Artificial microswimmers formed by liquid crystal droplets

我们实验项目的目的是研究表面活性剂水溶液中的自推进液晶液滴及其仿生能力。游动微生物，如浮游生物和单细胞生物，对它们的推进机制非常感兴趣，如趋化作用、自趋化信号和螺旋游泳，如观察到的精子、与土壤等界面和受限几何形状的相互作用，以及生物对流等集体过程，特别是关于海洋浮游生物动力学。在第一个资助期，我们已经表明，即使在一个非常简单的三组分液滴系统中，也可以观察和调整其中的许多特征。我们能够批量生产具有可重复速度和长运行时间的单分散液滴，这些液滴由于向列各向异性而表现出可切换的螺旋游泳，遵循表面活性剂胶束的化学趋化和自趋化梯度，与直线和曲面相互作用，可以通过重水混合物从沉积的浮游调整为自由浮游，其特点是大型集体中的限制和重力相关的对流。我们在光学显微镜下观察了微流体细胞中的这些液滴，以再现复杂和受限的几何结构，并开发了一种荧光光片显微镜，用于在真实三维条件下观察浮力液滴和大的对流系综。我们能够通过粒子图像测速仪对液滴内部和周围的流场进行成像，并通过偏光显微镜深入了解向列相结构。在第二个资助期，我们与优先方案的理论合作者合作，致力于解决基本制度的公开问题、进一步的应用和变化。我们想定量地了解游泳行为如何取决于液滴的内部结构，这可能会受到其他致线剂或手性掺杂剂的掺入、水滴形成向列壳的包裹体以及外部磁场的影响。螺旋轨迹的形态强烈依赖于负的自趋化作用，这抑制了自交叉，我们的目标是对这一影响进行建模和量化。在由准二维几何结构向三维几何结构的转变以及壁曲率变化的情况下，游泳者的壁面相互作用将被量化，这取决于运动水滴产生的流体动力场和磷光场。最重要的目的是彻底了解导致对流模式和集群形成的集体行为。使用我们的光片显微镜，我们将研究大型游泳者群体在不同有效重力和游泳者速度下的动力学，成像单个液滴的动力学和整体相的流场。

The aim of our experimental project is the study of self-propelled liquid crystal droplets in an aqueous surfactant solution and their biomimetic capabilities. Swimming microorganisms like plankton and single-cell organisms are of high interest with regard to their propulsion mechanisms, as in chemotaxis, autochemotactic signalling and helical swimming, as observed, e. g., for sperm, interactions with interfaces and confined geometries like soil, and collective processes like bioconvection, especially with respect to oceanic plankton dynamics. During the first funding period, we have shown that even in a very simple three component droplet system, many of these characteristics can be observed and tuned. We are able to mass produce monodisperse droplets with reproducible speed and long running times, which show switchable helical swimming due to their nematic anisotropy, follow chemotactic and autochemotactic gradients of surfactant micelles, interact with straight and curved surfaces, can be tuned from sedimented to free buoyant swimming by admixture of heavy water and which feature to confinement and gravity dependent convective flows in large collective ensembles. We observed these droplets in microfluidic cells under light microscopy to reproduce complex and confined geometries and developed a fluorescence light sheet microscope for the observation of buoyant droplets and large convective ensembles under real three dimensional conditions. We were able to image flow fields inside and around the droplets via particle image velocimetry and gained insight into the nematic structure via polarised microscopy. During the second funding period, in collaboration with theory collaborators from the priority programme, we aim to address open questions, further applications and variations of the basic system. We want to understand quantitatively how the swimming behaviour depends on the internal structure of the droplets, which can be affected by admixing other nematogens or chiral dopants, inclusion of water droplets to create nematic shells, and external magnetic fields. The morphology of the helical trajectories strongly depends on negative autochemotaxis, which inhibits self-crossing, an effect we aim to model and quantify. The wall interaction of the swimmers, depending on hydrodynamic and phoretic fields generated by the moving droplet, will be quantified at the transition from quasi two dimensional to three dimensional geometries and under variation of wall curvature. The most important aim is a thorough understanding of the collective behaviour resulting in the formation of convective patterns and clusters. Using our light sheet microscope, we will study the dynamics of large swimmer ensembles under varying effective gravity and swimmer velocity, imaging both individual droplet dynamics and the flow fields in the bulk phase.