  Characterizing structural inhomogeneity is an essential step in understanding
the mechanical response of amorphous materials. We introduce a threshold-free
measure based on the field of vectors pointing from the center of each particle
to the centroid of the Voronoi cell in which the particle resides. These
vectors tend to point in toward regions of high free volume and away from
regions of low free volume, reminiscent of sinks and sources in a vector field.
We compute the local divergence of these vectors, for which positive values
correspond to overpacked regions and negative values identify underpacked
regions within the material. Distributions of this divergence are nearly
Gaussian with zero mean, allowing for structural characterization using only
the moments of the distribution. We explore how the standard deviation and
skewness vary with packing fraction for simulations of bidisperse systems and
find a kink in these moments that coincides with the jamming transition.
