Motion and drag of a single bubble in super-purified water

We experimentally study the motion and shape of a single rising bubble in super-purified water. We develop an apparatus that is equipped with a measurement system for monitoring both the resistivity and amount of total organic carbon of water in order to evaluate the purity of water. We investigate the critical Reynolds number above which the rising bubble motion changes from rectilinear to zigzag or spiral motion. It is revealed that the critical Reynolds number is susceptible to the range of observation height in the experimental tank. Next, we investigate the effects of the amount of contaminant on bubble motion. The drag coefficient of bubbles in the case of low Reynolds numbers is greatly affected by a small amount of contaminant. We also compare the experimental results of both the terminal rising velocity and drag coefficient with theoretical results for ellipsoidal bubble obtained for high Reynolds number. For smaller bubbles, good agreement is found between the experimental and theoretical results. For larger bubbles, however, discrepancies are prominent. Finally, we examine the effects of the bubble shape on its motion. There exist significant discrepancies between the experimental and theoretical results of aspect ratio as a function of Weber number, even in the range where the experimental and theoretical results in both the terminal velocity and drag coefficient show good agreement.