Self Regulation in Active Fluids

1. Self Regulation in Active Fluids A. Gopinath, M. F. Hagan, and A. Baskaran, Martin Fisher School of Physics, Brandeis University Supported by NSF-DMR 0820492 Active fluids such as bacterial suspensions, cytoskeletal filaments interacting with motor proteins, and artificial microswimmers have been shown to exhibit a rich array of pattern formation phenomena. In this work we showed that the key mechanism for the emergence of structures is self-regulation. This is a consequence of the fact that the control parameter of the order disorder transition, namely the density, is itself dynamically modified by the order parameter that it induces. In the context of active polar fluids (where the symmetry breaking results in vector ordering) we have demonstrated the existence of solitary waves and asters (see Figure). In the context of active nematics we have shown the existence of self regulation, while the nature of emergent structures is the topic of ongoing investigation. Further, when there is a feedback between self regulation and activity, dynamical structures result; e.g. breathing asters in polar fluids (see Figure). Fig: Top: Emergent structures resulting from dynamic self regulation in a polar fluid. Left an aster and Right Solitary waves. Bottom: A breathing aster that dynamically deforms but resets itself due to feedback between self regulation and activity.