How hydrodynamics influences the collective motion of microswimmers: A particle-based simulation study

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Microswimmers are systems in non-equilibrium and show a variety of intriguing collective motional patterns ranging from density waves and convection rolls to turbulence. The collective dynamics of microswimmers, active rods, and filaments is often modeled on the basis of coarse-grained continuum theories or with particle-based simulations, where hydrodynamic flow fields are neglected. In the second period of the priority program we will continue with our study, how hydrodynamics influences the collective motion of microswimmers. We will use the parallelized computer code, developed in the first period, where a collection of the model swimmers called squirmers is simulated in a fluid environment using the simulation technique of multi-particle collision dynamics (MPCD).We will continue our investigation of the collective dynamics of microswimmers under gravity concentrating on the formation of floating rafts, which are seen in active emulsions, and on the effect of an aligning torque (bottom-heavy swimmers). Here, intriguing collective patterns such as convection rolls and plumes are expected reminiscent of bioconvection of microorganisms. We then glue squirmers together to form active rods and flexible filaments. They will enable us to explore how the full hydrodynamics influences collective patterns such as bacterial turbulence or density waves and the intriguing dynamics of liquid-crystal defects in systems of dense active bio-polymers. Real microswimmers typically move in a viscoelastic environment. We will thus implement a viscoelastic fluid in MPCD and show how this changes the motion of single and a collection of squirmers.