A novel non-invasive technique is described for measuring the viscoelastic modulus in compression/dilation of surfactant-covered interfaces. The new setup is a dynamic drop tensiometer in which a droplet a few mm in diameter is subjected to sinusoidal fluctuations of its volume. The setup differs from the conventional barrier-and-plate technique in that the fluctuations in area and tension are measured on one and the same small interfacial area, that homogeneity of deformation of this area is far more easily ensured and, finally, that the response time of the tension probe is much shorter than that of a Wilhelmy plate. Also, the area fluctuations can be superimposed on the transient change in area of a growing drop from the early stages of its life onwards, enabling us to study interfacial dynamics over a wide range of surface ages. First results were obtained with three different proteins adsorbed at a vegetable oil/water interface, and agreed within experimental error with conventionally measured moduli at the same interface. Up to quite high surface coverages, purely elastic behaviour was found. Moreover, results at different concentrations, frequencies and surface ages all coincided on a single modulus versus interfacial pressure curve characteristic for each individual protein at the given interface. In this range of surface coverages, reconformation phenomena within the protein layers occur so fast that a quasi-equilibrium state is reached in a time scale of 10 s. The characteristic curves at the oil/water interface were found to be distinctly different from those measured at the air/water interface. For example, bovine serum albumin moduli for oil/water were approximately half the values found for air/water at the same interfacial pressure, in the pressure range 0-10 mN m(-1). At higher interfacial pressures, values for different concentrations and surface ages still more or less coincided. Results at different frequencies, however, started to diverge, indicating the onset of viscoelastic behaviour at higher surface coverages for all three proteins.
