  We have performed a novel experiment on granular packs composed of
automatically swelling particles. By analyzing the Voronoi structure of packs
going through the jamming transition, we show that the local configuration of a
jamming pack is strikingly similar to that of a glass-forming liquid, both in
terms of their universal area distribution and the process of defect annealing.
Furthermore, we demonstrate that an unambiguous structural signature of the
jamming transition can be obtained from the pair correlation functions of a
pack. Our study provides insights into the structural properties of general
jamming systems.
