# Dresden 2020 – wissenschaftliches Programm

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# BP: Fachverband Biologische Physik

## BP 15: Active Matter III (joint session DY/BP/CPP)

### BP 15.7: Vortrag

### Dienstag, 17. März 2020, 15:30–15:45, ZEU 160

Transport coefficients of active particles: Reverse perturbations and response theory — •Thomas Ihle^{1}, Arash Nikoubashman^{2}, Alexander Unruh^{1}, Sven Stroteich^{1}, and Rüdiger Kürsten^{1} — ^{1}Institute for Physics, Greifswald University — ^{2}Institute of Physics, Johannes-Gutenberg-University Mainz

Müller-Plathe’s reverse perturbation method [Phys. Rev. E 59, 4894 (1999)] for shearing simple liquids is extended to the Vicsek model (VM) of self-propelled particles. It is shown how the shear viscosity ν and the momentum amplification coefficient λ, can be extracted from simulations by fitting to an analytical solution of the hydrodynamic equations for the VM. The viscosity consists of two parts, a kinetic and a collisional contribution. While analytical predictions already exist for the former [T. Ihle, J. Stat. Mech. 2016, 083205], a novel expression for the collisional part is derived by an Enskog-like kinetic theory [A. Nikoubahsman, T. Ihle, Phys. Rev. E 100, 042603 (2019)]. Using several methods to measure transport coefficients such as reverse perturbations, Green-Kubo relations and transverse current correlations, we find excellent agreement between the different methods and good agreement with theory. We introduce a novel kind of response theory that allows us to not only verify the analytical predictions of kinetic theory but also to efficiently obtain expressions for nonlocal (wavevector dependent) transport coefficients of active systems, avoiding tedious multiple-scale methods like the Chapman-Enskog expansion. The method is applied to the VM with metric and topological interactions as well as to a model with continuous time dynamics.