Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Experiments explain why some liquids are ‘fragile’ and others are ‘strong’

28.08.2014

‘Fragility’ provides a clue to the mystery of what happens when a liquid turns into a glass.

Only recently has it become possible to accurately “see” the structure of a liquid. Using X-rays and a high-tech apparatus that holds liquids without a container, Kenneth Kelton, PhD, the Arthur Holly Compton Professor in Arts & Sciences at Washington University in St. Louis, was able to compare the behavior of glass-forming liquids as they approach the glass transition. 


Mehran Moghtadai/GNU Free Documentation License

Glass is a liquid that has lost its ability to flow and so instead of taking the shape of its container can itself serve as a container.


Kelton

A levitated drop within the WU-BESL. Electrodes above and below the sample charge its surface by induction, and it rises in the evacuated chamber, hoisted by electrostatic attraction. This ‘containerless’ technique allows the drop to solidify without crystallizing.

The results, published in the August 6 issue of Nature Communications, are the strongest demonstration yet that bulk properties of glass-forming liquids, such as viscosity, are linked to microscopic ones, such as structure.

Although people have known how to make glass for thousands of years, the glassy state and the glass transition are still not fully understood. The method used to make most glasses provides a hint of the problem, Kelton said.

A liquid must first be cooled below its freezing temperature (supercooled) without crystallizing. As the temperature of the supercooled liquid drops further, the liquid becomes more and more viscous. Eventually it reaches a point where its molecules or atoms can’t move fast enough to accommodate changes in temperature, and portions of the liquid successively jam, or lock in place.

The transition from a liquid to a glass is not a phase transition, like the familiar conversion of water to ice. At the freezing point, water and ice are both states in thermodynamic equilibrium, meaning everything within them is in balance and nothing is driving them to change. Glass, on the other hand, is  not in equilibrium at any temperature.

Despite years of study, the process of glass formation still puzzles scientists. For the most part they have only been able to measure bulk properties of glass-forming liquids, such as viscosity and specific heat, and the interpretations they came up with depended in part on the measurements they took. But they were aware that these properties probably reflected changes in the liquid’s structure at an atomic level.

Understanding the glass transition is important, Kelton said, because glasses are far more common than people realize. “One day I noticed that some of my nuts-and-bolts papers on the crystallization of glasses were much more highly cited than I would have expected,” he said. “It turned out they were being cited by people in the pharmaceutical industry.”

Pharmaceutical companies have been developing “amorphous” (glassy) drugs for a variety of reasons. But one reason that amorphous drugs generally dissolve better in the body, so that lower doses are more effective.

But that’s just one of many hidden uses for glass science.


Fragile and strong liquids
Kelton chose to study a property called ‘liquid fragility,’ which appears to play a role in glass formation. The term ‘fragility’ was first coined in 1995 by Austen Angell, now a professor of chemistry at Arizona State University, who is known for his research on the physics of glasses and glass-forming liquids. Angell felt that a new term was needed to capture dramatic differences in the way a liquid’s viscosity increases as it approaches the glass transition.

The viscosities of some liquids change gradually and smoothly as they approach this transition. But as others are cooled, the viscosities change very little at first, but then take off like a rocket as the transition approaches.

Angell could only measure viscosity, but he called the first type of liquid “strong” and the second type “fragile” because he suspected a structural difference underlay the differences that he saw.

“It’s easier to explain what he meant in terms of the transition from a glass to a liquid rather than the other way around,” Kelton said. “Suppose a glass is heated through the glass transition temperature. If it’s a strong system, it ‘remembers’ the structure it had as a glass—which is more ordered than in a liquid—and that tells you that the structure does not change much through the transition. In contrast, a fragile system quickly ‘forgets’ its glass structure, which tells you that its structure changes a lot through the transition.

“That ‘s how Angell viewed it,” Kelton continued, “but it had never been experimentally shown. People argued that the change in viscosity had to be related to the structure—through several intermediate concepts, some of which are not well defined. What we did was hop over these intermediate steps to show directly that fragility was related to structure.”

Putting glass under the “microscope”
Kelton was able to look at structure because his team has built a new apparatus, the Washington University-Beamline Electrostatic Levitator, or WU-BESL, which was specifically designed to provide a kind of “microscope” to study the atomic structure of liquids, much like a traditional microscope can look into the body of a cell.  

