Interface relaxation oscillations

A third group of methods is the so-called dynamic methods, such as the oscillating jet, capillary wave, or oscillating drop/bubble methods. These methods are typically based on the evalnation of periodically applied interfacial stresses followed by interface relaxation and provide the means for determining the dilational rheology of the liquid-gas and liquid-liquid interfaces. 

It was determined, for example, that the surface tension of water relaxes to its equilibrium value with a relaxation time of 0.6 msec [104]. The oscillating jet method has been useful in studying the surface tension of surfactant solutions. Figure 11-21 illustrates the usual observation that at small times the jet appears to have the surface tension of pure water. The slowness in attaining the equilibrium value may partly be due to the times required for surfactant to diffuse to the surface and partly due to chemical rate processes at the interface. See Ref. 105 for similar studies with heptanoic acid and Ref. 106 for some anomalous effects. 

In the EHD impedance method, modulation of the flow velocity causes a modulation of the velocity gradient at the interface which, in turn, causes a modulation in the concentration boundary layer thickness. As demonstrated previously in Section 10.3.3 and Fig. 10.3 the experiment shows a relaxation time determined solely by the time for diffusion across the concentration boundary layer. Although there is a characteristic penetration depth, 8hm, of the velocity oscillation above the surface, and at sufficiently high modulation frequencies this is smaller than the concentration boundary layer thickness, any information associated with the variation of hm with w is generally lost, unless the solution is very viscous. The reason is simply that, at sufficiently high modulation frequencies, the amplitude of the transfer function between flow modulation and current density is small. So, in contrast to the AC impedance experiment, the depth into the solution probed by the EHD experiment is not a function... 

Further experiments to examine the ion exchange mechanism for the acceleration in the relaxation process of the chemical oscillation were also performed. We varied the kind of hydrophilic anions (Cl , Br and I ) in the oil phase and investigated the dependence of the desorption rate of DS ions on the standard free energy of transfer of those anions. Since the ion exchange should occur at the oil/water interface, we expected that the more the standard free energy of transfer of the anions from the interface to the water phase would decrease, the more efficiently the ion exchange would occur between the hydrophilic anions and DS ions, resulting in the acceleration of the desorption... 

In the present chapter current relaxation theories will be described first both damping of harmonically generated disturbances and relaxations to transient perturbations. Thereafter, experiments are described, based on the damping of capillary and longitudinal waves, oscillation behaviour of bubbles. Also transient relaxations with pendent drop and drop and bubble pressure measurements are shown. Finally, applications to different interfaces, using surfactants, surfactant mixtures, polymers and polymer/surfactant mixtures are discussed. 

It is well known (66) that the a-relaxation process of crystalline polymers consists of at least two processes, referred to as ai and U2 in the order of lower temperature, respectively. The ai-process (67-77) is pronounced in melt crystallized samples and is associated with the relaxation of grain boundaries, such as dislocation of lamellae with a frictional resistance related to disordered interface layers. The magnitude of the ai-process increases with the increase in the crystal defects. The o 2-process (71,73,78-83) is pronounced in single crystal mats and is ascribed to incoherent oscillations of the chains about their equilibrium positions in the crystal lattice in which intermolecular potential suffers smearing out. The magnitude of the Q 2-process increases with the increase in the lamellar thickness and/or the degree of crystallization (39). 

The drop-shape oscillation technique as developed by Tian et al. (206, 207) is another technique suitable for closing the gap in the experimental methods for liquid/liquid interfaces. This method is based on the analysis of drop-shape oscillation modes and yields again the matter-ex-change mechanism and the dila-tional interfacial elasticity. The method is similar to the transient relaxation methods applicable only for comparatively low oscillation frequencies. 

In the case of bulk-to-interface interchange, the effect should be less marked as the frequency of oscillation increases. In the second case (conformational changes), the effect will increase with increase of frequency. The relaxation times of the two processes should be very different. Slow reorientations are generally only observed for long chain, practically insoluble (irreversibly adsorbed) components. These are not subject to diffusional interchange. Conversely diffusion will be more important the greater the solubility and the shorter the chain length. 

Adsorbed monolayers of BSA at the liquid/liquid interface give rise to a phase difference, at certain frequencies, between the sinusoidal area oscillation and the interfacial tension variation. This contrasts sharply with the behavior at the liquid/air interface, where a previous investigation found no phase angle (14) for both spread and adsorbed films. Thus, relaxation processes can be shown to be important at the liquid/liquid interface for adsorbed... 

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