Single motion and collective behavior of self-propelling droplets driven by Marangoni flow

我们的目的是了解表面活性剂稳定的液滴结合改变表面张力的化学反应的自推进运动的一般原理。特别是，我们将探索基于Marangoni流的驱动机制，以及它是否适用于不同化学成分、液滴大小和液滴限制的油包水和水包油乳状液。我们还将识别特定参数的运行时间，并测量液滴周围连续相的流场。自行式推进机制和由此产生的流场应与理论结果和替代实验系统的行为进行比较。根据所使用的表面活性剂分子、液滴的绝对尺寸和限制条件，液滴的推进速度差别很大。通过适当地调整这些依赖关系，我们旨在探索不同尺寸但速度相同的液滴混合物的群集行为，反之亦然。预计作为液滴密度函数的集体行为显示了有趣的统计动力学和相变。随着对自推进机制和整体集体行为的理解，我们的目标是探索液滴在外部流场中的行为。我们预计自行流方向与外部流场的方向一致。利用周期性变化的外部流场，我们计划不仅探索液滴的定向，而且探索其定向的动力学。这些发现预计将揭开液滴/液滴相互作用的细节，从而进一步揭示集体行为的细节。

We aim at understanding the general principle of self-propelled motion of surfactant stabilized droplets in combination with a chemical reaction changing the surface tension. In particular we will explore the driving mechanism based on Marangoni flow and if it is universal both for water in oil and oil in water emulsions of different chemical composition, droplet size and droplet confinement. We will also identify the running time for particular parameter and measure the flow fields in the continuous phase around the droplets. The self-propelling mechanism and the resulting flow field shall be compared to theoretical results and the behavior found for alternative experimental systems. Depending on the used surfactant molecules, the absolute droplet size and the confinement, the propulsion velocity of the droplets differs significantly. By adjusting these dependencies appropriately we aim at exploring the swarming behavior for mixtures of droplets having different size but equal velocity and vice versa of droplets having equal size but different velocity. It is expected that the collective behavior as function of droplet density reveals interesting statistical dynamics and phase transitions. With the understanding of the self-propelling mechanism and the global collective behavior we aim at exploring the droplet behavior in an external flow field. We expect a directional orientation of the self-propelling flow direction with the external flow field. Using a periodically varying external flow field we plan to explore not only the directional orientation of the droplets but also the dynamics of their orientation. These findings are expected to unravel details of the hydrodynamic droplet/droplet interaction and thus further details of the collective behavior.