Scaling and dynamics of buoyant immiscible liquid jets in the laminar regime

While immiscible liquid jets play vital roles in applications from chemical reactors to environmental flows, understanding their dynamics has been hindered by optical distortions at fluid interfaces. Using simultaneous Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) with refractive index-matched fluids, this study provides comprehensive experimental validation of both inviscid and viscous analytical models for buoyant liquid-in-liquid jets in the laminar regime. The study uncovers two distinct scaling regimes: an inertia-dominated near field and a viscosity-governed far field, with buoyancy and jet Reynolds number controlling dynamics while surface tension and outer-phase viscosity play minor roles.