Physics of active matter: Coupled systems of active and passive matter

Abstract

Active matter represents an unconventional form of soft condensed matter, which is driven out of equilibrium by matter and energy fluxes. Active matter fascinates by the spontaneous emergence of spatiotemporal patterns. A prime example are systems of molecular motors interacting with cytoskeletal filaments inside biological cells. These motor-filament systems exhibit collective dynamics and self-organized pattern formation. Their hierarchical organization prompts novel theoretical approaches as well as multi-scale simulation frameworks to ultimately understand physical mechanisms of self-organization in active matter. An important motor-filament system are myofibrils, the almost crystalline force-generators of striated muscle. During myofibrillogenesis, myosin molecular motors and cytoskeletal filaments self-assemble into highly regular, yet dynamic structures. This self-assembly is now increasingly understood in terms of biomolecular players, but poorly understood in terms of physical mechanisms. A second important motor-filament system are motile cilia: these slender cell appendages contain stereotyped arrangements of dynein molecular motors. Collections of motile cilia can synchronize their bending waves into tissue-scale metachronal waves in cilia carpets on epithelial surfaces. These self-organized waves represent a striking example for the spontaneous emergence of temporal order that depends on broken symmetries. These prominent examples of active matter allow to tackle a general question concerning the physics of life: How do microscopic interactions at small scales give rise to self-organized dynamics and spatio-temporal patterns at large scales? Here, we propose a general modeling framework (Lagrangian Mechanics of Active Systems) for systems of coupled active and passive matter. Using this framework, we will study specific model systems (1: myofibrillogenesis, 2: cilia synchronization, 3: stochastic dynamics of microswimmers), to understand self-organized dynamics of active matter interacting with elastic substrates, viscous fluids, and chemical fields. We aim to disclose physical mechanisms of self-organized pattern formation in these systems, and investigate the robustness of these mechanisms to non-equilibrium fluctuations and quenched disorder. Each of the proposed biological model systems was selected to allow for quantitative comparison of theory and experiment. Access to experimental data to test our theoretical predictions is secured already within a trusted network of theory-experiment collaborations. With our research agenda, we aim to contribute to an ongoing endeavor of extending the boundaries of physics towards the domain of living matter.