Initial drop coalescence at a liquid–liquid interface

The initial coalescence of drops with bulk liquid in various outer fluids was investigated, focusing on the effect of outer fluid's viscosity. The inner fluids included both Newtonian and non-Newtonian liquids. A micro-particle image velocimetry (micro-PIV) system with 0.2 ms temporal and 5.2 μm spatial resolution quantified the flow field and mapped the viscosity distribution within the coalescing non-Newtonian drops. The temporary evolution of the liquid bridge width was tracked using both an electrical method with 0.8 μs resolution and an optical method with 10 μm resolution. In air, the rescaled bridge width Φ followed classical regimes with normalized time τ, exhibiting a viscous regime (Φ ~ τ) followed by an inertial regime (Φ ~ τ^1/2). In viscous outer fluids, both regimes are present, but a prolonged crossover regime was observed. An empirical fitting Φ ~ τ lnτ is found to satisfactorily describe the whole coalescence.