Dynamic Ordering in a Swarm of Floating Droplets Driven by Solutal Marangoni Effect

We review our recent results on a simple droplet system consisting of droplets of a mixture of two liquids. Although the system is composed of simple chemicals, it is capable of exhibiting complex collective behaviors as a cluster dynamics. We propose a simple inhomogeneous force model to analyze the experimental observations and discuss the mechanisms of the dynamic ordering in our system. The relevance of the observations to hierarchical ordering generally observed in systems of active elements is also discussed.