Temperature effect on single bubble velocity profile in water and surfactant solution

Surfactants called frothers control bubble properties in flotation. Two properties of single bubbles, the rise velocity profile (velocity vs. time from release) and terminal velocity, are investigated as a function of temperature (6–45 °C) in water and solutions of Triton X-100 (12.5×10−5 mol m−3) and Dowfroth 250 (0.06 and 0.25 ppm). The bubbles sizes (diameter) were <2 mm to ensure rectilinear motion. The terminal velocity in water, estimated from the maximum in the profile, increased with temperature and was reasonably predicted by the model due to Moore [J. Fluid Mech. 23 (1965) 749]. The terminal velocity in surfactant solution, estimated from the constant velocity stage of the profile, showed a lack of dependence on temperature when Reynolds number (Re)>130. This corresponds to the finding of Karamanev [AIChE J. 40 (1994) 1418], namely that the drag coefficient is constant (∼0.95) for this Re region. The profile showed the time to reach constant velocity tended to decrease as temperature increased. This is analyzed by considering factors that may increase the mass transfer rate of the surfactant. Some observations are offered of possible relevance to flotation.