Description
Despite the extensive investigations of jet formation and breakup in co-flowing microfluidics, far less attention has been given to how a microfluidic jet interacts with an interface between two immiscible fluids. Here we experimentally study the interaction between a microfluidic jet and a water-silicone oil interface in a nested, co-flowing capillary microchannel. An inertial jet of water is fired through silicone oil, towards the interface and we study the interaction between the jet and the interface post coalescence. By varying the Weber number of the jet and the channel confinement, four distinct regimes emerge within the parameter space: piercing, tunnelling via vortex ring formation, tunnelling via cascading vortex ring formation, and tunnelling via flow-focusing. We rationalise the distinct regimes by considering the balance between the jet inertia and the restoring surface tension of the interface, and the geometric confinement experienced by the intruding jet.