Study explains how sound waves travel through disordered materials

This work may lead to new heat- and shatter-resistant glass for smartphones and tablets.

Thanks to a newly developed theoretical model, it is now easy to understand the spread of vibrations through disordered materials, such as glass. Developed by the scientists from the University of Tsukuba, the model revealed that as the disorder’s degree increased, sound waves traveled less and less like ballistic particles and instead began diffusing incoherently.

The model explains the observed vibrations in a glass with a better agreement with experimental data. Scientists demonstrate that thinking about vibrations as individual phonons is only justified in the limit of long wavelengths. On shorter length scales, disorder leads to increased scattering, and the sound waves lose coherence.

Author Professor Tatsuya Mori Said, “We call these excitations ‘diffusions,’ because they represent the incoherent diffusion of vibrations, as opposed to the directed motion of phonons.”

“The equations for low frequencies start looking like those for hydrodynamics, which describe the behavior of fluids.”

Scientists later compared the predictions of the model with data obtained from soda-lime glass. They showed that they proved a better fit compared with previously accepted equations.

Co-authors Professor Alessio Zaccone, the University of Cambridge, and Professor Matteo Baggioli, Instituto de Fisica Teorica UAM-CSIC, say, “Our research supports the view that this phenomenon is not unique to acoustic phonons, but rather represents a general phenomenon that can occur with other kinds of excitations within disordered materials.”

In the future, scientists are planning to use disorder to improve the durability of glass for smart devices.

Journal Reference:
  1. Luigi Casella et al., Physics of phonon-polaritons in amorphous materials, The Journal of Chemical Physics (2021). DOI: 10.1063/5.0033371

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