Self-propelled particles in viscoelastic environments

以往关于主动悬浮的实验和理论研究大多是在牛顿液体中进行的，牛顿液体具有恒定的粘度和远低于任何与主动粒子运动相关的时间尺度的结构松弛时间。因此，周围的液体可以被认为保持热平衡，尽管存在自推进粒子。在SPP 1726的第二期中，我们想要重点关注形式稳定的微泳者在非牛顿流体中的行为。这是因为，非牛顿流体通常提供了活微生物的自然环境，但也为合成微泳者在未来的应用提供了现实条件。除了剪切稀化等非线性流变特性外，粘弹性流体还具有相当长的松弛时间(在颗粒运动的时间尺度上甚至更高)。然后，流体将被粒子运动驱动而失去平衡。正如PI小组最近证明的那样，粒子的定向运动和周围流体的微观结构松弛之间的耦合引起了相当意想不到的自推进粒子旋转扩散的显著增强(超过两个数量级)，表现出明显的对推进速度的依赖。在SPP的第二阶段，我们将在实验上进一步探索光激活的活性粒子在粘弹性流体中的运动。特别是，在较高的粒子浓度下，粒子-粒子相遇变得重要。由于中间流体的微观结构变形，我们预计，与牛顿液体相比，这种遭遇将发生很大变化，这将产生显著的后果，例如，团簇的形成以及靠近坚硬障碍物和墙壁的活跃运动。除了活性粒子的运动性恒定的情况外，我们还计划进行实验，即推进强度随时间周期性地调制。我们预计，在这样的条件下，对高斯涨落的强烈偏离，以及对非平衡热浴对活性粒子行为的影响的更好理解。

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.