Self-propelled particles in viscoelastic environments

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Most previous experimental and theoretical studies on active suspensions have been carried out in Newtonian liquids which are characterized by a constant viscosity and structural relaxation times far below any timescale relevant for active particle motion. Accordingly, the surrounding liquid can be considered to remain in thermal equilibrium despite the presence of self-propelling particles.In the 2nd period of the SPP 1726, we want to focus on the behavior of form-stable microswimmers in non-Newtonian fluids. This is motivated by the fact, that non-Newtonian fluids typically provide the natural surroundings of living microorganisms but also offer realistic conditions under which synthetic microswimmers will be operated in future applications.In addition to nonlinear rheological properties such as shear thinning, viscoelastic fluids are characterized by rather long relaxation times (being on the timescale of particle motion or even above). Then, the fluid will be driven out-of-equilibrium by the particle motion. As has been recently demonstrated by the group of the PI, the coupling between the particle's directed motion and the microstructural relaxation of the surrounding fluid, gives rise to a rather unexpected dramatic (more than two orders of magnitude) enhancement of the rotational diffusion of self-propelled particles which show a pronounced dependence on the propulsion velocity. In the second period of the SPP we will experimentally further explore the motion of light-activated active particles in viscoelastic fluids. In particular, at higher particle concentrations particle-particle encounters become important. Due to the microstructural deformation of the intermediate fluid, we expect such encounters to be largely changed compared to Newtonian liquids which will have pronounced consequences, regarding e.g. cluster formation but also active motion close to hard obstacles and walls. In addition to situations where the motility of active particles is constant, we also plan to perform experiments, where the propulsion strength is periodically modulated over time. We expect strong deviations from Gaussian fluctuations under such conditions and a better understanding of the consequences of a non-equilibrium thermal bath on the behavior of active particles.