Investigation of the mechanisms which govern bubble geometry in the spherical cap regime though application of particle image velocimetry

The significant variation in bubble geometry, as a function of size, has fascinated scientists for hundreds of years. However, a comprehensive explanation for these observations and the underlying physics has eluded researchers. Fluorescent particle image velocimetry has the potential to add new insight and is applied to examine the physics which cause larger bubbles to take on a spherical cap geometry. The experiments measured the velocity field around bubbles ranging from 0.5 mL to 4.0 mL under counterflow conditions in a vertical channel. The measurements showed the wake of a stable spherical cap bubble takes the form of a stable vortex ring, sized such that it approximately completes the sphere for with the bubble forms the cap. The vortex ring is followed by a streamwise cylindrical vortex tail with the two separated by a stagnation point. The flow around the bubble and its wake behaves similar to potential flow around a sphere. The transition volume from the elliptical regime to spherical cap regime is also investigated. Instead of an abrupt transition, a transition region is found where the bubbles are characterized by an unstable geometry. The vertical motion is accompanied by horizontal oscillation which coincides with the shedding of a vortex from the leading lateral edge. Finally, a model which describes bubble geometry in the spherical cap regime is proposed and tested.