论文标题
表面活性剂载有胶囊作为空气水接口处的Marangoni Microswimmers：对称性断裂和表面活性剂扩散和对流的自发推进
Surfactant-loaded capsules as Marangoni microswimmers at the air-water interface: Symmetry breaking and spontaneous propulsion by surfactant diffusion and advection
论文作者
论文摘要
我们通过在空气 - 水界面上通过半球藻酸盐胶囊来实现快速界面的Marangoni微型维数，这可以扩散地释放出水溶性分子（弱表面活性剂，例如聚乙烯甘油（PEG）），通过调制空气式水平式互动为“燃料”。对于许多不同的燃料，我们可以观察到对称性破裂和自发推进，尽管藻酸盐颗粒和发射是各向同性的。推进机制类似于肥皂或樟脑船，但是，这些机制通常在形状或排放中不对称以选择游泳方向。我们通过分析表面活性剂扩散和对流和流体流的耦合问题，其中包括表面活性剂诱导的流体Marangoni流量以及在空气水接口处的表面活性剂吸附，从而从低雷诺数字开始发展了Marangoni船推进的理论；我们还包括表面活性剂的可能蒸发。游泳速度取决于阻力和Marangoni部队的平衡。我们表明，自发对称性破坏导致推进的可能性超过了无量纲的表面活性剂发射率（PECLET数）。我们通过努塞尔特数字的概念得出了小子数和游泳速度之间的关系，并推广到更高的雷诺数。该理论解释了观察到的PEG-铝酸胶囊的游泳速度，并且我们根据樟脑的其他Marangoni船系统的差异，这主要是由液态空气界面蒸发造成的。胶囊Marangoni Microswimmers还表现出表面活性剂介导的排斥相互作用与墙壁，这可以通过在壁上的表面活性剂积累来定性地解释。
We present a realization of a fast interfacial Marangoni microswimmer by a half-spherical alginate capsule at the air-water interface, which diffusively releases water-soluble spreading molecules (weak surfactants such as polyethylene glycol (PEG)), which act as "fuel" by modulating the air-water interfacial tension. For a number of different fuels, we can observe symmetry breaking and spontaneous propulsion although the alginate particle and emission are isotropic. The propulsion mechanism is similar to soap or camphor boats, which are, however, typically asymmetric in shape or emission to select a swimming direction. We develop a theory of Marangoni boat propulsion starting from low Reynolds numbers by analyzing the coupled problems of surfactant diffusion and advection and fluid flow, which includes surfactant-induced fluid Marangoni flow, and surfactant adsorption at the air-water interface; we also include a possible evaporation of surfactant. The swimming velocity is determined by the balance of drag and Marangoni forces. We show that spontaneous symmetry breaking resulting in propulsion is possible above a critical dimensionless surfactant emission rate (Peclet number). We derive the relation between Peclet number and swimming speed and generalize to higher Reynolds numbers utilizing the concept of the Nusselt number. The theory explains the observed swimming speeds for PEG-alginate capsules, and we unravel the differences to other Marangoni boat systems based on camphor, which are mainly caused by surfactant evaporation from the liquid-air interface. The capsule Marangoni microswimmers also exhibit surfactant-mediated repulsive interactions with walls, which can be qualitatively explained by surfactant accumulation at the wall.
