Rise, bouncing and coalescence of bubbles impacting at a free surface

The rise, bouncing and coalescence processes of millimetric gas bubbles impacting at a free surface were studied experimentally. Single air bubbles were released from a syringe into a methacrylate tank filled with ethanol. The position and shape of the bubbles were measured from the images recorded by means of a high-speed camera. Bubbles with equivalent diameters d e < 0.47 mm rise until they touch the free surface and coalesce with it immediately. In contrast, bubbles with equivalent diameters d e ≥ 0.47 mm bounce repeatedly before the coalescence with the free surface occurs. We present results on the bubble terminal velocities, drag forces, and shape of the bubble during its steady rise before coalescence takes place. The oscillatory behavior of the bubble shape after the collision is described, and the position and velocities of bubbles during the bouncing process are analyzed. The motion of the bubble after the first bounce is found to be very similar to that of a damped oscillator. Bouncing time increases with bubble size, and so does the height of the bounce.