Fluidization of Active Amorphous Solids

Collections of self-propelled, or active, interacting particles self-organize, flow, or jam in ways that differ from their equilibrium counterparts in very interesting ways, making their study an exciting new direction in nonequilibrium statistical physics. Several natural examples of self-driven interacting particles arise in cellular and subcellular biological assemblies, where one typically encounters dense assemblies, unlike examples on larger scales such as bird flocks exhibiting exotic hydrodynamics. Flow properties of dense active matter, and their dynamical transitions from flowing to rigid states are of interest in investigating, e. g., motility transitions in tissues. Several interesting analogies with glass and jamming transitions in disordered condensed matter, as well as differences, have been actively explored. The presence of boundaries and interfaces is important in several examples of interest, which are expected to play a non-trivial role in active systems.

In this talk, I will describe results from computer simulations of the fluidization of rigid, disordered, assemblies of active particles, or active amorphous solids, with a particular focus on the role of confinement. Active amorphous solids undergo fluidization as the strength of the active forces increases, with the persistence times of the active forces playing an intricate role. Our results show that a remarkable analogy hold beteween such fluidzation and yielding in amorphous solids, and that confinement effects have a non-trivial influence on fliudization. I will describe an approach to coarse-grained modeling of mechanically induced cell state transitions based on the insights gained, and some key open questions arising from these investigations.

Goswami, Y., Shivashankar, G.V. & Sastry, S. Yielding behaviour of active particles in bulk and in confinement. Nat. Phys. 21, 817–824 (2025).

Maity, S., Bhaumik, H., Athani, S. et al. Fatigue failure in glasses under cyclic shear deformation. Nat. Phys. (2026). https://doi.org/10.1038/s41567-026-03174-x