Hydrodynamic effects on the oscillations of supported bubbles: Implications for accurate measurements of surface properties
Journal
Colloids and Surfaces A: Physicochemical and Engineering Aspects
Journal Volume
250
Journal Issue
1-3 SPEC. ISS.
Pages
367-384
Date Issued
2004
Author(s)
Abstract
Oscillating bubble techniques are commonly used to infer dynamic surface tensions (DST) from the measurements of the dynamic pressure differences across an interface. In inferring DST from such measurements, it is assumed that hydrodynamic efects either are negligible or can be approximated. In virtually all previous studies, the dynamic bubble shapes, which are set by the requisite balance of forces at the bubble surface, are taken to be nearly spherical and assumed to be governed by the static Young-Laplace (Y-L) equation alone. To examine these assumptions, the Navier-Stokes and continuity equations governing the flow of a liquid outside an axisymmetric bubble supported by a narrow capillary are solved simultaneously by a rigorous finite element method. Cases of constant surface tension (pure liquids or solutions with very fast adsorption) are tested to focus on understanding the effects of fluid motion on surface tension measurements. To test the capability of the computational algorithm on describing the hydrodynamics, computed and experimental velocity profiles are compared and found to be consistent, as are the apparent surface tensions. Parameters such as forcing frequency, forcing amplitude, chamber dimensions, and surface tension and viscosity of the liquid are varied to find the limits where pulsating bubbles depart from spherical and where hydrodynamic effects impact the determination of surface tension. In the commercial pulsating bubble surfactometer (PBS), the bubble shapes remain nearly spherical for low pulsation or oscillation rates (≤ 100 Hz) with moderate volume amplitudes. Under these conditions, however, two major hydrodynamic effects, due to the inertia of the bulk liquid, or to the liquid viscosity and the viscous forces acting on the chamber walls, are found to be important and can cause large errors in the surface tension measurements. For measurements of low surface tensions (≤ 5 mN/m) in a PBS, oscillating bubble methods that do not take into account shape deformations from the spherical are found to be inaccurate, and the results from dynamic bubble shape analysis (solving the Y-L equation for a nonspherical axisymmetric surface) are shown to provide another approach to obtain accurate results provided that hydrodynamic effects can be neglected. At higher frequencies (≥ 200 Hz), because of strong convection around the surface of the bubble, the bubble shapes become highly deformed. Further extensions of this algorithm are needed, to include the surfactant diffusion and adsorption effects by which the surface tension may change with time. © 2004 Elsevier B.V. All rights reserved.
SDGs
Type
journal article