Dilatational interfacial/surface rheology

First of all, surface rheology is completely described by four rheological parameters elasticity and viscosity of compression/dilatation and of shear. In every case surface flow is coupled with the hydrodynamics of the adherent liquid bulk phase. From interfacial thermodynamics we know that the integration over the deviation of the tangential stress tensor from the bulk pressure represents the interfacial tension y (after Bakker 1928). 

Additional information on interfacial layers can be gained from rheological and ellipsometry experiments. There is quite a number of different experimental setups used to determine surface rheological parameters (27). New possibilities to determine surface dilational parameters arise from oscillating-drop experiments. Using axisymmetric drop shape analysis (ADSA) the change in interfacial ten-... 

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. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

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). 

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... 

Dynamic properties of interfaces have attracted attention for many years because they help in understanding the behaviour of polymer, surfactant or mixed adsorption layers.6 In particular, interfacial rheology (dilational properties) is crucial for many technological processes (emulsions, flotation, foaming, etc).1 The present work deals with the adsorption of MeC at the air-water interface. Because of its amphiphilic character MeC is able to adsorb at the liquid interface thus lowering the surface tension. Our aim is to quantify how surface active this polymer is, and to determine the rheological properties of the layer. A qualitative and quantitative evaluation of the adsorption process and the dilata-tional surface properties have been realised by dynamic interface tension measurements using a drop tensiometer and an axisymmetric drop shape analysis. 

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... 

Interfacial rheology deals with the shear and dilatational mechanical behavior of adsorbed and deposited layers of surfactants, proteins, polymers, and other mixtures at fluid fluid interfaces and of monolayers at solid surfaces. The orientation of the adsorbed molecules,... 

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. 

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... 

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. 

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. 

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... 

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