Active matter: from collective self propelled rods to cell like particles
Active matter comprises systems with sustained energy uptake and dissipation of its constituents. This applies to systems across many scales. We study ensembles of self propelled rods (SPRs) in periodic boundaries and in confinement to mimic the collective behavior and dynamics of bacteria, and active filaments. Our models describe active systems that allow the propulsion to adapt to its environment. While SPRs with density dependent slowing down partially capture the behavior observed for bacteria, SPRs in ring like confinements can be considered as a minimal, soft matter model for cell motility. Phoretic microswimmers and genetically modified E. coli show density dependent reduced propulsion.This motivates the investigation of the collective behavior and dynamics of SPRs with density dependent propulsion force. The density dependent slowing down enhances polar ordering and cluster formation, and induces rod perpendicularity at cluster borders. As a model of cellular motility due to cytoskeletal activity, SPRs inside mobile, rigid circular confinement are considered, which build complex self propelled rings. The rod self organization gives rise to complex motility patterns, such as run and tumble and run and circle motion. Motility patterns observed for self propelled rigid rings are also observed for motile cells. Taking a further step towards a more realistic modeling of cell motility, we study SPRs inside mobile, deformable rings. In addition to ring motility, also ring deformability plays a crucial role in SPR alignment and cluster formation. Here, pulling forces at the back of the rings are crucial to recovering cell like shapes and motion. While our models do not take into account biochemical aspects of biological systems, they allow the identification of crucial mechanical aspects, and help to test different underlying mechanisms to interpret microscopic observations.