Viscoelasticity film tension

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. 

After the accomplishment of the above mentioned experiment on the nonlinear viscoelasticity of the DPPE thin film, we have tried to construct a new instrument for the measurement of the dynamic surface tension. We have noticed that, the blades used to change the surface area in the commercial instrument, did not show genuine triangle or sinusoidal trajectory but rather mathematically undefined. With our newly designed instrument, the time change of the surface area can be controlled according to a chosen function with the aid of a micro-computer. 

The high molar mass species reside mostly in the aqueous phase with a number of peptide groups residing in the oil/water interface [293]. Although these latter surfactants are less effective at reducing interfacial tension, they can form a viscoelastic membrane-like film around oil droplets or air bubbles. These tend to be used in the preparation of, for example, O/W emulsions. These trends are by no means exclusive, mixtures are the norm and competitive adsorption is prevalent. Caseinate, one of the most commonly used surfactants in the food industry, is itself a mixture of interacting proteins of varying surface activity [814],... 

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. ... 

Effect of Surfactant Type and Concentration. Surfactant concentration and type is of great importance for the stability of thin liquid films and for emulsion stability. Type and concentration of surfactants are responsible for the degree of lowering the interfacial tension and for the viscoelastic properties of droplet surface, as well as for the film thickness between two droplets. 

Nearly a decade later, Oldroyd [1953, 1955] proposed a constitutive model similar to that of Frohlich and Sack, vahd at small deformations. The model considered low concentration of monodispersed drops of one Newtonian hquid in another. The interfacial tension and the viscoelastic properties of the interfacial film were incorporated by means of convected derivatives. The model provided the following relation for the complex modulus ... 

Prestrain has the additional benefit of improving the frequency response of many elastomer films. The drop off in acmation strain with frequency is less pronounced for acrylic films that have been prestrained. The increased tension in the film increases the modulus and reduces the viscoelastic namre of the films. 

The bonding of a viscoelastic material (a film of glue) onto a solid surface can only be expected, then, when the surface tension of the liquid is lower than the critical surface tension 7. of the solid body. According to equation (13-3), these two quantities are related to the contact angle S and the interfacial surface tension between solid and adhesive film. Since a chemical variation in the surface can also cause the surface tension to change, it is often possible to obtain better bonding through chemical modification of a surface. An example of this is the oxidation of the surface of polyolefins [see the critical surface tensions of poly(ethylene) and poly-(vinyl alcohol) in Table 13-3]. 

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