A NEW OSCILLATING BUBBLE TECHNIQUE FOR MEASURING SURFACE DILATIONAL PROPERTIES

A new oscillating bubble device has been developed and used for measur ing the surface dilational properties of surfactant, protein and surfa ctant/protein adsorption layers in the frequency range 1-300 Hz. Withi n a closed measuring chamber, a small hemispherical bubble is produced at the tip of a capillary. A piezoelectric driver generates sinusoida l 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 c hanges in the radius and surface tension. The pressure amplitudes are monitored by a sensitive pressure transducer which is mounted at the b ottom 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 p hase angle Phi between deformation and surface tension. This novel tec hnique can also be used for adsorption kinetics studies at the air/wat er and oil/water interfaces. Experimental results on the frequency dep endence 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 de pendence was observed in the studied frequency interval. A mixed gelat in/anionic surfactant adsorption layer shows an almost linear dependen ce of Delta gamma(f) in the frequency range 50-300 Hz. This increase i n the stress amplitude (Delta gamma) with the deformation rate is a ch aracteristic feature of an intrinsic viscosity.