Connect with us

Quantum

Physicists Chip Away at a Mystery: Why Does Glass Exist?

Published

on

In 2008, Miguel Ramos read in the newspaper that 110-million-year-old amber bearing pristine Mesozoic insects had been discovered a few hours’ drive from Madrid, where he lived. A physicist who specializes in glass, Ramos had wanted for years to get his hands on ancient amber. He contacted the paleontologists working at the site, who invited him to visit.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research develop­ments and trends in mathe­matics and the physical and life sciences.

“They provided me with the clear samples that are not good for them,” he said. “They have no interesting insects or whatever … but they are perfect for me.”

Ramos spent the next several years intermittently working on measurements of the ancient glass. He hoped that the fossilized tree resin, after aging for so long, might approach a hypothetical form of matter known as ideal glass.

For decades, physicists have dreamed of this perfect amorphous solid. They desire ideal glass not so much for its own sake (though it would have unique, useful properties) but because its existence would solve a deep mystery. It’s the mystery posed by every window and mirror, every piece of plastic and hard candy, and even the cytoplasm that fills every cell. All of these materials are technically glass, for glass is anything that’s solid and rigid but made of disordered molecules like those in a liquid. Glass is a liquid in suspended animation, a liquid whose molecules curiously cannot flow. Ideal glass, if it exists, would tell us why.

Samples of amber in Ramos’s lab.Photograph: James Rajotte/Quanta Magazine

Inconveniently, ideal glass would take so long to form that it may not have done so in all of cosmic history. Physicists can only seek indirect evidence that, given unlimited time, it would. Ramos, an experimental physicist at the Autonomous University of Madrid, hoped that after 110 million years of aging, the Spanish amber might have started to show glimmers of perfection. If so, he would know what the molecules in ordinary glass are really doing when they appear to do nothing.

Ramos’s amber measurements are part of a surge of interest in ideal glass. In the past few years, new methods of making glass and simulating it on computers have led to unexpected progress. Major clues have emerged about the nature of ideal glass and its connection to ordinary glass. “These studies provide renewed support for the hypothesis of the existence of an ideal-glass state,” said Ludovic Berthier, a physicist at the University of Montpellier who was centrally involved in the recent computer simulations.

But the emerging picture of ideal glass only makes sense if we set aside one piece of evidence.

“Indeed,” Berthier said, “the amber work stands out as difficult to rationalize.”

The Paradox of Glass

When you cool a liquid, it will either crystallize or harden into glass. Which of the two happens depends on the substance and on the subtleties of the process that glassblowers have learned through trial and error over thousands of years. “Avoiding crystallization is a dark art,” said Paddy Royall, a glass physicist at the University of Bristol in the United Kingdom.

The two options differ greatly.

Crystallization is a dramatic switch from the liquid phase, in which molecules are disordered and free flowing, to the crystal phase, in which molecules are locked in a regular, repeating pattern. Water freezes into ice at zero degrees Celsius, for instance, because the H2O molecules stop jiggling around just enough at that temperature to feel each other’s forces and fall into lockstep.

Other liquids, when cooled, more easily become glass. Silica, for example—window glass—starts as a molten liquid well above 1,000 degrees Celsius; as it cools, its disordered molecules contract slightly, crowding a bit closer together, which makes the liquid increasingly viscous. Eventually, the molecules stop moving altogether. In this gradual glass transition, the molecules don’t reorganize. They simply grind to a halt.

Illustration: Lucy Reading-Ikkanda/Quanta Magazine

Exactly why the cooling liquid hardens remains unknown. If the molecules in glass were simply too cold to flow, it should still be possible to squish them into new arrangements. But glass doesn’t squish; its jumbled molecules are truly rigid, despite looking the same as molecules in a liquid. “Liquid and glass have the same structure, but behave differently,” said Camille Scalliet, a glass theorist at the University of Cambridge. “Understanding that is the main question.”

Advertisement

A clue came in 1948, when a young chemist named Walter Kauzmann noticed what became known as the entropy crisis, a glassy paradox that later researchers realized ideal glass could resolve.

