Effect of wall inclination on the near wall rising behaviour of air bubbles in still water*

Path instability of rising bubbles is a well-established phenomenon. We conducted experiments using high speed photography to analyze the dynamics of buoyant air bubbles(3-4 mm) rising near an inclined wall(50-120° with horizontal) in still water. The flow properties were computed using image processing. As previously observed, the bubble shows a bouncing trajectory with constant amplitude and frequency, when the wall was placed vertically.  As the wall inclination angle was reduced, the amplitude of the bounces decreased. After a critical angle, the bouncing motion transitioned into a sliding motion in which the bubble steadily rises under the wall. There have been no reported studies on the motion of a bubble rising along an obtusely inclined wall. For such conditions, we found that upto a critical inclination angle, the bubble bounces on the wall once and then it rises upwards. On increasing the angle beyond this critical value, the bubble rises straight upward with negligible wall effect. Thus, the bubble motion can be divided into four different regimes depending on the wall inclination – sliding (0-54°), bouncing (54-90°), one bounce and rise away (90-114°) and straight rising (114-180°). For air bubbles in water, the transition angles are 54° and 114°.