# Soft Spots in Disordered Colloidal Packings

Like
liquids, solids can flow under applied shear stresses.  In crystalline solids, figuring out vulnerable regions where the material will break under stress is
well-established—they are typically controlled by a population of defects in
the crystal structure known as dislocations.
In disordered solids, however, defects are everywhere, making the task
of identifying such vulnerable spots
much more daunting.

When
a disordered solid is placed under mechanical load, it can fail via localized
rearrangements of particles.  These
rearrangements are what allow the solid to flow.  We showed that there is a link between the
nature of vibrations in disordered packings and mechanical failure.  We studied the vibrations of thermosensitive colloidal particles in a dense packing
before it was placed under compression, identifying a population of soft spots.
We then showed that these localized regions are indeed vulnerable to
rearrangement.  Thus, we are now able to
identify the vulnerable regions in a dense, disordered packing before it fails under a load. We are now examining the ways in which these results apply to disordered atomic packings.

In a
dense disordered packing, the particles labeled in dark blue belong to soft
spots that have a propensity to fail via particle rearrangements.  The red arrows mark particles that actually
rearrange when the system is placed under a mechanical load.  This image shows that rearrangements tend to
occur at soft spots.