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

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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.