Capillary Break-up of Liquid-Liquid interfaces: (map of misery)2

Studies of capillary breakup phenomena of liquid filaments are classically related to liquid-air interfaces; however, there is a variety of applications where liquid-liquid interfaces are involved. The presence of a surrounding medium of different viscosity and different interfacial tension significantly changes the dynamics, so that the classical descriptions need to be reconsidered. Pioneering studies on this topic focused usually on the thinning and breakup behaviour near pinch-off in dripping experiments, exploiting a large range of viscosity ratios. For this, different similarity solutions were proposed to describe the universal pinch-off for well-defined regimes of the Navier-Stokes equations. The aim of the current work is to explore the thinning behaviour away from the proposed self-similar final stage of pinch-off for a wide range of inner and outer fluids, combining inertia controlled, viscous and intermediate fluids. To this end, capillary thinning of filaments with liquid-liquid interfaces is investigated, using a modified version of a capillary breakup setup. This novel liquid-in-liquid technique allows to accurately control the instability induced in the filament and to vary the step-stretch profile. A high-resolution optical setup enables to get information about the complete shape of the interface throughout the capillary breakup process. Results will be presented, focussing on the whole parameter range of inner and outer fluid types, to demonstrate transitions from viscosity controlled to the self-similar thinning for a viscous inner fluid and an inertia-viscous outer fluid. Secondly, we show that the capillary thinning of inviscid fluids can be investigated in higher detail, thanks to the slowed-down dynamics arising from the lower interfacial tension. The proposed framework will pave the way towards studies of active interfaces, stress and deformation rates determination using techniques like particle tracking, X-ray scattering and birefringence in capillary breakup experiments.