Bubble mobility in seawater during free-rise, bouncing, and coalescence with the seawater-air interface

The way bubbles in seawater interact with the seawater-air interfaces plays an important role in many naturally occurring processes. Here we use high-speed video imaging to investigate how millimeter-sized bubbles free-rise, bounce, and coalesce with the seawater-air interface in seawater samples collected from the Red Sea coastal area. The samples salinity and organics content are typical for the open oceans and seas. To evaluate the seawater-air interface mobility, which is one of the fundamental properties determining the bubble hydrodynamic interactions, we measure the free-rise velocity of the bubbles and their bouncing trajectories from the interface. Our experiments demonstrate that both the bubble interface and the pool seawater-air interface are fully mobile, indicating that the presence of high concentration of electrolytes and organics in the seawater does not affect the interfaces mobility during free-rise and bouncing for these bubble sizes. In contrast, the presence of electrolytes in the seawater is shown to increase the characteristic time for the final bubble coalescence with the interface from milliseconds to seconds. Whereas the delay of the bubble coalescence by the high concentration of the electrolyte is a well-known phenomenon, here we evaluate this effect for single bubbles in actual seawater samples to separate the effects of the electrolyte and organic components. • We investigated millimeter-sized bubbles in seawater, during free-rise, bouncing and coalescence. • The bubble interface and the seawater-air pool interface remain fully mobile during free-rise and bouncing. • The final coalescence in seawater occurs much more slowly compared to pure water. • The dramatic increase in the coalescence times could indicate switch from mobile to immobile interfaces before coalescence.