DILATIONAL VISCOELASTIC PROPERTIES OF FLUID INTERFACES .2. EXPERIMENTAL-STUDY

A longitudinal wave technique combined with tracer particle measurements offers a promising method for the measurement of dalational,viscoelastic properties of fluid interfaces containing surface active agents and other chemical additives. In this technique, a longitudinal wave disturbance is imparted to an interface by means of a sinusoidafly oscillating barrier, and the resulting response of the interface is measured in terms of amplitude and time tag of the tracer particle motion relative to the original disturbance. An apparatus constructed for this purpose is capable of measuring the dilational elastic properties from 10-2 to 10 dyne/cm, and the viscosides from 10-3 to 10 surface poise. Surface viscoelasticity measurements have been conducted on decanoic acid solutions-representing an adsorbed monolayer system; and on stearic acid films-representing a deposited monolayer system. A significant observation from the measurements on stearic acid monolayers in the range of 35-45 Å2/molecule was that the delational surface viscosity of these monolayers was 15-400 times greater than the corresponding shear viscosity. This experimental evidence, reported for the first time in this study, indicates the extent of the importance of interfacial dilational properties in comparison with interfacial shear properties. © 1979.