Frontiers

Under Pressure

Arjun Yodh and Andrea Liu tame the disordered solid.
November 2011

The lazy melody of a wind chime; the roar of a gong; the chirp of a bell—what do these sounds all have in common? Each is produced by the organized vibrations of atoms in an ordered solid, also known as a crystal. While all solids contain flaws, defects in crystals manifest in easily recognizable patterns. Non-crystals, or disordered solids—materials like glass and sand—don’t merely contain flaws, they consist entirely of them. Arjun Yodh, James M. Skinner Professor of Science and the Director of The Laboratory for Research on the Structure of Matter and Andrea Liu, Hepburn Professor of Physics, both of the Physics and Astronomy Department, are working to understand how vibrations in these disordered solids might be linked to their mechanical failure. This research, carried out by postdocs Ke Chen, Wouter Ellenbroek, Zexin Zhang at Penn, Lisa Manning of Princeton University, and graduate student Peter Yunker, was published recently in the form of two articles in Physical Review Letters.

“If you give a little push to a pendulum or a ball on a spring, there is natural back-and-forth movement or vibration that follows,” Yodh says. “In crystals, atoms behave like many balls connected by many springs, and they vibrate together in an organized way. In a disordered system like a glass, however, the atoms are arranged more randomly and their vibrations are quite different as a result.”

In order to study the connection between vibrations and mechanical response, the team subjected a novel test solid, consisting of small plastic balls packed together, to a mechanical force. Mechanical stress was created because the particles change size when heated and “press” differently on each other. With the help of video microscopy and computer analysis, Physics and Astronomy postdoc Ke Chen determined which particles moved as a result of the compressive stress and compared these motions to the natural vibrations of the disordered solid.

They discovered that particles in the disordered solids exhibited localized low-frequency vibrations in “soft spots.” The locations of these soft spots turned out to be markers for defects or weak points with a propensity to rearrange when stressed.

“If you give a little push to a pendulum or a ball on a spring, there is natural back-and-forth movement or vibration that follows. In crystals, atoms behave like many balls connected by many springs, and they vibrate together in an organized way.” – Arjun Yodh

“If you try to bend a glass rod, it will inevitably break—that much is common sense,” Liu says. “The key is to understand what atomic events led up to that failing in that precise location.”

The future implications of Yodh and Liu’s research are far-reaching. The ability to develop disordered solids tough enough to withstand stress at their weak points could have a host of potential benefits to a wide swath of industries. It might also aid in safety testing, particularly in complex machines like vehicles that might contain metallic glass or other non-crystals that are subject to accumulative defects.

“If you monitor these incremental flaws on a microscopic level,” Liu says, “you can conceivably rethink entire future designs with reinforcement in mind.”

In addition to their research on disordered solids, Yodh and Liu are involved in numerous other groundbreaking projects. Yodh is developing optical technology to pass light through various parts of the body in order to analyze blood oxygen and flow levels. This could eventually lead to easy analysis and monitoring of strokes and tumors, or various other conditions with vascular cues.

Liu is studying how cells coordinate cell division across the early embryo of the fruit fly, a process that she believes involves mechanical coupling to generate propagating wavefronts of cell division.