Studying the Shape and Path of Air Bubbles Rising in Clean Water.

Abstract

This study looks at the shape and path of air bubbles (diameter 0.1 ? 0.2 cm) rising in clean water. In previous experiments [1], bubbles were generated using capillary tubes of different sizes; the bubble shapes were recorded by a high speed camera. This summer, software routines developed in Matlab are used to (1) find the contours of the images, (2) fit the images to a function, and (3) obtain information such as bubble diameter and aspect ratio. Two stable shapes were observed for the bubbles in this diameter range ? ellipsoidal and spherical. The analysis suggests that the shape the bubbles assume depends on the size of the capillary tube from which bubbles detach. The bubbles become ellipsoidal if the capillary tube is much smaller than the bubble diameter, and spherical if the tube is of size comparable to the bubble diameter. It is found that the bubble velocity and trajectory are different for the two bubble shapes. Spherical bubbles move notably slower than ellipsoidal ones of equivalent diameter. When the bubble diameter exceeds a threshold, the straight path of spherical bubbles changes to a zigzag path, while that of ellipsoidal bubbles becomes a spiral. Reference: [1] Mingming Wu and Morteza Gharib, 'Experimental investigations on the shape and motion of small bubbles rising in clean water', Preprint, 2001..