Experimental studies on the shape and motion of air bubbles in viscous liquids

Abstract

This paper is concerned with single bubble dynamics over a wide experimental data set in stagnant water and glycerol aqueous solution. The bubble trajectory in three-dimensional space was deduced by analyzing the bubble characteristics in two-dimensional plane captured by a single high-speed camera and the bubble trajectory in the water was ascertained by this method. Bubble shape, trajectory and terminal velocity in water, which closely related and strongly influenced by each other, are not only determined by bubble diameter, but also dramatically influenced by nozzle diameters, i.e. the bubble dynamics is sensitive to the disturbance of detachment process. The bubbles remain spherical and rise up rectilinearly when their diameters are small enough. Otherwise, the bubbles begin to deform to ellipsoidal, oblate ellipsoidal, or cap shape, with surface wobbling strongly, and proceed in a zigzagging, helical, nearly zigzagging or helical motion. In this case, periodic oscillations occur at the velocity and aspect ratio of the bubbles. Moreover, there exists an inverse function relationship between them, a large deformation, i.e. a small aspect ratio will lead to a high velocity and vice versa. However, in glycerol aqueous solution, bubble shape, trajectory and velocity is stable under a certain bubble diameter. In the water, bubble shape is mainly dominated by the inertial force and surface tension, and the influence caused by the viscous force could be neglected. The influence of gravity should be taken into consideration if the bubble diameter is large enough. However, the bubble shape is mainly dominated by the viscous force, surface tension and inertial force in the glycerol aqueous solution. Available correlations in the literature do not give fully satisfactory results in predicting aspect ratio, and new correlations combined Weber number with Eötvös number and Weber number with Reynolds number were proposed to correlate bubble shape in water and glycerol aqueous solution, respectively, showing that a good relevance between them in the range of the present experimental data set.