Expansion and collapse of an initially off-centered bubble within a narrow gap and the effect of a free surface

Abstract

A non-equilibrium oscillating bubble within a narrow gap in between two flat perspex plates has, in earlier studies, been reported to show complex behavior when it is centered between the plates. We study here what happens when the bubble is created in an off-centered position with respect to the mid-plane between the plates. The effect of the non-dimensional gap-width, i.e. the gap width normalized by the maximum bubble radius, and a second parameter which indicates the degree of offset from the center are studied. A high-speed imaging technique which utilizes a single camera and a set of mirrors to simultaneously capture both the side-view and the front-view of the bubble dynamics is employed in this study. Results show that while the direction in which the bubble forms a jet depends on the non-dimensional gap-width with a change of direction observed at a value of 0.5, the degree of offset does not seem to have any influence on the jet direction. Additionally, the behavior of a centered bubble between two plates which are positioned near a free surface is also presented. The presence of three boundaries near the bubble affects both the jetting behavior of the bubble and the formation of a spray above the free surface resulting in interesting bubble dynamics.