QUADRUPOLE OSCILLATIONS OF DROPS FOR STUDYING INTERFACIAL PROPERTIES

An acoustic method for suspending a drop in another liquid and deforming the drop has been combined with an optical detection scheme to measure the free quadrupole oscillation frequency and damping constant of the drop. The experimental results are compared with the theoretical predictions to deduce the dynamic interfacial properties. The systems studied are hexane drops of different radii in pure water and hexane drops in sodium dodecyl sulfate (SDS) aqueous solutions of different concentrations (0.164 to 5.9 mM). The data for the pure water system support the theoretical predictions. For hexane drops in SDS aqueous solutions, it is found that the most important interfacial property is the Gibbs elasticity. Furthermore, the measured damping constants for SDS concentrations lower than 1.75 mM (cp. CMC 7.6 mM) are well described by employing an ideal equation of state for surfactants at the interface. The interfacial tension between hexane and SDS aqueous solutions of different concentrations is also reported. The method, which is nonperturbative and requires a very small amount of sample, may supplement other methods to measure the interfacial properties at a liquid-liquid interface. © 1990.