In front of the solidifying interface, the solidifying boundary layer plays an important role in determining whether foreign particles could be engulfed or not by the solidifying interface. In this study, we have investigated how the boundary layer influences the behavior of the foreign particles by directly observing the behavior of fine bubbles in front of a vertical solidifying interface of of the water solution. In the case of surfactant water solution, bubbles in the vicinity of the solidifying interface were found to move horizontally toward the solidifying interface, and then were engulfed by it. However, this phenomenon was not observed in the case of NCl water solution. Analysis indicated that inside the boundary layer with a surfactant concentration gradient, a surface tension gradient around the interface of the bubble and the solution was formed. We have concluded that the surface tension gradient generates a driving force on the bubble, causing the bubble to move towards the solidifying interface. Experimental results were found to be well in accord with theoretical analysis.
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