Kauzmann knew that the more slowly you cool a liquid, the more you can cool it before it transitions into glass. And slower-formed glass ends up denser and more stable, because its molecules had longer to shuffle around (while the liquid was still viscous) and find tighter, lower-energy arrangements. Measurements indicated a corresponding reduction in the entropy, or disorder, of the slower-formed glass—fewer ways its molecules could be arranged with the same low energy.

Extrapolating the trend, Kauzmann realized that if you could cool a liquid slowly enough, you could cool it all the way down to a temperature now known as the Kauzmann temperature before it fully hardened. At that temperature, the resulting glass would have an entropy as low as that of a crystal. But crystals are neat, orderly structures. How could glass, disordered by definition, possess equal order?

No ordinary glass could, which implied that something special must happen at the Kauzmann temperature. Crisis would be avoided if a liquid, upon reaching that temperature, attained the ideal-glass state—the densest possible random packing of molecules. Such a state would exhibit “long-range amorphous order,” where each molecule feels and affects the position of every other, so that in order to move, they must move as one. The hidden long-range order of this putative state could rival the more obvious orderliness of a crystal. “That observation right there was at the heart of why people thought there should be an ideal glass,” said Mark Ediger, a chemical physicist at the University of Wisconsin, Madison.

According to this theory, first advanced by Julian Gibbs and Edmund DiMarzio in 1958, ideal glass is a true phase of matter, akin to the liquid and crystal phases. The transition to this phase just takes too long, requiring too slow a cooling process, for scientists to ever see. The ideal-glass transition is “masked,” said Daniel Stein, a condensed matter physicist at New York University, by the liquid becoming “so viscous that everything is arrested.”

“It’s sort of like looking through a glass darkly,” Stein said. “We can’t get to [ideal glass] or see it. But we can theoretically try to create accurate models of what’s going on there.”

A New Glass

Unexpected help has come from experiments. There was never any hope of forming ideal glass by cooling a liquid, the glassmaking method humans have used for millennia. You’d have to cool a liquid impossibly slowly—perhaps even infinitely slowly—to keep it from hardening before it hit the Kauzmann temperature. But in 2007, Ediger, the Wisconsin physicist, developed a new method of glassmaking. “We figured out there was another way to make glasses that are high density and close to the ideal-glass state by a completely different route,” he said.

Ediger and his team discovered that they could create “ultra-stable glasses” that exist in a state somewhere between ordinary and ideal. Using a method called vapor deposition, they dropped molecules one by one onto a surface as if they were playing Tetris, allowing each molecule to settle into its snuggest fit in the forming glass before the next molecule came down. The resulting glass was denser, more stable, and lower in entropy than all of the glasses throughout human history. “These materials have the properties that you would expect if you took a liquid and cooled it over the course of a million years,” Ediger said.

Another property of ultra-stable glass would eventually reveal the most promising road map to ideal glass.

Advertisement

Two groups, one of them led by Miguel Ramos in Madrid, identified that property in 2014, when they found that ultra-stable glass departs from a universal characteristic of all ordinary glass.

Vapor-deposited glass can have different properties depending on the temperature at which it is created. In this sample, researchers maintained a temperature gradient across the sample, which led to the rainbow effect. The ultrastable glass is toward the middle of the sample.Photograph: Diane Walters/University of Wisconsin-Madison

Physicists have known for decades that ultra-cold glass has a high heat capacity—the amount of heat needed to raise its temperature. Glass can take way more heat than a crystal can near absolute zero, with a heat capacity that’s directly proportional to the temperature.

Theorists including Phil Anderson, the revered Nobel Prize-winning condensed matter physicist, suggested an explanation in the early 1970s. They argued that glass contains many “two-level systems,” little clusters of atoms or molecules that can slip back and forth between two alternative, equally stable configurations. “You can imagine a whole bunch of atoms kind of shifting from one configuration to a very slightly different configuration,” said Frances Hellman of the University of California, Berkeley, “which just doesn’t exist in a crystalline material.”

Although the atoms or molecules are too boxed in by their neighbors to do much switching on their own, at room temperature, heat activates the two-level systems, providing the atoms with the energy they need to shuffle around. This activity diminishes as the glass’s temperature drops. But near absolute zero, quantum effects become important: Groups of atoms in the glass can quantum mechanically “tunnel” between the alternative configurations, passing right through any obstacles, and even occupy both levels of the two-level system at once. The tunneling absorbs a lot of heat, producing glass’s characteristic high heat capacity.

