Subscribe free to our newsletters via your
. Space Industry and Business News .

PETRA Reveals Coexisting Structures in Glass
by Staff Writers
Hamburg, Germany (SPX) Apr 12, 2013

X-ray scattering pattern from colloidal glass. Credit: Dmitry Denisov.

The craft of glassmaking extends way back in time. It was over five-thousand years ago when mankind learned how to make glass. Even prior to this discovery, humans had been using naturally occurring glass for tool making. Despite this long and rich history and widespread use of glass, surprisingly little is known about the interplay between the mechanical properties of glasses and their inner structures.

For the first time, researchers from Amsterdam University (The Netherlands) and DESY have now monitored subtle structural changes in a glass made from microscopic silica spheres, which they exposed to shear stress.

Using a unique experimental setup at DESY's PETRA III X-ray source, the scientists discovered coexisting structural states in the glass and related them to its flow behavior. The research was published in the journal "Scientific Reports".

Glasses are substances that can transition from a fluid state into a non-fluid glassy state. Unlike water, which freezes from a fluid into an ordered solid, the glassy phase retains a fluid-like structure with little order. Nevertheless, the material becomes highly viscous or even hard in the glassy state.

Numerous other materials, including metallic alloys, polymers, and colloids, exhibit a liquid-glass transition. "Toothpaste, for instance, behaves similarly to glass, and so do foams, gels, and cremes," says Dmitry Denisov, the study's first author from Peter Schall's group at Amsterdam University.

Model for more complex systems
In their study, Denisov and his colleagues prepared a glass from silicon dioxide, or silica. They added silica spheres that were a mere fifty-millionths of a millimeter in diameter to water.

Much like fat particles in milk, the spheres are dispersed throughout the water, forming what is known as a colloid. When the spheres make up 58% of the entire volume, the dispersion's motion arrests - the colloid is in its glassy phase. For smaller volume fractions, the colloid remains in its liquid state.

"Our spherical silica beads all have approximately the same diameter and the glass can be mathematically described rather easily," Denisov explains. "Hence, our system is a good starting point for modeling more complex systems with a liquid-glass transition."

The researchers were particularly interested in studying a phenomenon termed "shear banding". When they shear the glass by moving its horizontal layers parallel to one another, the stress response in the glass is divided into two regions, or bands, in which particles move with different speeds.

"Shear banding has been known for a long time. The different bands can be made visible with confocal microscopy, and mechanical measurements show that something happens in the sample," says Denisov.

"However, when we examined the interactions between particles in the different bands, we found the interactions to be the same. In fact, the structural differences between the two bands are so small that people initially thought they did not exist." To prove otherwise, the research team turned to a more sensitive method.

Unique experimental setup
At PETRA's experimental station P10, the researchers filled their sample between two horizontal plates. One of the plates was stationary while the other was rotated to shear the sample.

With this setup, called a rheometer, the scientists tracked the glass' mechanical response, including changes in its viscosity. Simultaneously, PETRA's intense X-ray beam traveled through the sample, examining its inner structure.

When X-rays scatter off particles inside a sample, they form a characteristic scattering pattern behind it, from which researchers can gain knowledge of the sample's structural order and the average particle-particle distances.

"When we altered the shear rate in our experiment, we were able to see how the average distance between silica spheres in the glass varied," says DESY scientist Bernd Struth. "These structural changes have never been seen before."

Key to the experimentalists' success was the unique geometry of the setup, which Struth designed together with DESY engineer Daniel Messner. The PETRA lightsource emits X-rays in the horizontal plane. However, liquid samples have to be probed vertically, since they have to be placed horizontally in order to prevent them from dripping out of the setup. X-rays are not easily redirected by a mirror, but a unique optical element deflects the beam so that it traverses the sample in a vertical direction. "The structural changes in the glass are very small," Struth points out.

"We can only measure them precisely when we average over a small amount of sample, which is only achievable in our geometry."

The researchers determined that shear stress in the colloidal glass results in a higher structural order and an increased vertical distance between particles. Thereby, layers of the suspension glide along each other with more ease and, thus, shear flow in the sample is facilitated. "Normally, you would expect the shear stress in the sample to increase with the applied shear rate," Struth says.

In fact, this is what the researchers observed when they started rotating the rheometer's plate faster and faster. "Above a certain shear rate, however, the mechanical response of the glass no longer changes. When we continue increasing the shear rate, the shear stress remains constant until we reach a second distinct shear rate, above which the stress becomes larger again."

