Force measurements on rising bubbles

The dynamics of millimetre-sized air bubbles rising througli still water are investigated using ultrasound velocimetry combined with high-speed video. From measurements of speed and three-dimensional trajectories we calculate time-resolved precise drag and lift forces on the bubble, which give rise to planar zigzag and three-dimensional spiralling motion. Temporal correlations of forces and the oscillatory bubble motions, particularly the lift force, emphasize the importance of the wake vortices found in previous studies.