  Microstructural dynamics in amorphous particle packings is commonly probed by
quantifying particle displacements. While rigidity in particle packings emerges
when displacement of particles are hindered, it is not obvious how the
typically disordered displacement metrics connect to mechanical response.
Particle rotations, in contrast, are much less sensitive to confinement
effects, while still sensitive to the mechanics of the packing. So far, little
attention has been paid to connect microscopic rotational motion to mechanics
of athermal amorphous packings. We demonstrate through experimental
measurements that particle packing mechanics can be directly linked to the
rotational motion of even round particles in a sheared packing. Our results
show that the diffusive nature of rotational dynamics is highly strain
sensitive. Additionally, there is substantial spatial correlation in rotation
dynamics that is a function of the particle friction and packing density.
Analysis of our measurements reveals that particle rotation dynamics plays an
essential role in amorphous material mechanics.
