Potential repulsive force acting on a pair of slightly different size in-line bubbles at intermediate Reynolds number

Various studies have been reported about a pair of bubbles rising in-line. Two same size bubbles rise keeping a constant distance in the numerical or theoretical prediction due to the balancing between the potential repulsive and viscous attractive forces, and the distance is called equilibrium distance. On the other hand, a motion which trailing bubble deviate from a vertical line was reported in experiments and the deviated distance was larger than the equilibrium distance. We considered that this is due to the slight difference of two bubbles diameter in the experiment. In this study, we observed the motion of two bubbles rising in-line experimentally with precise measurement of bubble sizes. As a result, two bubbles rose linearly just after bubble generation. After that, bubbles deviated from a vertical line. When the trailing bubble was larger than the leading bubble, two bubbles deviated from a vertical line to opposite direction. We believe this deviation is caused by repulsive force due to potential effect. On the other hand, when the leading bubble was larger than trailing bubble, only the trailing bubble deviated from a vertical line. We considered that the deviation of the trailing bubble was caused by lift force due to wake of the leading bubble.