In active materials energy is injected at the scale of individual units, instead of at the boundaries as in sheared materials, and the motion is not governed by a thermal bath. At low and intermediate densities, broken time-reversal invariance can lead to interesting and well-studied phenomena like giant number fluctuations and motility-induced phase separation. In contrast, at high densities such macroscopic density fluctuations are suppressed, and the structure of the material is very similar to a disordered solid, while the dynamics exhibit interesting flows with non-trivial features such as long-range spatial correlations in the velocity field, intermittency, and avalanche dynamics.Inthe first lecture, I will discuss several frameworks that have been developed to understand the origin of long-range spatial correlations in the particle velocities/displacements [1,2]. In the second lecture, I will discuss intermittency, avalanches, and yielding in dense active matter, including recent work highlighting a direct link between active matter and sheared systems in the limit of infinite persistence [3,4].
References: Szamel, G., & Flenner, E. (2021). Long-ranged velocity correlations indense systems of self-propelled particles. EPL (Europhysics Letters), 133(6), 60002.
 Henkes, S., Kostanjevec, K., Collinson, J. M., Sknepnek, R., & Bertin, E. (2020). Dense active matter model of motion patterns in confluent cell monolayers. Nature communications, 11(1), 1-9.
 Mandal, R., Bhuyan, P. J., Chaudhuri, P., Dasgupta, C., & Rao, M. (2020). Extreme active matter at high densities. Nature communications, 11(1), 1-8.
 Morse, P. K., Roy, S., Agoritsas, E., Stanifer, E., Corwin, E. I., & Manning, M. L. (2021). A direct link between active matter and sheared granular systems. Proceedings of the National Academy of Sciences, 118(18)