| There have been many studies about the motion of biological units. The way they obtain momentum from surroundings has attracted attention and many theoretical models have been proposed. Recently, the nonbiological systems that move spontaneously are actively studied as model systems for the biological motion. Spontaneous droplet motion induced by Marangoni effect is one of such systems. Symmetry of interfacial tension field around a surfactant-containing droplet is broken breaks due to the nonlinear effect by the Marangoni flow. The droplet shows spontaneous translational motion when the Peclet number is larger than the critical value. Compared to translational motion, it seems to be difficult to induce the rotational motion using interfacial tension for the following reason: Since the flow speed at the interface is proportional to the gradient of interfacial tension, the uniformly rotating flow around a circular droplet can not be induced. We propose a system that shows spontaneous rotation of a droplet induced by the Marangoni effect. The critical Peclet number is calculated and compared with the experimental results. We also investigated the dynamics of the group of rotating droplets using a simple mathematical model. When the density of the droplet is large enough, droplets accumulate and made the lattice of vortices. |
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