Interpreting the Dynamic Interaction between a Very Small Rising Bubble and a Hydrophilic Titania Surface

Abstract

We interpret recent measurements of time-varying interference fringe intensities observed between rising bubbles (diameter 15−120 μm) and a horizontal hydrophilic titania plate to determine the rate of approach of such bubbles to the surface. The bubbles remain spherical because of high Laplace pressures and small buoyancy forces so the approach velocity remains in the Stokes flow regime. The rate of approach of the bubble to the titania plate is controlled by buoyancy force and surface forces such as van der Waals and electric double layer interactions together with a hydrodynamic force arising from the drainage of the thin water film between the bubble and the titania surface. The dynamics are found to be consistent with the no-slip hydrodynamic boundary condition at the surface of the bubble and at the titania plate. However, far from the titania plate the terminal velocities of the rising bubbles suggests that the bubble surfaces are fully mobile with zero tangential stress.