In a sense the WU-BESL isn’t all that different from the familiar light microscope. But instead of being clipped on a stage, the supercooled sample is levitated in a vacuum to avoid contact with a container or even a floating speck of dust, which could make it suddenly crystallize. 

And instead of probing the sample with visible light, which has wavelengths too long to resolve atoms, it probes them with high intensity X-rays. During an experiment, the WU-BESL is carried to Argonne National Laboratory outside of Chicago and installed at the Advanced Photon Source, a particle accelerator that produces an intense beam of X-ray photons.

Once levitated, the sample is melted with a high-power laser and allowed to cool. As its temperature decreases toward the glass transition, the sample is exposed to the X-ray photons and detectors measure the intensity of scattered photons as a function of the scattering angle.

By analyzing the scattered data, the scientists obtain a plot of “the structure factor,“ an oscillating line with peaks of rapidly diminishing height that contains information on atom locations. Kelton and his colleagues focus on the first of these peaks, which corresponds most directly with the change in the liquid’s average structure.

To test Angell’s idea, the scientists heated and then cooled many samples of metallic glass-forming liquids, some considered strong and the others fragile, in the WU-BESL. The strong liquid’s structure factor evolved gradually from that of a liquid to that of a glass. But the evolution of the fragile liquid’s structure factor accelerated abruptly as it approached the glass transition.

It was just as Angell had suspected. The rate of atomic ordering in the liquid near the transition temperature determines whether a liquid is fragile or strong.

These results help us understand some fundamental physics, but Kelton also points out that they have a practical importance, providing glass manufacturers with a new way to search for good glass formers. Strong liquids seem to be good glass formers that “want” to form glasses and ignore inducements to crystallize. Fragile liquids are generally bad glass formers that can be coaxed to make glass only by using extreme manufacturing techniques.

Since 2010, Apple Inc. has had an exclusive rights agreement with Liquidmetal Technologies, a company that make metallic glasses, Kelton said. In May 2014, Apple was granted a patent for “printing” metallic glass bezels for smartphones, which will be stronger than plastic ones, but more flexible than metal ones.

“By learning how liquid structure is related to the formation of liquid glasses,” Kelton said, “we open the door a bit wider to the invention of new metallic glasses for novel applications.”

Diana Lutz | Eurek Alert!
Further information:
https://news.wustl.edu/news/Pages/27286.aspx

Further reports about: amorphous formation glass liquids metallic photons properties structure temperature viscosity

More articles from Physics and Astronomy:

nachricht Climate cycles may explain how running water carved Mars' surface features
02.12.2016 | Penn State

nachricht What do Netflix, Google and planetary systems have in common?
02.12.2016 | University of Toronto

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Novel silicon etching technique crafts 3-D gradient refractive index micro-optics

A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...

Im Focus: Quantum Particles Form Droplets

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.

“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...

Im Focus: MADMAX: Max Planck Institute for Physics takes up axion research

The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.

The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...

Im Focus: Molecules change shape when wet

Broadband rotational spectroscopy unravels structural reshaping of isolated molecules in the gas phase to accommodate water

In two recent publications in the Journal of Chemical Physics and in the Journal of Physical Chemistry Letters, researchers around Melanie Schnell from the Max...

Im Focus: Fraunhofer ISE Develops Highly Compact, High Frequency DC/DC Converter for Aviation

The efficiency of power electronic systems is not solely dependent on electrical efficiency but also on weight, for example, in mobile systems. When the weight of relevant components and devices in airplanes, for instance, is reduced, fuel savings can be achieved and correspondingly greenhouse gas emissions decreased. New materials and components based on gallium nitride (GaN) can help to reduce weight and increase the efficiency. With these new materials, power electronic switches can be operated at higher switching frequency, resulting in higher power density and lower material costs.

Researchers at the Fraunhofer Institute for Solar Energy Systems ISE together with partners have investigated how these materials can be used to make power...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ICTM Conference 2017: Production technology for turbomachine manufacturing of the future

16.11.2016 | Event News

Innovation Day Laser Technology – Laser Additive Manufacturing

01.11.2016 | Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

 
Latest News

UTSA study describes new minimally invasive device to treat cancer and other illnesses

02.12.2016 | Medical Engineering

Plasma-zapping process could yield trans fat-free soybean oil product

02.12.2016 | Agricultural and Forestry Science

What do Netflix, Google and planetary systems have in common?

02.12.2016 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>