Several years after Ediger figured out how to make ultra-stable glass, Hellman’s group at Berkeley and Ramos’ in Madrid independently set out to study whether it might depart from that universal heat capacity near absolute zero. In their respective experiments, they probed the low-temperature properties of ultra-stable silicon and ultra-stable indomethacin (a chemical that’s also used as an anti-inflammatory drug). Sure enough, they found that both glasses had far lower heat capacity than usual near absolute zero, in line with a crystal’s. This suggested that ultra-stable glass has fewer two-level systems to tunnel between. The molecules are in especially snug configurations with few competitors.

Ramos cools amber down to temperatures close to absolute zero to test how closely it approaches the state of ideal glass.Photograph: James Rajotte/Quanta Magazine
Advertisement

If ultra-stable glass’s exceptionally low heat capacity really does come from having fewer two-level systems, then ideal glass naturally corresponds to the state with no two-level systems at all. “It’s just perfectly, somehow, positioned where all the atoms are disordered—it doesn’t have a crystal structure—but there’s nothing moving at all,” said David Reichman, a theorist at Columbia University.

Furthermore, the drive toward this state of perfect long-range amorphous order, where each molecule affects the positions of all others, could be what causes liquids to harden into the glass we see (and see through) all around us.

In this emerging picture, when a liquid becomes a glass, it’s actually attempting to transition to the ideal-glass phase, drawn by a fundamental pull toward long-range order. The ideal glass is the endpoint, Royall said, but as the molecules try to crowd closer together, they get stuck; the increasing viscosity prevents the system from ever reaching the desired state.

Recently, groundbreaking computer simulations were used to test these ideas. Simulating ultra-stable glass on a computer used to be infeasible because of the extraordinary computing time required for the simulated molecules to crowd together. Two years ago, though, Berthier found a trick that allowed him to speed up the process by a factor of 1 trillion. His algorithm picks two particles at random and swaps their positions. These shake-ups help the simulated liquid stay unstuck, allowing molecules to settle into snugger fits—just as the ability to swap two ill-fitting shapes would help in Tetris.

In a paper that’s under review for publication in Physical Review Letters, Berthier, Scalliet, Reichman and two co-authors reported that the more stable the simulated glass, the fewer two-level systems it has. As with Hellman’s and Ramos’ heat capacity measurements, the computer simulations suggest that two-level systems—competing configurations of groups of molecules—are the source of glass’s entropy. The fewer of these alternative states there are, the more stability and long-range order an amorphous solid has, and the closer it is to ideal.

The theorists Vassiliy Lubchenko of the University of Houston and Peter Wolynes of Rice University suggested back in 2007 that ideal glass should have no two-level systems. “I’m quite happy with Berthier’s result,” Wolynes said by email.

The Amber Anomaly

But then there’s that amber.

Ramos and his collaborators published their comparisons of old and “rejuvenated” samples of the yellow glass in Physical Review Letters in 2014. They found that the 110-million-year-old amber had grown about 2 percent denser, in line with ultra-stable glass. This should suggest that the amber had indeed stabilized over time, as little groups of molecules slipped, one by one, into lower-energy arrangements.

But when the Madrid team cooled the ancient glass nearly to absolute zero and measured its heat capacity, the results told a different story. The aged amber had the same high heat capacity as new amber—and all other ordinary glass. Its molecules seemed to be tunneling between just as many two-level systems as usual.

Why didn’t the number of two-level systems drop over time as the amber stabilized and became denser? The findings don’t fit.

“I really like the experiments on amber, but making an amber glass is sort of a messy process,” said Ediger, the originator of the vapor-deposition method. “It’s basically tree sap that over time chemically changes and solidifies as well as ages.” He thinks impurities in the Spanish amber might have sullied the heat capacity measurements.

