| Coalescence is a process where two fluidic bodies merge into one. Coalescence will occur, for example, after a raindrop impacts on a pool of water or when petroleum oil is collected in bulk after separating it from water. However, the topological transition and the short length and time scales associated with coalescence made it difficult to model analytically or simulate numerically. Hence, predictions of any coalescence phenomena are typically made through empirical or ad-hoc methods which can be cumbersome and costly. The objective of this investigation was to obtain detailed velocity field measurements using the particle image velocimetry (PIV) method on a simple two-fluid flow undergoing a coalescence process. These measurements will provide test cases against numerical simulations.; The flow was modeled experimentally by releasing a single drop through a less dense ambient until it achieved a terminal velocity before impacting onto a liquid/liquid interface. Viscosity of the less dense ambient was varied to study the effect of Reynolds number on the flow. The drop approach and impact were also studied to observe their influence on the coalescence process. Index matching and a slight camera inclination were employed to eliminate optical distortions. Sequences of images and velocity fields were obtained using a high-speed video camera to study flow evolution during impact and coalescence.; The experimental measurements showed that coalescence is decoupled from impact for the liquid/liquid combinations studied here. Prior to impact, a wake existed upstream of each drop due to viscous and pressure drag. Inside the drop, the fluid circulated around a vortex ring whose diameter extends near the drop boundary. After impact, the viscously-induced circulation inside the drop dissipated completely due to wake impingement and drop deformation. The interfacial rupture which led to coalescence was typically off-axis, followed by a retraction of the thin film free edge with velocities of more than ten times the impact velocity. Vorticity was generated near the film free edge due to excess pressure inside the drop and expanded outward along with the retracting film. The drop collapse rate was influenced by a contracting capillary wave that pinched the upper drop surface. |
