Date of this Version
J. M. Davidson, "Experimental study on the effect of air flow on soap bubble formation," MS thesis, Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, 2013
Soap bubbles are a common interfacial fluid dynamics phenomenon having applications such as buoyant hollow spherical fillers and flow visualization of large scale airflows. In contrast to the dynamics of liquid drops in gas and gas bubbles in liquid, the dynamics of soap bubbles has not been well documented, possibly because soap bubbles have gas-liquid-gas interfaces. Having the thin-liquid-film interface seems to alter the characteristics of the bubble/drop creation process. Thus, the main objective of this study is to experimentally examine how airflow develops and interacts with the soap liquid film as the film stretches and finally collapses to pinch-off.
A soap bubble blowing apparatus was constructed to consistently reproduce soap bubble blowing process with various soap ring sizes, and high-speed videography was utilized to investigate the blowing process. The minimum radius of the neck of the soap liquid tube was measured when approaching pinch-off. It is seen that the minimum neck radius is approximately proportional to the time left till pinch-off by a scaling exponent of about 2/3. The exponent is 1/3 for a gas bubble in liquid and 2/3 for a liquid drop in gas. This indicates that liquid drops collapse more slowly than gas bubbles because the liquid is more viscous and that soap bubbles show slower collapses due to two gas-liquid interfaces.
One observed difference in soap bubble pinch-off due to the different ring size was that the pinch-off process followed the scaling law less as the ring became smaller. Based on this observation, a hypothesis was proposed that airflow outside the soap liquid tube causes variances in the pinch-off of the tube. To test the hypothesis, particle image velocimetry was used to observe the effects of airflow on the growing soap liquid tube. It is shown that the smaller rings more interfere with the airflow causing perturbation and thus increase the deviation from the scaling law, which supports our hypothesis.
Advisor: Sangjin Ryu