In this study, the injection of a viscoplastic (yield stress) fluid into a quiescent Newtonian medium is experimentally investigated, using a range of experimental methods, including high-speed camera imaging, laser-induced fluorescence (LIF), and ultrasound Doppler velocimetry (UDV) techniques. The case of our consideration is the continuous downward injection of a Carbopol solution, as a core fluid, through an eccentric inner pipe, placed inside a vertical closed-end pipe filled with a glycerin-water solution. The injected fluid is always heavier than the in-situ fluid and the fluids are miscible. As the injection continues, a filament of the core fluid immersed in the in-situ fluid is formed, for which it has been shown that there exist three distinct flow patterns, namely, the breakup, coiling, and bulging (buckling) regimes. Our focus in this study is to analyse the detailed flow features and sub-regimes at longer times, from an experimental perspective. To generalize our results, they are presented versus the governing dimensionless numbers of the flow, i.e. the buoyancy number, the Bingham number, the viscosity ratio, and the falling height. The breakup regime is analysed in terms of the yielding, necking and pinch-off process, the length of filaments, and their subsequent falling behaviours, the coiling regime in terms of the regular, free and irregular coiling behaviours, and the bulging (buckling) regime in terms of the appearance of a detached falling part and the maximum penetration depth. The results of this work can find applications, in particular, in the plug and abandonment (e.g. cementing processes) of oil and gas wells and 3D printing technologies.
