In order to understand the behaviors of the stretching gas-liquid interface and the effects of surfactants, the coalescence of nitrogen bubble pairs issued from neighboring nozzles submerged in water has been studied by means of high-speed photography. It is shown that small amount of surface active additive is able to increase the coalescence time greatly. The transition from 0 to 100 percent bubble coalescence during their stay at the nozzle mouths happens over a very narrow concentration range. The central value of this concentration range increases with the increase of bubbling frequency, which is proportional to the heat flux in the case of nucleate boiling.
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