Abstract

Materials composed of active particles, where energy is injected at the scale of the individual constituent parts, are well-studied in the limits of low- and intermediate densities where particles are always exchanging neighbors, and recent work has also focused on the interesting statistical properties of active solids, where neighbors are locked in place. In this talk I will instead focus on the regime where dense active materials transition between solid and fluid-like states, driven by increasing active forces. Open statistical physics questions include: what is the nature of the yielding transition — does it share similarities with the random field Ising model like passive sheared solids, or is it entirely different? Is its behavior dominated by the disorder in the field of active forces, or the annealed disorder of the particle packing itself? Are plastic events still localized as in amorphous passive matter, and if so, can they be predicted? Is there something akin to an energy landscape and if so, what are its properties? Is there a constitutive law for such a material? From a practical standpoint, answers to these questions set the stage for controlling active forces in order to generate shape-shifting solids with the ability to rapidly and programmably alter their material properties.