Self-propelled particles in viscoelastic environments

以前大多数关于主动悬浮液的实验和理论研究都是在牛顿液体中进行的，这些液体的特点是粘度恒定，结构松弛时间远远低于与主动粒子运动有关的任何时间尺度。因此，尽管存在自推进粒子，但可以认为周围的液体保持在热平衡状态。在spp1726的第二阶段，我们想把重点放在非牛顿流体中形式稳定的微游泳者的行为上。这是因为，非牛顿流体通常为活微生物提供了自然环境，但也为合成微游泳器在未来的应用中提供了现实条件。除了非线性流变特性，如剪切变薄，粘弹性流体的特点是相当长的松弛时间（在粒子运动的时间尺度上甚至更高）。然后，流体将被粒子运动所驱动而失去平衡。正如PI小组最近所证明的那样，粒子的定向运动和周围流体的微观结构弛豫之间的耦合，导致了一个相当意想不到的戏剧性（超过两个数量级）的自推进粒子的旋转扩散增强，这显示出对推进速度的明显依赖。在SPP的第二阶段，我们将通过实验进一步探索光活化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，粒子与粒子的相遇变得很重要。由于中间流体的微观结构变形，我们预计与牛顿流体相比，这种相遇将有很大的改变，这将产生明显的后果，例如团簇形成，以及靠近坚硬障碍物和墙壁的主动运动。除了活性粒子的运动是恒定的情况外，我们还计划进行实验，其中推进强度随时间周期性调制。我们预计在这种条件下会出现高斯波动的强烈偏差，并能更好地理解非平衡热浴对活性粒子行为的影响。

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.