The explanation for the region of constant stress is shear banding. "In this region, two bands with different velocities and viscosities coexist," explains Denisov. "The bands change dynamically with the shear rate, but when we measure the shear stress in the sample, averaged over the two bands, the stress is constant."

Small changes, big effect
However, what exactly happens structurally in the shear banding region? "The structural parameters that we obtained from X-ray scattering correlate with the mechanical observations. The order and the average particle-particle distance in the two bands are different, suggesting that two different structures coexist," Denisov says.

"Previous studies indicated that the structure may be the same in the different bands. We proved that this is not the case." Since the observed structural changes are very small, they have not been resolved in earlier studies.

In general, the researchers established that small structural variations in the colloid glass have a large impact on its flow behavior. For the smallest shear rate applied in their experiment, the viscosity of the glass was ten thousand times larger than the viscosity at the largest shear rate.

In contrast, the observed structural change over the same range was only less than three percent. "Small changes in the glass structure on the microscopic scale correspond to large changes in the mechanical properties on the macroscopic scale," Denisov summarizes.

"Our methodology enabled us to observe structural modifications in a glassy state that we know relatively little about," Struth emphasizes. "The new data can be used in simulations that will improve our understanding of such systems."

Given the variety of systems with a liquid-glass transition and the widespread use of glasses in human culture, gaining insights into the intimate link between structure and flow behavior of glasses is an exciting perspective.

"Resolving structural modifications of colloidal glasses by combining x-ray scattering and rheology'"; Denisov, D., Dang, M.T., Struth, B., Wegdam, G. and Schall, P.; Scientific Reports (2013, 6, 1631); DOI: 10.1039/C2JA30119A


Related Links
Deutsches Elektronen-Synchrotron
Space Technology News - Applications and Research

Comment on this article via your Facebook, Yahoo, AOL, Hotmail login.

Share this article via these popular social media networks DiggDigg RedditReddit GoogleGoogle

Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News

Accidental discovery may lead to improved polymers
Toronto, Canada (SPX) Apr 09, 2013
Chemical Engineering Professor Tim Bender and Post-Doctoral Fellow Benoit Lessard's discovery of an unexpected side product of polymer synthesis could have implications for the manufacture of commercial polymers used in sealants, adhesives, toys and even medical implants, the researchers say. Bender and Lessard discuss their discovery in "Boron subphthalocyanine polymers by facile coupling ... read more

Florida Tech professors present 'dark side of dark lightning' at conference

PETRA Reveals Coexisting Structures in Glass

Northrop Grumman G/ATOR Radar System Completes Government Testing at Wallops Island

Accidental discovery may lead to improved polymers

Boeing Delivers FAB-T Test Units to US Air Force

Fourth Lockheed Martin MUOS Satellite Entering System Test as Communication Module and Multi-Beam Antenna Installed

Advancing secure communications: A better single-photon emitter for quantum cryptography

Northrop Grumman Awarded U.S. Navy Contract to Upgrade, Enhance NGC2P Tactical Data Link Processor

Ecuador to launch first homemade satellite

Arianespace receives the second Vega for launch from French Guiana

Future Looks Bright for Private US Space Ventures

Europe's next ATV resupply spacecraft enters final preparatio?ns for its Ariane 5 launch

Smithsonian dedicates new exhibition to navigation

Extreme Miniaturization: Seven Devices, One Chip to Navigate without GPS

Down the slopes with space app in your pocket

Lockheed Martin Team Completes Delta Preliminary Design for Next GPS III Satellite Capabilities

Dassault and India in Rafale deal standoff

Israel boosts air force 'pack of leopards

More delays in Brazil air force upgrades

Fasten seatbelts for bumpier flights: climate study

Redesigned Material Could Lead to Lighter, Faster Electronics

A step toward optical transistors?

New 'transient electronics' disappear when no longer needed

World Record Silicon-based Millimeter-wave Power Amplifiers

Kazakhstan to launch first remote sensing satellite this year

Raytheon brings automation and virtualization to NASA's Earth Observing System

Ball Aerospace Begins Integration Phase for DigitalGlobe's WorldView-3 Satellite

RADARSAT-1 Malfunction

Albania to hold referendum on waste imports

Smog-eating pavement on greenest street in America

Latin America looks to earn from e-waste

Russia seeks Baltic pollution partnerships

The content herein, unless otherwise known to be public domain, are Copyright 1995-2014 - Space Media Network. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA Portal Reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement,agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement