Nonlinear plastic modes reveal defects in solids with pressure gradients

Glasses are characterized by structural disorder and unique thermal, mechanical, and dynamical behavior. Several studies have shown that these features are governed by populations of defects in the system's disordered microstructure. In contrast to crystalline defects, glassy defects are difficult to identify from local structural information. This challenge is further complicated in active disordered solids, such as human crowds, cell collectives, and active colloids, which have internally generated stresses, strong gradients in pressure, and noisy dynamics. In recent work, we examined the structure and dynamics of a simulated active particle model for human crowds, where stable disordered reference configurations with pressure gradients are formed in the limit of persistent self-propulsion. We discovered that normal mode analysis, derived from a harmonic approximation of the system's potential energy, is not sufficient to predict rearrangements in dense regions of these packings, in contrast to previous findings. Here, we utilize anharmonic approximations of the potential energy to identify populations of defects in a similar class of particle packings. Further, we seek to compare the capability of harmonic and anharmonic structural predictors in forecasting plastic motion in disordered solids with global pressure gradients. The methods we develop in this work may be generalizable to predict dynamics in more complex non-hamiltonian active materials.