Simultaneous internal and external flow field measurement of buoyant oil droplet

In this study, simultaneous flow field measurement of internal and external flow field under 2 different liquids was investigated using time-resolved three-dimensional particle tracking velocimetry (4-D PTV). We generated a silicone oil droplet with tracers of oil phase from a nozzle in a 16-sided water tank. The water tank was filled with a mixture of water and glycerin with tracers of mixture phase. The mixture contained 50 weight percentage of glycerin to match refractive index of 2 liquids, which prevents distortion of the tracer particles in the oil droplet. All the particles were illuminated by 2 blue LEDs and diffusing sheets opposite to 4 high-speed cameras. The individual particles trajectories were tracked using Shake-The-Box (STB) algorithm with iterative particle reconstruction (IPR), and the Lagrangian particle trajectories were reconstructed to Eulerian flow field using Vortex-In-Cell sharp (VIC#) method. We observe the edge of droplet is not detected due to the refractive index matching. While the droplet rises, it generates upward flow at the center of the droplet and the flow goes down along the interface of the droplet. The flow induces large vortex generation in the droplet. Also, the surrounding mixture flow seems to be similar to the results of air bubbles. We observe the droplet pushes the mixture away along its interface, and the flow is entrained to the trailing region of the droplet.