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Bubble formation and growth in a liquid is an important process in many industries. Bubble formation regime determines the essential characteristics of the bubble formation process such as bubble volume, shape, and formation time. The formation regime changes from single to double and multiple by increasing the volumetric gas flowrate. In the present research, various regimes of air bubble formation in pure water and glycerin solutions (glycerin 92%, glycerin 96%, and glycerin 100%) were observed by conducting experiments, and they have been defined in terms of the dimensionless Froude (Fr) and Bond (Bo) numbers. The liquids that have been used in the experiments provide a wide range of viscosity from 0.001 to 1.07 Pa·s. However, the surface tension remains approximately constant. In these experiments, relatively small needle sizes (lower than 0.6 mm), Bond numbers smaller than 0.05, and Froude numbers smaller than 70,000 were used, and the boundaries between various regimes were determined. The results indicate that the Froude number associated with the boundaries between various regimes decreases by increasing Bond number. In addition, for a given needle diameter, the air flowrate at which the regime changes from single to double is lower in glycerin solutions than in water. The mentioned flowrate decreases by increasing the liquid viscosity. Finally, based on the governing equations and experimental results of this study, a new correlation has been obtained to estimate the volume of the first bubble at the moment of the detachment in the double coalescence regime.

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