LONGITUDINAL SURFACE-WAVES AT LIQUID INTERFACES - MEASUREMENT OF MONOLAYER VISCOELASTICITY

We have studied compression waves excited at liquid-air and liquid-liquid interfaces by an oscillating wire. These waves are detected by the surface tension modulation that they produce when the interface is covered by a surfactant monolayer. The measurement of the wavelength and damping of the waves allows the determination of the compressional elasticity modulus and the corresponding surface viscosity for the monolayer. We present results for myristic acid at the free surface of water which we compare with results obtained previously with capillary wave devices operating at different frequencies. The differences between the coefficients measured at different frequencies are large (five-decade variations for the surface viscosity in the frequency range studied). The origin of the relaxation phenomena responsible for this frequency variation remains to be elucidated. We also present results on myristic acid and sodium dodecyl sulfate at the interface between water and octane. These data are compared with a model by Lucassen and van den Tempel [J. Lucassen and M. van den Tempel, Chem. Eng. Sci., 271 (1972) 1283; J. Colloid Interface Sci., 41 (1972) 491].