Effects of gravity level on bubble detachment, rise, and bouncing with a free surface

Abstract

Bubble detachment, rise, and bouncing upon impact with a free surface is studied experimentally in variable gravity conditions. Previous investigations focused on the effects of fluid properties such as viscosity or surface tension on the rise and bouncing dynamics. Gravity force is a crucial factor in the detachment, rise and bouncing processes. However, the effect of different gravity levels has never been studied experimentally. In this paper we analyze the role of gravity in the detachment, rise velocity and bouncing motion of millimetric bubbles colliding with a free surface. Single air bubbles in ethanol are detached from a nozzle by the buoyancy force. After reaching a terminal velocity, the rising bubble interacts with the free surface in a bouncing process prior to coalescence. The equivalent bubble diameter at detachment decreases as the gravity level increases, in agreement with the theoretical prediction. An expression for the terminal velocity as a function of gravity is proposed. The terminal velocity is found to increase with the gravity level, although bubbles are smaller at higher values of gravity. The bouncing process has been modelled by a damped oscillator, in which the free surface acts as an elastic membrane. An expression for the frequency of bouncing as a function of gravity has been obtained, showing a good agreement with the experimental results. The motion of the bubble during the bouncing process can be approximated by an underdamped oscillator even if viscosity is negligible. Therefore, viscosity is not the main responsible for damping, which is probably due to energy transfer from the bubble to the fluid in the form of vortex and surface waves generation.