Dynamics of an oil-coated bubble rising in a quiescent water medium

We experimentally investigated the rising dynamics of oil-coated compound bubbles at various oil fractions in a quiescent water medium. Three-dimensional particle tracking velocimetry was used to characterize the trajectories of the bubbles, and particle image velocimetry was used for complementary flow characterization. Results show that the oil-coated bubbles undergo a zigzagging path with a steady oscillation pattern at comparatively low oil fractions. In contrast, damped oscillations occur at high oil fractions, which do not happen in clean gas bubbles. The oil coating changes the rising dynamics of the bubble mainly by adjusting the bubble surface boundary condition and effective density. A lightly coated bubble experiences a smaller shape deformation, similar drag coefficient, a larger frequency, and smaller amplitude in the path oscillation compared to a clean gas bubble. In addition, the increase of oil fraction results in reduced shape deformation and drag coefficient with a lower frequency and amplitude of the path oscillation. Estimation of the forces using a Frenet reference frame shows that the wake-induced lift and drag decreased with oil fraction, and became negligible for bubbles with damped oscillations. Overall, our work contributes to the fundamental understanding of the rising dynamics of oil-coated bubbles with various oil fractions and viscosities.