Influence of added dye on Marangoni-driven droplet instability

Multiphase flows are challenging systems to study, not only from a fundamental point of view, but also from a practical one due to the difficulties in visualizing phases with similar refraction indices. An additional challenge arises when the multiphase flow to be observed occurs in the submillimetric range. A common solution in experimental fluid dynamics is the addition of a color dye for contrast enhancement. However, added dyes can act as surface-active agents and significantly influence the observed phenomena. In the recently reported Marangoni bursting phenomenon, where a binary droplet on top of an oil bath is atomized into thousands of smaller droplets by Marangoni stresses, visualization is particularly challenging and the use of color dyes is naturally tempting. In this work, we quantify experimentally the significant influence of added methyl blue dye, as well as the obtained contrast enhancement. We find that the reaction of the system to such an additive is far from trivial and brings an opportunity to learn about such a complex phenomenon. Additionally, we propose a simple method to analyze the contrast enhancement as a function of dye concentration, with the aim of finding the minimum concentration of dye for successful imaging in similar systems.