Interfacial rheology viscoelasticity

The measurement of rheological properties at the surface of a solution or the interface between a solution and, for example, a biological film is called surface or interfacial rheology. In this technique also, experiments are performed either in tension, compression or shear, and phenomena observed in bulk rheology such as flow and viscoelasticity are also observable. An introduction to the techniques available and some key findings are discussed by Warburton. ... 

It should be emphasised that polymeric surfactants prevent the coalescence of water droplets in the multiple emulsion drops, as well as coalescence of the latter drops themselves. This is due to the interfacial rheology of the polymeric surfactant films. As a result of the strong lateral repulsion between the stabilising chains at the interface (PHS chains at the W/O interface and PEO chains at the O/W interface), these films resist deformation under shear and hence produce a viscoelastic film. On approach of the two droplets, this film prevents deformation of the interface so as to prevent coalescence. 

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. 

Viscoelastic and viscosimetic measurements have been performed on model systems and on commercial products [29]. Water-in-silicone emulsions have been prepared, in which the oil phase consisted of decamethylcyclopentasiloxane (cyclomethicone, D5) and the surfactant used was a branched type silicone copolymer. The interfacial rheology study of such surfactants revealed that the ability of these eopolymers to effectively stabilize water-in-D5 emulsions is a result of a process involving the nucleation, growth, and accumulation of surfactant-rich particulates at the D5/water interface [55]. 

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.. 

The chapter next dealt with the rheology of emulsions stabilized by polymeric surfactants. The factors affecting the rheology of emulsions were briefly discussed. This was followed by a section on interfacial rheology and its correlation with emulsion stability. The bulk rheology of oil-in-water and water-in-oil emulsions stabilized by polymeric surfactants was described. Both steady-state and viscoelastic investigations were described. These emulsions show a transition from predominantly viscous to predominantly elastic response as the frequency of oscillation exceeds a critical value. This allows one to obtain... 

Adsorbed protein molecules interact at the interfaces to form viscoelastic films. The viscoelastic properties of protein films adsorbed at fluid interfaces in food emulsions and foams are important in relation to the stability of such systems with respect to film rupture and coalescence. Interfacial rheology techniques are very sensitive methods to measure the viscoelastic properties of proteins, thereby evaluating the protein-protein or protein-surfactant interactions at the interfaces. There was an excellent review about the principal and methods of interfacial rheology [17]. 

The interfacial rheology of protein adsorption layers has been intensively studied in relation to the properties of foams and emulsions stabilized by proteins and their mixtures with lipids or surfactants. Detailed information on the investigated systems, experimental techniques, and theoretical models can be found in Refs. [762-769]. The shear rheology of the adsorption layers of many proteins follows the viscoelastic thixotropic model [770-772], in which the surface shear elasticity and viscosity depend on the surface shear rate. The surface rheology of saponin adsorption layers has been investigated in Ref. [773]. 

The deep-channel viscometer could also be adapted for measurement of the nonlinear interfacial rheological behaviour of the film [52]. In this case several small tracer particles are placed on the fluid interface at different radial positions and the angular velocities are determined from measurements of the period of revolution. When used to measure viscoelastic properties, the deep-channel viscometer is operated in an oscillatory mode, in which case the floor of the viscometer is oscillated sinusoidally. Simultaneous measurements of the phase angle between the surface motion and the oscillating motion of the bottom dish, and the surface-to-floor amplitude ratio, may permit determination of the viscoelastic properties of the fluid interface, presuming knowledge of an appropriate rheological model [52]. 

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). 

Protein-polysaccharide complexation affects the surface viscoelastic properties of the protein interfacial layer. Surface shear rheology is especially sensitive to the strength of the interfacial protein-polysaccharide interactions. Experimental data on BSA+ dextran sulfate (Dickinson and Galazka, 1992), asi-casein + high-methoxy pectin (Dickinson et al., 1998), p-lactoglobulin + low-methoxy pectin (Ganzevles et al., 2006), and p-lactoglobulin + acacia gum (Schmitt et al., 2005) have all demon-... 

Palierne JF (1990) Linear rheology of viscoelastic emulsions with interfacial-tension. Rheol Acta 29 204-214... 

Hallworth and Carless (1 ) discuss several possibilities for the effect of light liquid paraffin on the stability of emulsions with light petroleum or chlorobenzene as the main components. They seem to prefer an explanation previously advanced by them and several other authors for the effect of fatty alcohol, namely that the increased stability is due to the formation of an interfacial complex between the additive and sodium hexadecyl sulphate. The condenced mixed film will resist coalescence primarily by virtue of its rheological properties. With mixed films of the present type, the importance of the film viscoelasticity lies in its ability to maintain electrical repulsion between approaching droplets by preventing lateral displacement of the adsorbed ions. The effective paraffinic oil has chains at least as long as those of the alkyl sulphate and will be associated by van der Waals forces with the hydrocarbon chain of the alkyl sulphate at the interface. 

Blend morphology commonly depends on the weight fraction and viscoelastic properties of each component, the interfacial tension between components, the shape and sizes of the discontinuous phase, and the fabrication conditions and setup. Most rheological experiments applied to homogeneous melts can also be similarly applied to these immiscible blends [55,63,88,89]. The viscoelastic properties arising from these studies should be labeled with a subscript apparent since the equations used to translate rheometer transducer responses to properties incorrectly assume that the material is homogeneous. Nevertheless, these apparent properties are often found to be excellent metrics of fabrication performance. 

The possible development of gradients in the components of the interfacial stress tensor due to flow of an adjacent fluid implies that the momentum flux caused by the the flow of liquid at one side of the interface does not have to be completely transported across the interface to the second fluid but may (partly or completely) be compensated in the interface. The extent to which this is possible depends on the rheological properties of the interface. For small shear stresses the interface may behave elastically or viscoelastically. For an elastic interfacial layer the structure remains coherent the layer will only deform, while for a viscoelastic one it may or may not start to flow. The latter case has been observed for elastic networks (e.g. for proteins) that remciln intact, but inside the meshes of which liquid can flow leading to energy dissipation. At large stresses the structure may yield or fracture (collapse), leading to an increased flow. 

We tentatively suggest that the frequently observed increase in stabilizing efficiency of proteins after they have been denatured in solution could be because they are aggregated and adsorb as aggregates, resulting in an interfacial layer that exhibits very quickly highly viscoelastic and solidlike rheological properties. 

Mohammed et al. [55] published a series of papers in which they probed the rheology, interfacial tension, surface pressure, and compressional modulus of crude oil-water interfaces with and without added demulsifiers under thermal and electrical fields. They observed that with short aging times of the crude oil-water interface, the interface rheology could be characterized as substantially viscous, but with very little elastic character. They demonstrated that thick viscoelastic films tended to accumulate in aged films and that some demulsifiers prevented such accumulations. 

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