Jammed Beads Simulate Glass Transition
Daniel S. Burgess
University of Chicago researchers have experimentally determined that the forces on granular systems differ qualitatively depending on whether the particles are flowing or jammed. The findings offer insight into the behavior of glasses, pointing to the possibility that these materials display an analogous structural signature related to temperature as they change from a rigid to a fluid state.
To dynamically measure the forces on a granular system subjected to different stresses, scientists monitored the changes in the polarization of light reflected from a photoelastic force transducer underneath a column of tens of thousands of beads at various rates of rotation.
This well-known phenomenon, the glass transition, has resisted attempts to study it using x-ray and neutron-scattering techniques. Eric I. Corwin, a graduate student in the university's department of physics, characterized it as one of the great mysteries in science and noted that solving it might enable the development of novel materials.
In the work, the scientists observed a rotating column of tens of thousands of 3-mm-diameter soda-lime glass beads atop a 0.25-mm-thick sheet of photoelastic polymer. The stress caused by beads pressing on the polymer induced a rotation in the polarization of incident light provided by a 500-W bulb from a slide projector and passing through a circular polarizer. Therefore, by imaging the size and intensity of the spots of light reflected from the sheet through an oppositely polarized circular polarizer with a digital video camera, the researchers dynamically measured the contact forces that the beads generated.
By shearing the top of the granular column using a rotating piston -- and thereby injecting energy into the system -- the investigators caused the beads at the outer edge of the column to transition from a jammed to a flowing state. They discovered a clear signature in the plotted distribution of the contact forces. For particles in the jammed state, the distribution decayed exponentially. For those in the flowing state, it indicated that the system was in equilibrium.
The size and intensity of the reflected spots of light, imaged through an oppositely polarized circular polarizer with a digital video camera, indicated the magnitude of the contact force on the photoelastic plate.
Granular and glassy materials are presumed to be analogous because both display flowing or jammed behavior depending on the amount of energy in the system, Corwin explained. In glass in the rigid state, the atoms are stuck and cannot move past each other. As the temperature rises, they gain the energy to push over and around each other and local obstacles, and the glass becomes fluid. The implication of the researchers' findings, thus, is that glass should exhibit a signature similar to that in the granular system.
Beyond addressing the fundamental question of how glass behaves as it does, Corwin suggested that there are potential practical consequences of the work. A better understanding of granular systems, he noted, may enable more efficient handling and processing of grains and of construction materials such as gravel and sand.
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