The dynamics of compound drops at high Reynolds numbers: Drag, shape, and trajectory

The behaviour of compound drops rising at high Reynolds numbers, roughly 60 to 700, was studied experimentally for a wide range of diameter ratios and three different combinations of the internal and external fluids. Two rising regimes were identified, namely a rectilinear and an oscillatory trajectory. The governing effects in each motion regime were discussed. The compound drops presented mild shape distortions in all the measurements, suggesting that the internal fluid movement might play an essential role in the motion transition. The compound drops presented a stable path when the internal bubble is small with viscous effects governing the motion. The internal bubble reduced the viscous dissipation and the drag coefficient exhibited intermediate values compared to correspondent single-fluid drops and bubbles. For a large internal bubble, the path of the compound drops becomes oscillatory and viscous effects are diminished. The oscillation is characterized by a relative motion of the external fluid relative to the inner bubble, thus increasing the drag coefficient. Mechanistic models for the drag coefficient based on the governing dimensionless numbers of the flow were proposed, and they showed a good agreement with the measurements.