Interfacial dilational rheology

When an element of area covered with soluble material is subject to surface contraction, some of the material escapes into the bulk phase and returns when the interface is expanded in interfacial dilational rheology (Murray and... 

Wijmans, C.M., Dickinson, E. (1998). Simulation of interfacial shear and dilatational rheology of an adsorbed protein monolayer modeled as a network of spherical particles. Langmuir, 14, 7278-7286. 

A large variety of techniques for measuring interfacial rheological parameters exists. A first classification can be made between techniques suited for measuring interfacial shear and interfacial dilational properties. A recent review of the various techniques can be found in a review by Miller et ah ). In order to prevent undue fragmentation of the text, here we shall also describe techniques that work better for Gibbs monolayers. 

While dilatational rheology plays an important role in short-term stability of dispersions shear viscosity may contribute appreciably to the long-term stability (Murray and Dickinson, 1996 Murray, 1998, 2002). The shear characteristics of the interfacial film are governed by the composition and structure of the adsorbed material. In addition, surface shear viscosity is a very sensitive technique to analyze the competitive adsorption of protein and water-soluble LMWE at the air-water interface (Murray and Dickinson, 1996 Murray, 1998, 2002 Bos and van Vliet, 2001). 

In fact, Equation 5.281 describes an interface as a two-dimensional Newtonian fluid. On the other hand, a number of non-Newtonian interfacial rheological models have been described in the literature. Tambe and Sharma modeled the hydrodynamics of thin liquid films bounded by viscoelastic interfaces, which obey a generalized Maxwell model for the interfacial stress tensor. These authors also presented a constitutive equation to describe the rheological properties of fluid interfaces containing colloidal particles. A new constitutive equation for the total stress was proposed by Horozov et al. ° and Danov et al. who applied a local approach to the interfacial dilatation of adsorption layers. 

Loglio G, Pandolfini P, Tesei U, and Noskov B (1998b) Measurements of interfacial properties with the axisymmetric bubble-shape analysis technique effects of vibrations. Colloids Surfaces A 143 301-310 Loglio G, Pandolfini P, Miller R, Makievski AV, Ravera F, Ferrari M and Liggieri L (2001) "Drop and Bubble Shape Analysis as Tool for Dilational Rheology Studies of Interfacial Layers", in "Novel Methods to Study Interfacial Layers", Studies in Interface Science, Vol. 11, D. Mobius and R. Miller (Eds.), Elsevier, Amsterdam, pp 439-485... 

In the book by Joos [16] as well as in original papers, some special cases of this general approach have been discussed. It was shown that such stress relaxation experiments are well suited for studying the dilational rheology of interfacial layers, which yield the dilational elasticity as a function of the effective surface age teff... 

As mentioned above relaxation techniques are additional methods suitable to get insight into the mechanism of adsorption processes. Moreover, these methods represent the experimental tools to determine the dilational rheology of interfacial layers. The general principle of relaxation methods is the small disturbance of the interfacial layer, which has reached the equilibrium state beforehand. Particular methods are suitable to detect characteristic times of relaxations processes as they work each in a specific frequency range. This paragraph discusses briefly the most frequently used and very recently developed methods. 

The present chapter gives also detailed introduction to a large number of experimental methods, suitable for studying dynamic interfacial tensions. The methods are discussed in terms of the available time window. There are methods which complement each other such that a time interval from less than 100 microseconds up to hours and days of adsorption time can be covered (about ten orders of magnitude). The relaxation methods, also suitable for detecting the adsorption mechanism of surfactant s adsorption provide in addition the dilational rheology of interfacial layers. It is discussed that in particular these dilational rheological studies are most informative in respect to adsorption mechanisms, as the data interpretation includes the thermodynamic model as well as the adsorption dynamics. 

Many experiments have been proposed for measuring the interfacial shear viscosity and elasticity and interfacial dilatational viscosity and elasticity at gas/liquid and liquid/ liquid interfaces [22]. Interfacial shear viscosities of different oil/aqueous systems have been studied worldwide. Some experimental results indicate that low interfacial shear viscosities do not necessarily imply that an emulsion will be unstable [23]. The dilatational rheology is based on area changes due to an expansion or compression of a fluid surface and stress relaxation experiments. The experiment results show that the interfacial dilatational properties can be much higher than the interfacial shear properties for the same system [15,24-27]. This makes researchers believe that interfacial dilatational viscosity and elasticity may have a better relationship with the stability of the emulsion than with interfacial shear properties. 

Wasan and his research group focused on the field of interfacial rheology during the past three decades [15]. They developed novel instruments, such as oscillatory deep-channel interfacial viscometer [20,21,28] and biconical bob oscillatory interfacial rheometer [29] for interfacial shear measurement and the maximum bubble-pressure method [15,29,30] and the controlled drop tensiometer [1,31] for interfacial dilatational measurement, to resolve complex interfacial flow behavior in dynamic stress conditions [1,15,27,32-35]. Their research has clearly demonstrated the importance of interfacial rheology in the coalescence process of emulsions and foams. In connection with the maximum bubble-pressure method, it has been used in the BLM system to access the properties of lipid bilayers formed from a variety of surfactants [17,28,36]. 

