Characterizing particle rearrangements in sheared highly polydisperse materials

Abstract: We compare three particle-scale measures of rearrangement in highly polydisperse materials under driven flow: (a) nonaffine motion defined relative to the time-averaged mean flow, (b) changes in nearest-neighbor connectivity, and (c) $D<sup>2_{\min}$, which measures nonaffine motion relative to an affine deformation fitted locally in space and time [Falk and Langer, Phys. Rev. E 57, 7192 (1998)]. We apply these measures to previously published two-dimensional simulations [Jiang, Sussman, and Weeks, Phys. Rev. E 108, 054605 (2023)] and granular-flow experiments [Illing and Weeks, Phys. Rev. E 111, 045422 (2025)] with polydispersities up to $δ\approx0.50$. Changes in connectivity and $D<sup>2_{\min}$ both require a definition of neighboring particles, making the choice of neighborhood nontrivial in highly polydisperse systems. For detecting changes in connectivity, we recommend radical Delaunay triangulation, which provides a size-aware topological definition of neighbors. For calculating $D<sup>2_{\min}$, we recommend a size-aware pairwise cutoff distance method. We further show that changing the neighborhood definition can reverse the apparent dependence of $D<sup>2_{\min}$ on particle size in experimental data. Thus, trends in $D<sup>2_{\min}$ cannot be interpreted independently of the neighborhood used to calculate it. Overall, the three measures presented provide complementary information about rearrangements in highly polydisperse systems.