Waves at Interfaces

Registration of high-frequency waves on the surface by the interference methods

Capillary waves are frequently used to measure the surface tension of liquids. However, this approach has not found wide application in the manufacture of modern commercial tensiometers because of the limitations imposed by capillary wave excitation techniques and the labor input associated with its practical implementation. In this paper we introduce a modified version of the capillary wave method which allows one to avoid the existing limitations and disadvantages. The distinguishing features of the proposed technique are as follows: acoustic wave generation and application of an interferometry technique for 3D surface profile reconstruction. A dynamic speaker with controlled vibration frequency and amplitude is used to produce acoustic vibrations. Application of a conventional Fizeau interferometer and the spatial phase shifting method makes it possible to perform surface form measurements with a high accuracy. For calculating wavelengths and the damping co-efficient, the surface profile is fitted with a decaying cylindrical wave equation. The accuracy of surface tension measurement by the modified capillary wave technique is 0.3 %. Owing to the non-contact way of wave generation and the small amounts of the examined fluid, the proposed method can be used in different studies.

Dilational rheology of adsorbed surfactant layers--role of the intrinsic two-dimensional compressibility

The frequency and concentration dependencies of the dilational elasticity and viscosity of adsorbed surfactant layers is yet not quantitatively understood. Especially at a surface coverage above 50% the measured high frequency limits of the elasticity pass typically a maximum or reach a plateau while classical models expect an exponential increase to very high values. The consideration of an intrinsic two-dimensional compressibility allows a much better description of experimental data. There are various surfactant systems discussed in literature following this new model. Although the impact of these interfacial characteristics on the dilational rheology is significant, the adsorption isotherm and equation of state change only slightly.

Surface Elasticity and Viscosity from Oscillating Bubbles Measured by Automatic Axisymmetric Drop Shape Analysis

The pendant drop/sessile drop instrument developed by our group and based on video image analysis has been enhanced to measure oscillating drops and bubbles at a rate up to 25 pictures per second. Data analysis has been developed to analyze the results from sinusoidal oscillations in terms of dilatational surface elasticity and viscosity. The polymers ethylhydroxyethyl cellulose (EHEC) and two different poly(oxyethylene)-poly(oxypropylene) ABA block copolymers (PE6200 and PE6800) have been investigated at the air-water interface regarding rate of adsorption and surface dilatational properties. The polymers give surface pressures in the region 20-30 mN m-1, the surface elastic moduli are between 4 and 30 mN m-1, and the viscous moduli are generally low, from 0 to 6 mN m-1. The elastic moduli increase with increasing frequency, but both the slope and the concentration dependency vary. For the most water-insoluble polymer, EHEC, the modulus increases with polymer concentration, the relatively hydrophobic polymer PE6200 shows the opposite behavior, and the most water-soluble polymer, PE6800, shows a maximum in the equilibrium elasticity. These observations can be explained by the changes in the molecular orientation in the surface layer as reflected in pi vs A isotherms. The PE6200 polymer also shows a higher viscous modulus, which may be explained by diffusional transport between surface and bulk. Copyright 1998 Academic Press.

Waves at liquid surfaces: coupled oscillators and mode mixing

The dispersion of surface waves on liquids has been reconsidered in the frame work of currently established linear theory. Mixed excitations, which are neither capillary nor dilational in nature, can occur due to the coupling between the lossy oscillator formed by the vertical and horizontal motions of a surface element. The results emphasize that the capillary and dilational waves are only approximately transverse and longitudinal in nature and that it cannot, in general, be correct to neglect the coupling between these degrees of freedom. In the present case mode mixing only occurs when a particular surface viscosity - that affecting shear normal to the surface - exceeds a critical value. Experimentally accessible tests of the predicted mode mixing are proposed, which would further test the physical reality of the surface viscosity involved.