Destabilizing Marangoni flow in an evaporating binary liquid lens

Evaporating multi-component droplets are ubiquitous in nature. They are very complex systems, governed by many parameters, such as volatility and surface tension, to name but two.

One example of an unstable multi-component evaporating system is Marangoni Bursting as first studied by Keiser et al. [PRL 118, 074504 (2017)].

Surface tension and evaporation induce an alcohol/water droplet floating on an oil bath to burst into daughter droplets. The different volatilities and surface tensions of the two components generate a Marangoni flow, which leads to an instability at the droplet rim.

Here we present novel experimental observations of the flow in the mother droplet using high-speed microscopic imaging to characterize contact line motion, rim instability, droplet pinch-off, and the internal flow field. The Marangoni-driven velocity increases with an increasing alcohol concentration φ_0 in the droplet, confirming previous studies. However, towards the unstable contact line the flow slows down and, independent of φ_0, reaches an unexpected plateau value of constant velocity.

We compare our experimental results with numerical simulations, which offer us – previously inaccessible – insights into the phenomenon, such as the interface shape between binary mother droplet and oil bath.