Interfacial relaxation

For effective demulsification of a water-in-oil emulsion, both shear viscosity as well as dynamic tension gradient of the water-oil interface have to be lowered. The interfacial dilational modulus data indicate that the interfacial relaxation process occurs faster with an effective demulsifier. The electron spin resonance with labeled demulsifiers suggests that demulsifiers form clusters in the bulk oil. The unclustering and rearrangement of the demulsifier at the interface may affect the interfacial relaxation process. 

The variation of the chemical composition of the substrate (not realized in a continuous tunable fashion) leads to drastic modifications of surface fields exerted by the polymer/substrate (i.e.,II) interface [94,97, 111, 114,119]. The substrate may, for instance, change contact angles with the blend phase from zero to a finite value. As a result the final morphology changes from a layered structure of Fig. 5b into a column structure of Fig. 5c [94,114]. On the other hand our very recent experiment [16] has shown that the surface fields are temperature dependent. Therefore, although it has been shown that surface-induced spinodal decomposition yields coexisting bilayer structure (Fig. 5b) at a singular temperature [114,115], that in principle may not be necessary true for other temperatures. This motivated our comparative studies [107] on coexistence compositions determined with two techniques described above interfacial relaxation and spinodal decomposition. 

Surface induced spinodal decomposition leads, for properly controlled surface fields, to a two layer structure characteristic for coexisting phases. Hence it may be used to determine the coexisting conditions in a more convenient way that with the interfacial relaxation method as the initial bilayer geometry may be avoided. In practical terms the overall composition of the whole thin film may be much better controlled in experiments involving spinodal decomposition. Therefore in experiments studying the equilibrium composition vs depth pro-... 

In order to understand the reactivity of ceria surfaces and the interaction of it with metal particles or adsorbates, it is of fundamental interest to know its surface structure and the extent or type of defects present. Even though the film may be an oriented single crystal, there is still the question of whether the surface is terminated in oxygen anions, Ce cations, a mixture or in defects associated with the termination. Charge neutrality, interfacial relaxation and dielectric discontinuities may modify the properties of an oxide surface. Also the ability of the surface to adsorb or give up oxygen, as well as the structure, clustering and reactivity of defects may be expected to depend upon the surface orientation and structure. 

The aim of Chapter 3 is to demonstrate the complexity of the coupling of surface rheology and bulk transport processes. It also demonstrates the close link of surface rheology and interfacial relaxation processes. 

In this chapter specific theories and experimental set-ups for interfacial relaxation studies of soluble adsorption layers are presented. A general discussion of relaxation processes, in bulk and interfacial phases, was given in Chapter 3. After a short introduction, in which the important role of mechanical properties of adsorption layers and the exchange of matter for practical applications are discussed, the main differences between adsorption kinetics studies and relaxation investigations are explained. Then, general theories of exchange of matter and specific theories for different experimental techniques are presented. Finally, experimental setups, based on harmonic and transient interfacial area deformations, are described and results for surfactant and polymer adsorption layers discussed. 

Before starting with the description of present theories of interfacial relaxations, the difference from adsorption kinetics studies has to be pointed out. The general difference lies in the composition of the adsorption layer. Adsorption kinetics processes, described in previous chapters usually start from an uncovered interface. The species with the highest concentration and surface activity adsorb first. The best measure to estimate the rate of adsorption at the beginning of the process is the ratio of surface concentration r over bulk concentration c. To compare the adsorption rate of two surface active compounds a simplification of Eq. (4.85) can be used, from which the time t needed by a surfactant to reach 95% of the equilibrium adsorption results. 

It was shown by Loglio et al. (1991a) that the most useful disturbance for interfacial relaxation experiments is the trapezoidal area change. For time regimes realised in most of the transient relaxation experiments the trapezoidal area change can be approximated adequately by a square pulse. For the square pulse area change we obtain ... 

There are many experimental techniques for studying interfacial relaxations of soluble adsorption layers. Except for the wave damping techniques, these methods are developed and used only by individual research groups. Up to now, no commercial set-up exists and therefore, relaxation experiments are not so wide spread. New developments in this field will probably increase the number of investigators studying the dynamic and mechanical properties of adsorption layers, since instruments are easy to construct and data handling is relatively simple. In this section, wave damping and other harmonic methods as well as transient relaxation techniques will be described. 

Interfacial relaxations of HA at the water/decane interface are performed with the same technique (Miller et al. 1993c, d). The results for three subsequent square pulse disturbances of the adsorption layer of 0.02 mg/ml HA are shown in Fig. 6.22. 

Figure 7. Dynamic mechanical spectra of the copolymer cast from dioxane (o) and chloroform ( ) onto glass braids. Arrows A, Aand A" indicate the onset of side chain motion (10,), B, interfacial relaxation (3) and C, the...

This chapter will also present a selected number of experiments representative for the various models discussed. This refers especially to surfactant systems where recently new phenomena have been observed and explained, i.e. the possibility of changes in the orientation of adsorbed molecules are alternatively the formation of aggregates at the interface. Adsorption kinetics as well as interfacial relaxation experiments will be reported and the results discussed in terms of the specific parameters of these new theories. This will include also some data on proteins as particular type of surface active molecules able to change their conformation at an interface and hence changing the molar area at the interface. 

Interfacial relaxation methods are typically based on a perturbation of the equilibrium state of an interfacial layer (equilibrium within the interfacial layer and with the adjacent bulk phases) by small changes of the interfacial area. The small relative change in area is defined by... 

In recent years, several theoretical and experimental attempts have been performed to develop methods based on oscillations of supported drops or bubbles. For example, Tian et al. used quadrupole shape oscillations in order to estimate the equilibrium surface tension, Gibbs elasticity, and surface dilational viscosity [203]. Pratt and Thoraval [204] used a pulsed drop rheometer for measurements of the interfacial tension relaxation process of some oil soluble surfactants. The pulsed drop rheometer is based on an instantaneous expansion of a pendant water drop formed at the tip of a capillary in oil. After perturbation an interfacial relaxation sets in. The interfacial pressure decay is followed as a function of time. The oscillating bubble system uses oscillations of a bubble formed at the tip of a capillary. The amplitudes of the bubble area and pressure oscillations are measured to determine the dilational elasticity while the frequency dependence of the phase shift yields the exchange of matter mechanism at the bubble surface [205,206]. 

We want to give only two examples of interfacial relaxation methods. The whole field of interfacial relaxations and rheology is so broad and of strong practical relevance that this topic deserves a whole book. At first, two examples, a harmonic and a transient experiment will be shown as example for slow relaxation experiments, while as second we will present results of experiments performed under ground and microgravity conditions, respectively, based on the principle of oscillating bubbles. 

Dielectric relaxation and dielectric losses of pure liquids, ionic solutions, solids, polymers and colloids will be discussed. Effect of electrolytes, relaxation of defects within crystals lattices, adsorbed phases, interfacial relaxation, space charge polarization, and the Maxwell-Wagner effect will be analyzed. Next, a brief overview of... 

Since the optical constants were not measured in situ in the same experiments the results may be considered as a rough evaluation only (for the same reason the independence of extracted values for da/d<7 on a> was not checked). Somehow the results shown in Fig. 18 seem to be in line with general expectations of the interfacial relaxation theory sketched in the upper corner of the figure the meaningfulness of the obtained absolute values (for In, 1 pC causes a variation of a by 0.26 a.u.) is discussed in Ref [145],... 

Fig. 18 The evaluated interfacial relaxation, extracted from the data on s-polarized electroreflectance, as a function of electrode potential. The insert plots show schematically, the shape of the a(a) and do(

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