Interfacial phenomena of the interaction between a liquid–liquid interface and rising bubble

We experimentally investigate the interfacial phenomena involved in the occasions when a rising bubble interacts with the interface between two immiscible liquids, by varying the bubble size (3.0–8.0 mm; i.e., covering the straight, zigzagging, and rocking trajectories) and the viscosity ratio ( $$\Lambda =1.38$$ and $$12.8$$ ) of liquids (silicone oil and water–glycerin mixture as the upper and lower liquids, respectively). Our major focus is to understand how the dynamics of a rising bubble and subsequent deformation of the interface are determined, based on the spatiotemporal variation of interface and velocity fields simultaneously measured by using the two-phase particle image velocimetry and laser-induced fluorescence, respectively. When the viscosity ratio is small ( $$\Lambda =1.38$$ ), even the bubble of the same size, under the path instability, passes through the interface quite differently depending on the colliding angle. This is because the flow dragged by the bubble and wake vortices change according to the bubble position in the oscillating rise path. As the viscosity ratio increases to 12.8, the effect of path instability becomes negligible owing to the enhanced viscous dissipation, so that only the bubble in a rocking motion can escape the liquid interface instantly. By estimating the momentum flux, we find that the bubble-induced momentum transferred to the interface dissipates fast as the viscosity ratio increases. In addition, the maximum height (deformation) of the liquid interface is closely related to the escape dynamics of the bubble. We think the present results will enhance our understanding of how the complex interaction between moving fluid interfaces is determined, and further the effective way of controlling it.