Dynamics of topological defects in bacterial active matter

• Applicants: Professor Dr. Markus Bär; Dr. Sebastian Heidenreich
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 396653815

This project aims to identify and describe topological defects in swarming bacterial colonies. We propose to analyze the statistics and dynamics of defects in order to characterize the bacterial collective motion. Physically, topological defects are indicative of the underlying physical mechanisms. Biologically, they are claimed to be related to layer formation and possibly to biofilm initiation. Thus, the physics of defects is coupled to the biological evolution of the colony. The proposal combines experimental (done in Israel) and theoretical (done in Germany) aspects. Experimentally, we will study the behavior of Bacillus subtilis and related strains in different two main setups – monolayer and multilayer. Monolayer experiments allow to systemically vary the average density as a control parameter, facilitating a detailed study of clustering and phase separation in two dimensions (2D). Multilayer colonies (3D) display intriguing collective motion like mesoscale turbulence at approximately constant overall densities. Theoretically, we will derive and study a unified continuum model for active fluids, with polar and nematic interactions to model the different experimental systems. Data analysis methods shall be used to extract the defect dynamics from experiments as well as simulations to calibrate the model equations. The results of experiments and unified continuum models will be compared regarding the observed defect dynamics. Whereas prior work has focused on the characterization of single defects, our experimental set-ups and modelling approaches will allow us to study also dynamics and collective organization of many defects in different biologically relevant situations potentially shedding light on their role in the large-scale organization of bacterial swarming and their impact on morphological transitions like biofilm formation.

• DFG Programme: Research Grants
• International Connection: Israel
• International Co-Applicants: Professor Dr. Gil Ariel; Professor Dr. Avraham Beer