Distinct from solid particle, bubble is soft matter, which tends to adjust its shape according to motion, and according to the work of Tomiyama, for bubble with diameter larger than 1.3mm, the rising path would change from rectilinear to zig-zag or helical, which make the determination of the terminal velocity more difficult. Even for the bubble with diameter larger than 0.075 mm reported by Pawliszak, terminal velocity scatter started.

The existence of surfactants or contaminants and different initial deformations of bubbles introduced by varying capillary or nozzle diameters were generally considered to be the two main reason account for the terminal velocity scatter.

In this paper, the rising characteristics of small bubbles with 1 < Re < 100 were investigated. Especially the motion characteristic right after the departure. Comparisons were made between three bubbles with almost the same diameter (0.58 ± 0.008mm), the same aspect ratio (0.98 ± 0.01), it was found that the only differences lied on oscillation frequencies of aspect ratio. And the higher oscillation frequency of aspect ratio was, the higher steady-state velocity the bubble would achieve. Therefore, it was thought that the deformation is one of the ways for bubble to accelerate itself, i.e., bubbles accelerated itself through the oscillations of aspect ratio. The oscillation of aspect ratio is of high possibility to be one reason for velocity scatter.

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