The measurement of dilational properties of liquid surfaces

For measurements of viscoelastic properties of monolayer covered interfaces, the longitudinal wave technique is used. The longitudinal wave is generated in the plane of the interface, using a horizontal oscillating barrier. The wave propagation depends on the values of the viscoelastic parameters of the monolayer. The present paper deals with a new method of determining the wave parameters. Using small tracer particles, the displacement of the monolayer, as a function of the distance to the oscillating barrier, is made visible and recorded by video cameras. From these recordings the wave parameters and viscoelastic properties are evaluated. A comparison is made between results obtained with this method and those obtained from the measurements of the varying surface tension. This latter method has until now normally been used to determine the wave parameters. Results on cholesterol and decanoic acid monolayers indicate that both methods lead to similar results within experimental error. It is shown that in case of reflected waves the combination of the results of the two methods, as described earlier by Crone et al., leads to significantly more accurate values of the wave parameters.