Interfacial rheological behavior

In Chapter 17, we discuss rheological properties, in particular viscosity and elasticity, of colloidal systems. These properties are at the basis of quality characteristics such as strength, pliancy, fluidity, texture, and other mechanical properties of various materials and products. In addition to bulk rheology, rheological features of interfaces are discussed. Interfacial rheological behavior is crucial for the existence of deformable dispersed particles in emulsions and foams. Emulsions and foams, notably their formation and stabilization, are considered in more detail in Chapter 18. 

As mentioned above, interfacial films exhibit non-Newtonian flow, which can be treated in the same manner as for dispersions and polymer solutions. The steady-state flow can be described using Bingham plastic models. The viscoelastic behavior can be treated using stress relaxation or strain relaxation (creep) models as well as dynamic (oscillatory) models. The Bingham-fluid model of interfacial rheological behavior (27) assumes the presence of a surface yield stress, cy, i.e.. 

Caustic Waterflooding. In caustic waterflooding, the interfacial rheologic properties of a model crude oil-water system were studied in the presence of sodium hydroxide. The interfacial viscosity, the non-Newtonian flow behavior, and the activation energy of viscous flow were determined as a function of shear rate, alkali concentration, and aging time. The interfacial viscosity drastically... 

Interfacial rheology deals with the flow behavior in the interfacial region between two immiscible fluid phases (gas-liquid as in foams, and liquid-liquid as in emulsions). The flow is considerably modified by surface active agents present in the system. Surface active agents (surfactants) are molecules with an affinity for the interface and accumulate there forming a packed structure. This results in a variation in physical and chemical properties in a thin interfacial region with a thickness of the order of a few molecular diameters. These... 

Several factors can be identified as being crucial for the foaming of immiscible polymer blends the blend morphology, the phase size of the blend constituents, the interfacial properties between the blend partners, and, last but not least, the properties of the respective blend phases such as the melt-rheological behavior, the glass transition temperature, the gas solubility, as well as the gas diffusion coefficient. Most of these factors also individually influence the melt-rheological behavior of two-phase blends. 

Distillate cut 3 obtained from bulk separation is still dark colored which may indicate the presence of small quantities of asphaltic material (20). Figure 3 indicates that it is possible that a composite of compounds, besides carboxylic acids, may be required to yield optimal recovery. In order to understand the mechanism of oil recovery, the contribution of a given individual fraction to ultra-low surface tension characteristics and the contribution of various combinations of individual fractions contribute greatly to viscosity behavior. Therefore, interfacial rheology may be dependent on the appropriate composition of the crude oil. 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

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,... 

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]. 

We have already mentioned the importance of controlling the stability (i.e.. the tendency of the particles to remain unaggregated in suspension as exposed to the thermodynamic requirement of free energy reduction by decreasing the interfacial area of the system), and hence the sedimentation and redispersibility behavior, of the particles. One of the possibilities of achieving such control is to suitably modify the viscosity and rheological behavior of the vehicle we will analyze this in the next paragraph and focus now on other methods. 

In the following discussions the published experimental findings are presented interrelatedly first in terms of internal oil chemistry at the interface and instabilities based on its composition, secondly in terms of effects of water chemistry, and thirdly in terms of demulsifier interaction. We include the activity of interfacial components involved in the structure of the protective skin, the behavior(s) of this structure with changes to water chemistry or solvency, or the effects of changes in film stmeture itself due to modification of relative proportions of interfacially active components. In some examples, developments in interfacial rheology, which is both a tool for understanding stable films and a means of rationalizing the effects of demulsifiers in demulsification, are discussed interrelatedly. Films may be sensitive to crude oil type, gas content, aqueous pH, salt content, temperature, age, and the presence of demulsifiers. Demulsifier performance is also influenced by many of these variables. 

Thus, the discussions that follow include several important factors. These are that (1) the activity of interfacial components is involved in the structure of the protective skin (2) the behavior of this structure changes with water chemistry or solvency due to mass transfer and interfacial dissipation effects (3) the changes in structure may be due to modification of the relative proportions of components and (4) for understanding stable films and as a means of measuring demulsification, one may adapt the new developments in interfacial rheology as tools. These are all factors considered in past studies and which are described in flie following sections. 

The results of flic interfacial rheological studies on asphaltene adsorption at oil-water interfaces teach us a great deal about the behavior of asphaltenes and their role in emulsion stabili2ation. The conclusions drawn are based largely on the assumption that the rheological properties measured, namely flic elastic film modulus G are directly related to the surface excess concentration of asphaltenes. F. It is understood diat die elastic modulus actually depends on both the surface excess concenlration and the relative conformation (i.e., coimectivity) of the adsorbed asphaltenes. It is further understood that a minimum adsorbed level is required to observe a finite value of G and that the relationship between G and G is not linear. With these caveats in mind, we can conclude die following ... 

At surfactant concentrations near the CMC, an enhanced blocking of the surface and change in interfacial rheology, and thus reduction of mass transfer may occur [67]. However, even low surfactant concentrations with species of high adsorption affinity may show the similar effect. As to this, it is essential to know the adsorption behavior of single or mixed adsorption layers to properly predict their impact on interfacial transfer kinetics. 

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