Terminal velocity of single bubbles in surface tension force dominant regime

Terminal velocity VT of a single bubble rising through an infinite stagnant liquid in surface tension force dominant regime was investigated theoretically and experimentally. A theoretical VT model, which is applicable to a distorted spheroidal bubble with a high bubble Reynolds number, was deduced from a jump condition and a potential flow theory for a flow about an oblate spheroid. Experiments were conducted using air and water to measure bubble trajectories, shapes and velocities. As a result, it was confirmed that (1) the primal cause of widely scattered VT in this regime is not surfactant concentration but initial shape deformation, (2) small initial shape deformation results in a low VT and a high aspect ratio, whereas large initial shape deformation results in a high VT and a low aspect ratio, (3) the primal role of surfactants in this regime is to cause the damping of shape oscillation, by which a contaminated bubble behaves as if it were a clean bubble with low initial shape deformation, and (4) the proposed model gives good predictions of VT for single distorted bubbles.