Investigation of the oscillating bubble technique for the determination of interfacial dilatational properties
A mathematical framework is developed for measuring the dilatational rheological properties of gas-liquid interfaces by analyzing small amplitude radial oscillations of an air bubble blown into a liquid substrate. Contrary to earlier analyses which customarily require the bubble radius change during the oscillation, the present study is based on the readily measurable quantities of bubble pressure change during the oscillation and phase lag between the bubble pressure and the relative displacement of the oscillation generating device. To expose the extensive interrelationships among the Various experimental parameters that have a profound effect on bubble dynamics, the analysis is focused on the influence of the variable gas volumes at the sides of the differential pressure transducer, the gas phase compressibility, the dynamic behavior of the transducer membrane and the bubble surface being only a segment of a sphere, as they play a key role in the response of the system. A linear viscoelastic surface behavior is assumed which includes intrinsic surface viscosities and Gibbs elasticities. New data are deduced from previous measurements on the dilatational surface properties of stearic acid monolayers which, being practically insoluble in water, offer an ideal experimental system without the complications due to bulk-interface interactions. For a fixed frequency of oscillation, a virtually constant dilatational surface viscosity is determined throughout the employed concentration range. Yet, surface viscosity is found to slightly increase when the oscillation frequency increases. In addition, for the range of the examined parameters, no firm evidence of a dilatational surface elasticity is observed. (C) 1999 Elsevier Science B.V. AII rights reserved.