System for automatically tracking changes in interfacial tension

A technique is presented for automatically monitoring changes of the interfacial tension between two liquids when a surfactant is added to one of them. A drop of one liquid is acoustically levitated in the other liquid and also is acoustically driven into quadrupole shape oscillations. The frequencies associated with several phase angles (between the drop's oscillation and the drying force) near the quadrupole resonance are measured, allowing the interfacial tension to be calculated given the drop's diameter and the densities of the two liquids. A feedback scheme is used to control the driving frequency, such that the phase is locked at a selected angle as the surface-active molecules pack into the interface and the resonance frequency changes. Therefore, the history of the interfacial tension after the drop is injected can be obtained. Some data for the interfaces between water-surfactant solutions and some common hydrocarbon liquids will be presented. The final equilibrium interfacial tensions will be compared with existing data from the literature. [Work supported by ONR.]