Individual and collective dynamics of chemically-active droplets

Chemically-active droplets slowly solubilizing in a surfactant-laden solution can spontaneously self-propel as a result of the non-linear transport of surfactant molecules and micellar compounds by the interfacial flows induced by these slowly-diffusing solutes. Many recent experiments have reported the intriguing individual behaviour of these droplets, in particular the emergence of straight, curly and chaotic trajectories. Furthermore, such droplets are fundamentally anti-chemotactic : they avoid their own chemical trail as well as the proximity of other droplets. In experiments on multiple-droplet interactions, this results in complex avoiding trajectories. Motivated by these observations, we use theoretical modeling and numerical simulation to rationalise these individual and collective dynamics. In this presentation, I will present some of our recent work on the emergence of complex individual behaviour and on the nonlinear interplay of hydrodynamic and chemical interactions in droplet collisions. In particular, based on a first-of-a-kind analysis of the complete hydro-chemical interactions in a two-droplet collision, I will show how such pairwise collisions may lead to the emergence of a variety of collective dynamical regimes, as well as alignment and scattering of the droplets.