Some approaches analyzed directly flic influence of flic stabilizing adsorption layers and concluded that diere is a dependence of the stability of an emulsion on flic interfacial concentration and the sum of inter-molecular interactions (8—10). Murdoch and Leng (11) pointed out the role of bulk and interfacial rheological parameters to describe these processes. This concept was further treated by several authors (12—14). A very comprehensive approach was given by Wasan and co-workers (15,16) who considered the surface shear and dilational rheology, and also some hy-drody-namic parameters in their analysis of emulsion films. 

Rheology is the study of the deformation and flow of materials under the influence of an applied stress. The interfacial rheology of a surfactant film normally accounts for the interfacial viscosity and elasticity of the film. The interfacial viscosity can be classified with interfacial shear viscosity and interfacial dilational viscosity. Films are elastic if they resist deformation in the plane of the interface and if the surface tends to recover its natural shape when the deforming forces are removed. The interfacial elasticity can also be classified with interfacial shear elasticity and interfacial dilational elasticity (6, 7, 12). Malhotra and... 

The mechanical properties of asphaltene films at interfaces can be probed by a variety of rheological techniques. These methods provide valuable insight into the origins of stability of asphaltene emulsions and into the role of concentration, and solvation by resins and aromatic solvents on the adsorption and self-assembly of asphaltenes. Miller et al. provide a comprehensive review of methods for probing interfacial dilational and shear properties of adsorption layers at liquid interfaces (72). They describe devices that measure surface velocity profiles (indirect methods) or determine torsional stress values (direct methods). Indirect... 

Loglio G, Pandolflni P, Miller R, Makievski AV, Ravera R, Ferrari M, Liggieri L (2001) Drop and bubble shape analysis as a tool for dilational rheological studies of interfacial... 

Moreover, this model can also involve parameters that refer to the interfacial rheological properties, such as the interfacial dilation modulus and the interfacial shear modulus (Jacobs et al. 1999). Consequently, quantitative measurements of... 

Drop and Bubble Shape Analysis as a Tool for Dilational Rheological Studies of Interfacial Layers... 

The interfacial tension response to transient and harmonic area perturbations yields the dilational rheological parameters of the interfacial layer dilational elasticity and exchange of matter function. The data interpretation with the diffusion-controlled adsorption mechanism based on various adsorption isotherms is demonstrated by a number of experiments, obtained for model surfactants and proteins and also technical surfactants. The application of the Fourier transformation is demonstrated for the analysis of harmonic area changes. The experiments shown are performed at the water/air and water/oil interface and underline the large capacity of the tensiometer. 

In this chapter first the theoretical basis of the drop and bubble shape method is described, and then details of its practical use are given. The various functionalities of the instrument are demonstrated then in the form of examples, such as dynamic surface and interfacial tensions of surfactant and protein solutions, and the dilational rheology of some selected systems. 

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. 

Tirf, ) are normalized by their values at pH = 5. The experimental data show that both the surface elasticity, Ec, and relaxation time, increase with increase of pH. The interfacial dilatational viscosity, y d, exhibits a maximum at pH = 6. A similar peak of the interfacial shear viscosity of BSA at pH = 6 has been observed by Graham and Phillips [188] at petroleum ether—water interface. The results in Fig. 9 demonstrate a marked influence of the ionic strength on the rheological parameters. 

As a rule, dynamic processes with fluid interfaces are accompanied by interfacial dilatation, compression, and/or two-dimensional flows in the surfactant adsorption mono-layer. These processes are affected by the interfacial rheological properties, such as surface (Gibbs) elasticity, dilatational, and shear-surface viscosity, and adsorption relaxation time see Sec. III.F. The interfacial rheological properties are especially important for the foaminess of surfactant solutions and the emulsion preparation by homogenization. 

Interfacial Shear Rheology In contrast to the dilational technique, the surface shear methods are direct determinations of the mechanical properties of an interface. The simplest approach is a two-dimensional adaptation of standard three-dimensional viscoelastic measurements performed on a standard rheometer. The only difference is the sensitivity and the geometry. Figure 1.11 shows the geometries commonly used for oil-water interfaces. 

There are various direct measurements of micellar solutions giving access to the dynamics rate constants - mainly based on disturbance of the equilibrium state by imposing various types of perturbations, such as stop flow, ultrasound, temperature and pressure jump [14,15[. This aspect is also not further elaborated here we focus instead on the impact of micellar kinetics on interfacial properties, to demonstrate that tensiometry and dilational rheology are suitable methods to probe the impact of micellar dynamics. The first work on this subject was published by Lucassen already in 1975 [16[ and he showed that the presence of micelles in the bulk have a measurable impact on the adsorption kinetics, and hence on the dilational elasticity, when measured by a longitudinal wave damping technique. Subsequent work demonstrated the effect of micellar dynamics on non-equilibrium interfacial properties [17-29]. The physical idea of the impact of micellar dynamics on the dynamic properties of interfacial layers can be easily understood from the scheme given in Figure 13.1. 

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