Researchers plan to do further experiments on amber, as well as lab-made and simulated glass, hoping to uncover more details of two-level systems and to get closer to the putative ideal state. Reichman noted that it may never be possible to prove its existence with complete certainty. “Maybe one day we will know, at least on the computer, how to precisely pack particles in a way that would be the ideal glass we are looking for,” he said. “But we would then have to wait a very long time—too long—to see if it remains stable.”

Editor’s Note: Ludovic Berthier and David Reichman have received funding from the Simons Foundation, which also supports Quanta, an editorially independent publication. Simons Foundation funding plays no role in their coverage.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.


Read more: https://www.wired.com/story/physicists-chip-away-at-a-mystery-why-does-glass-exist/

Quantum

Achieving superlubricity with graphene

Published

on

Sometimes, experimental results spark enormous curiosity inspiring a myriad of questions and ideas for further experimentation. In 2004, Geim and Novoselov, from The University of Manchester, isolated a single layer of graphene from bulk graphite with the “Scotch Tape Method” for which they were awarded the 2010 Nobel Prize in Physics.  This one experimental result has branched out countless times serving as a source of inspiration in as many different fields.  We are now in the midst of an array of branching-out in graphene research, and one of those branches gaining attention is ultra low friction observed between graphene and other surface materials.  

Much has been learned about graphene in the past 15 years through an immense amount of research, most of which, in non-mechanical realms (e.g., electron transport measurements, thermal conductivity, pseudo magnetic fields in strain engineering).  However, superlubricity, a mechanical phenomenon, has become the focus among many research groups. Mechanical measurements have famously shown graphene’s tensile strength to be hundreds of times that of the strongest steel, indisputably placing it atop the list of construction materials best for a superhero suit.  Superlubricity is a tribological property of graphene and is, arguably, as equally impressive as graphene’s tensile strength.

Tribology is the study of interacting surfaces during relative motion including sources of friction and methods for its reduction.  It’s not a recent discovery that coating a surface with graphite (many layers of graphene) can lower friction between two sliding surfaces.  Current research studies the precise mechanisms and surfaces for which to minimize friction with single or several layers of graphene. 

Research published in Nature Materials in 2018 measures friction between surfaces under constant load and velocity. The experiment includes two groups; one consisting of two graphene surfaces (homogeneous junction), and another consisting of graphene and hexagonal boron nitride (heterogeneous junction).   The research group measures friction using Atomic Force Microscopy (AFM).  The hexagonal boron nitride (or graphene for a homogeneous junction) is fixed to the stage of the AFM while the graphene slides atop.  Loads are held constant at 20 𝜇N and sliding velocity constant at 200 nm/s. Ultra low friction is observed for homogeneous junctions when the underlying crystalline lattice structures of the surfaces are at a relative angle of 30 degrees.  However, this ultra low friction state is very unstable and upon sliding, the surfaces rotate towards a locked-in lattice alignment. Friction varies with respect to the relative angle between the two surface’s crystalline lattice structures. Minimum (ultra low) friction occurs at a relative angle of 30 degrees reaching a maximum when locked-in lattice alignment is realized upon sliding. While in a state of lattice alignment, shearing is rendered impossible with the experimental setup due to the relatively large amount of friction.

Friction varies with respect to the relative angle of the crystalline lattice structures and is, therefore, anisotropic.  For example, the fact it takes less force to split wood when an axe blade is applied parallel to its grains than when applied perpendicularly illustrates the anisotropic nature of wood, as the force to split wood is dependent upon the direction along which the force is applied.  Frictional anisotropy is greater in homogeneous junctions because the tendency to orient into a stuck, maximum friction alignment, is greater than with heterojunctions.  In fact, heterogeneous junctions experience frictional anisotropy three orders of magnitude less than homogeneous junctions. Heterogenous junctions display much less frictional anisotropy due to a lattice misalignment when the angle between the lattice vectors is at a minimum.  In other words, the graphene and hBN crystalline lattice structures are never parallel because the materials differ, therefore, never experience the impact of lattice alignment as do homogenous junctions. Hence, heterogeneous junctions do not become stuck in a high friction state that characterizes homogeneous ones, and experience ultra low friction during sliding at all relative crystalline lattice structure angles.

