Axial velocity profiles of single bubbles in water/frother solutions

The axial velocity profile (local velocity versus time or position) of single bubbles in the presence and absence of frother (Dowfroth 250, MIBC and pine oil) was measured in a water column over a distance of 4 m. Temperature along the column was maintained uniform and constant at 30 degrees C by a temperature-controlled water jacket. Single bubble sizes 0.9, 1.5, 2.2 and 2.7 mm in diameter, covering a range of interest in flotation, were studied. A bubble generation frequency was selected such that velocity was independent of frequency. To follow the bubble during its rise, a video camera supported on a track and capable of moving vertically at adjustable speeds was employed. In the absence of frother, the profiles showed two stages: first the velocity increased rapidly and then decreased continuously over the 4 m. In the presence of frother, a third stage was reached, a constant (terminal) velocity. The terminal velocity was not dependent on frother concentration but, was a function of frother type. The shape and motion (path) of the bubble were also recorded. The observations are qualitatively explained in terms of time-dependent adsorption of surfactants, either present as contaminants in tap and distilled water or deliberately added in the case of frother.