Dynamics of oil-coated bubbles rising in a quiescent liquid*

Compounded bubbles with a liquid phase in another continuous bulk phase are produced in a wide range of natural and industrial processes, such as gas released from natural seeps in the deep sea and froth flotation with oily bubbles. In these configurations, how does the coating influence the bubble rising dynamics remains largely unexplored. Here we take the oil-coated bubble as an example to experimentally investigate its rising dynamics with various oil fractions in a quiescent water medium. We find that the oil coating changes the rising dynamics of the bubble mainly by modifying the bubble surface boundary condition and effective density. A lightly-coated bubble experiences a smaller shape deformation and similar drag coefficient compared to a clean gas bubble, and the increase of oil fraction results in reduced shape deformation and drag coefficient. Estimation of the forces using a Frenet reference frame shows that the wake-induced lift decreased with oil fraction, based on which the effect of oil coatings on the path oscillation frequency and amplitude are further rationalized. Overall, our work contributes to the fundamental understanding of the rising dynamics of oil-coated bubbles with various oil fractions and viscosities.