A bubble cap (400) has a cap (430) with at least one slot (495) and a riser (420), configured with a skirt height (460) of at least 4 cm, such that 1.5*Skirt Height (cm)+[Slot Length (cm)-Exposed Slot Height (cm)] is greater than or equal to 7.5, and posed such that a liquid fluid and a gas flow co-currently upwardly in a space between the riser (420) and the cap (430).