High-speed visualization of soap bubble blowing and image-processing-based analysis of pinch-off dynamics

Soap bubble blowing has long been an amusement for humans, and the process involves pinch-off similarly to liquid drops and gas bubbles. To visualize the pinch-off process of soap bubble blowing, we built an apparatus consisting of air jet flow and thin soap film on a circular ring, and replicated human soap bubbling. High-speed videography captured growing soap film tube and following pinch-off, and the minimal neck radius of the tube was measured based on image processing. Scaling law analyses show that regardless of the ring diameter, the scaling exponent of soap bubble pinch-off is about 2/3, which is similar to that of soap film bridge. Also, the speed of the airflow into the tube was evaluated based on volume calculation of the soap film tube, and the Reynolds number of the airflow was estimated to be 1060-2970, which suggests that soap bubbling may involve Bernoulli suction effect.