Presumably, to increase applicability, upscaling to much larger loads will be necessary. A large scale cost effective method to dramatically reduce friction would undoubtedly have an enormous impact on a great number of industries.  Cost efficiency is a key component to the realization of graphene’s potential impact, not only as it applies to superlubricity, but in all areas of application.  As access to large amounts of affordable graphene increases, so will experiments in fabricating devices exploiting the extraordinary characteristics which have placed graphene and graphene based materials on the front lines of material research the past couple decades.

Source: https://quantumfrontiers.com/2020/03/24/achieving-superlubricity-with-graphene/

Continue Reading

Quantum

Erratum: Analytic model of the energy spectrum of a graphene quantum dot in a perpendicular magnetic field [Phys. Rev. B 78, 195427 (2008)]

Published

on

COVID-19 has impacted many institutions and organizations around the world, disrupting the progress of research. Through this difficult time APS and the Physical Review editorial office are fully equipped and actively working to support researchers by continuing to carry out all editorial and peer-review functions and publish research in the journals as well as minimizing disruption to journal access.

We appreciate your continued effort and commitment to helping advance science, and allowing us to publish the best physics journals in the world. And we hope you, and your loved ones, are staying safe and healthy.

Source: http://link.aps.org/doi/10.1103/PhysRevB.95.039901

Continue Reading

Quantum

Erratum: More realistic Hamiltonians for the fractional quantum Hall regime in GaAs and graphene [Phys. Rev. B 87, 245129 (2013)]

Published

on

COVID-19 has impacted many institutions and organizations around the world, disrupting the progress of research. Through this difficult time APS and the Physical Review editorial office are fully equipped and actively working to support researchers by continuing to carry out all editorial and peer-review functions and publish research in the journals as well as minimizing disruption to journal access.

We appreciate your continued effort and commitment to helping advance science, and allowing us to publish the best physics journals in the world. And we hope you, and your loved ones, are staying safe and healthy.

Source: http://link.aps.org/doi/10.1103/PhysRevB.92.159902

Continue Reading
Blockchain41 mins ago

Interactive Brokers’ Clients Made Fewer Trades Than April and March

Blockchain44 mins ago

Bitcoin Is a Peaceful Protest: Crypto Leaders On The Minneapolis Riots Following George Floyd’s Death

Blockchain53 mins ago

Cardano’s upcoming Shelley launch may spur price

Cannabis1 hour ago

Is Cannabis Recession-Proof? We’re About to Find Out.

Blockchain1 hour ago

Cash or Plastic? Countries Where Crypto Debit Cards Are Fair Game

Blockchain1 hour ago

Bitcoin Price Prediction: BTC/USD Stabilizes Above $9,500 As The Bulls Struggling To Conquer $9,800

Blockchain1 hour ago

Exclusive: Binance Korea to Integrate Coinfirm Real-Time AML Monitoring

Blockchain1 hour ago

Moderna (MRNA) Shares Up 0.37% amid Investors’ Concerns about Stock Sales

Cannabis1 hour ago

Louisiana OKs medical marijuana reform, but minimal market boost seen

Blockchain1 hour ago

Leading US Crypto Exchange Bittrex Lists WINGS

Cannabis1 hour ago

Star signs and cannabis strains: June 2020 horoscopes

Blockchain1 hour ago

Litecoin, ETC, HBAR: Price Analysis, June 1

Blockchain1 hour ago

Twitter’s Bitcoin Sentiment Suggests a Price Breakout is Imminent: Here’s Why

Blockchain2 hours ago

Miners Have Been Selling More Bitcoin Than They Generate, Recent Data Suggests

Blockchain2 hours ago

Tether Integrates with OMG Sidechain to Decrease Load on Ethereum

Blockchain2 hours ago

Swiss Bank Maerki Baumann Launches Crypto Custody and Trading

Blockchain2 hours ago

Chainlink, Ethereum, Cardano and DigiByte Among 27 Altcoins Outperforming Bitcoin (BTC) in 2020

Cannabis2 hours ago

Riots in Los Angeles Affecting Weed Delivery

Blockchain2 hours ago

How to Learn Any Skill Fast

Blockchain2 hours ago

Leading US Crypto Exchange Bittrex Global Lists WINGS

Trending