A new oscillating bubble technique for measuring surface dilational properties

A new oscillating bubble device has been developed and used for measuring the surface dilational properties of surfactant, protein and surfactant/protein adsorption layers in the frequency range 1-300 Hz. Within a closed measuring chamber, a small hemispherical bubble is produced at the tip of a capillary. A piezoelectric driver generates sinusoidal oscillations of the bubble volume, and consequently produces changes in the surface area and the radius. The piezoelectric driver can also produce arbitrary changes of area. This method allows the measurement of the pressure difference across the curved bubble surface, due to changes in the radius and surface tension. The pressure amplitudes are monitored by a sensitive pressure transducer which is mounted at the bottom of the chamber. One obtains information on the deformation from the pumped volume, on the change in surface tension from the pressure amplitudes, on the rate of deformation from the frequency and on the phase angle Phi between deformation and surface tension. This novel technique can also be used for adsorption kinetics studies at the air/water and oil/water interfaces. Experimental results on the frequency dependence of the dilational elasticity of cetyltrimethylammonium bromide solutions are in good agreement with a diffusional exchange of matter theory. Gelatin behaves as an almost insoluble layer; no frequency dependence was observed in the studied frequency interval. A mixed gelatin/anionic surfactant adsorption layer shows an almost linear dependence of Delta gamma(f) in the frequency range 50-300 Hz. This increase in the stress amplitude (Delta gamma) with the deformation rate is a characteristic feature of an intrinsic viscosity.