Flow instabilities due to Marangoni stresses in ternary systems*

We study the flow patterns resulting from a flow instability of a ternary system in a co-flow configuration inside a channel, where the jet consists of a binary mixture (oil and solvent), and the surrounding sheath flow of the third component (water). Due to a solvent-exchange process at the jet boundary, small oil droplets nucleate close to the inlet, while further downstream large oil-rich droplets on the scale of the jet diameter form. Using high-speed imaging, we demonstrate that the resulting droplets grow over time, and are able to migrate upstream, either engulfed axisymmetrically by the jet, or in an oscillatory motion around the jet center axis. We rationalize these observations by demonstrating a similar instability in a binary system of an ethanol jet in water, where a gas bubble is introduced at the jet center axis, and show that the driving force of the droplet or bubble motion is due to solutal Marangoni stresses acting at the fluid interface. Our results emphasize the complex interaction between fluid flow, species transport and interfacial properties, and allow for a better understanding of channel flows of ternary systems.