Interfacial shear viscosity

A study on a commonly used demulsifier, namely, a phenol-formaldehyde resin, elucidated how various parameters such as interfacial tension, interfacial shear viscosity, dynamic interfacial-tension gradient, dilatational elasticity, and demulsifier clustering affect the demulsification effectiveness [1275]. 

The interfacial shear viscosities are measured by the deep channel viscous traction surface viscometer (5) at the Illinois Institute of Technology. The oil-water equilibrium tensions are measured by either the spinning drop or the du Nouy ring (6) method. 

Our results suggest that the lowering of interfacial shear viscosity, although necessary, is not a sufficient criterion for effective demulsification. In addition, a demulsifier must also rapidly dampen any fluctuations in the oil-water interfacial tension. 

Figure 8.13 Effect of the method of preparation of the complexes of sodium caseinate (CN) + dextian sulfate (DS) on the time dependence of interfacial shear viscosity, r s, and interfacial shear elasticity, Gs (frequency 0.1 s 1) ( ) CN + DS, mixed layer, freshly prepared complexes (O) CN + DS, mixed layer, 24-h-old complexes ( ) CN + DS, bilayer. Aqueous solutions contained 0.5 wt% CN and 1 wt% DS in 20 mM imidazole buffer at pH = 6. Reproduced from Jourdain et al. (2009) with permission.

Interfacial Viscosity. In a clean system in which two pure liquids produce an interface, the viscosity of the interface should be the same as the bulk solution viscosity. However, surfactant or impurity adsorption at an interface can cause a resistance to fiow to occur that can be measured as the interfacial shear viscosity. This viscosity is defined as the ratio between the shear stress and the shear rate in the plane of the interface (12), Methods used to make these measurements include a viscous traction surface viscometer (J2), droplet-droplet coalescence (J3), the rotating ring viscometer (14), and surface laser light scattering (9). 

Extending the analogy with bulk rheology, for linear shear deformation of an interface it is possible to define a surface (or interfacial) shear viscosity rj° and a surface (or interfacial) shear modulus G°. In a Cartesian co-ordinate system, with again the z-axis normal to the interface... 

Interfacial shear properties can be determined either by steady state or by oscillatory measurements (see sec. 3.6f). In the latter case dynamic shear moduli are obtained in the former the interfacial shear modulus or the interfacial shear viscosity is obtained. 

Indirect measurements. Indirect methods for determining the interfacial shear viscosity include procedures in which the liquid underneath, together with the monolayer are brought into motion and the ensuing dissipation consists of a bulk and interfacial contribution. Theory is required to compute these two contributions and to subtract the former. An old method for determining the interfacial shear viscosity is the canal surface viscometer. It is based on the determination of the flow rate of a film through a narrow canal or slit under eui applied two-dimen-... 

The effects of interfacial tension, interfacial charge and interfacial viscosity on coalescence, and emulsion stability for crude oil emulsions in alkaline solutions have been assessed. It was observed that the NaOH concentration which yields higher interfacial shear viscosity also results in higher emulsion stability. 

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. 

Solid (or rigid) films that under compression form relatively insoluble skins and possess high interfacial shear viscosity. 

Different techniques for the study of shear rheology of interfacial layers have been developed over the years however, they are mostly suited for liquid/gas inter faces. The early instruments were constructed to measure the interfacial shear viscosity under constant shear conditions. In more complex systems, nonlinear effects, shear-rate dependencies of the viscosity, and viscoelastic properties are... 

Figure 23 Interfacial shear viscosity of ABPI (bean protein) at the water/hexadecane interface protein concentrations 10 % ( ), 2 X 10">% (A), 3 X 10">% (p), 4 X 10 % (T).

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 interfaeial shear viscosity is measured with an SVR I S Interfaeial Viscoelastic Meter (Kyowa Kagaku Co. Ltd, Japan). The schematic of flie measuring part of the interfacial viscoelastic meter is shown in Fig. 1. The results show that the higher the interfaeial shear viscosity of the interfacial film between jet fuel and water the more stable are the emulsions stabilized by the asphaltene or resin fractions from crude oils (18). We also found that flie value of the interfacial shear viscosity is affected by the following factors (see below). 

Figure 2 Interfacial shear viscosity of the interfacial film between jet fuel and the synthetic formation water. The inter-faciaUy active fractions separated from Daqing crude oil T= 20°C.

Figure 5 Influence of wax particles on interfacial shear viscosity of the interfacial film between a model oil and distilled water. The model oil consisted of 5 or 20% Daqing crude oil in jet fuel. Shear...

Figure 7 Interfacial shear viscosity of the interfacial fihn between jet fuel and distilled water. The interfacially active fractions separated from JUin crude oil. C = 2% shear rate 0.0159 s. ...

In comparing Fig. 7 with Fig. 8 it can be seen that, when the interfacially active fiactions from Jilin crude oil are used as emulsifiers and the oil phase is jet fuel, the synthetic formation water gives an interfacial shear viscosity higher than that of distilled water when the temperature is below 30°C. However, it is reversed when the temperature is higher than 35°C. This result indicates that the interfacial shear viscosity is affected by the properties of water the ions in the water may play an important role in the properties of the interfacial film. 

The above experimental results show that wax particles can affect the rheological properties of the interfacial film between the water and oil phases. In order to confirm these phenomena we added a synthetic wax to the jet fuel/syn-thetic formation-water system and took the interfacially active fractions from Daqing crude oil as emulsifier. The content of the synthetic wax in jet fuel was 5% and the melting temperature of the synthetic wax was 54-56°C. The results shown in Fig. 11 demonstrate that the interfacial shear viscosity increased as the temperature rose to the range between 20 and 30° C, and the interfacial shear viscosity decreased when the temperature was higher than 30°C. It is obvious that the presence of synthetic wax particles at the interface makes the properties of the interfacial film greatly different from those shown in Fig. 2. 

Figure 12 shows that when the temperature is lower than 30°C, the interfacial shear viscosity increases as the shear rate increases. This phenomenon shows a dilatant behavior (shear thickening) as in a three-dimension system (12). These results fiirther prove that wax particles can contribute to the rheological properties of the interfacial film between... 

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