into an easy-to-solve formula.

“For the last 100 years, the convention was to choose height,” Beroz says. “But as a droplet deforms, its height changes, and therefore the mathematical complexity of the problem is inherent in the height. On the other hand, a droplet’s volume remains fixed regardless of how it deforms in the electric field.”

By formulating the equations using only parameters that are “fixed” in the same sense as a droplet’s volume, “the complicated, unsolvable parts of the equation cancel out, leaving a simple equation that matches the experimental results,” Beroz says.

Specifically, the new formula the team derived relates five parameters: a droplet’s surface tension, radius, volume, electric field strength, and the electric permittivity of the air surrounding the droplet. Plugging any four of these parameters into the formula will calculate the fifth.

Beroz says engineers can use the formula to develop techniques such as electrospraying, which involves the bursting of a droplet maintained at the orifice of an electrified nozzle to produce a fine spray. Electrospraying is commonly used to aerosolize biomolecules from a solution, so that they can pass through a spectrometer for detailed analysis. The technique is also used to produce thrust and propel satellites in space.

“If you’re designing a system that involves liquids and electricity, it’s very practical to have an equation like this, that you can use every day,” Beroz says.

Materials provided by Massachusetts Institute of Technology