Interfacial Rheology and Stability

Stability. There are other factors such as viscosity that influence the dynamics of droplet growth. We take these factors into consideration using the general Navier-Stokes equation to mathematically describe droplet expansion  

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Danov, K.D., Ivanov, I.B., and Kralchevsky, P.A., Interfacial rheology and emulsion stability, in Second World Congress on Emulsion, Paris, 1997, 2-2, 152. 

To study the inter-relationship between rheological characteristics of interfacial layers and stability of water-in-oil concentrated emulsions, model emulsions containing 50% water and 50% soap in n-decane were prepared and their properties studied. In our previous work (3,4) high stabilities of emulsions stabilized by aluminum and iron soaps were observed, rheological properties of stabilizing interfacial layers being compared with the stability of drops. 

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... 

The results of the experiments can be evaluated as follows. Eucarol emulsifiers proved to be more advantageous with respect to pharmaceutical use than Tween 60, as they are produced with the use of raw materials of plant origin and they do not contain ethoxy groups. They were accumulated on the boundary to a greater extent than Tween 60, they elicited a greater decrease in interfacial tension and also had better wetting properties. These favourable properties were also manifested in better rheological and stability characteristics. 

To characterize emulsion systems, it is necessary to obtain fundamental information on the liquid/liquid interface (e.g. interfacial tension and interfacial rheology) and properties of the bulk emulsion system, such as droplet size distribution, flocculation, coalescence, phase inversion and rheology. The information obtained, if analyzed carefully, can be used for the assessment and (in some cases) prediction of the long-term physical stability of the emulsion. 

Modern attempts to formulate a quantitative theory of emulsions and emulsion stability have looked most closely at the nature of the interfacial region separating the two immiscible phases, especially the chemical and physical nature of the adsorbed film, the role of mixed films and complex formation, interfacial rheology, and steric and electronic factors at the interface. The theoretical foundations for current ideas concerning emulsion formation and stability are presented in several of the references cited in the Bibliography. A few of the most basic ideas, however, are presented below. 

The role of various surfactant association structures such as micelles and lyotropic liquid crystals (372), adsorption-desorption kinetics at liquid-gas interfaces (373) and interfacial rheology (373) and capillary pressure (374) on foam lamellae stability has been studied. Microvisual studies in model porous media indicate... 

In considering the impact of thermodynamically favourable interactions between biopolymers on the formation and stabilization of food colloids, a number of regular trends can be identified. One of the most important aspects is the effect of complexation on interfacial properties, including rates of adsorption and surface rheological behaviour. 

Dickinson, E. (2001). Milk protein interfacial layers and the relationship to emulsion stability and rheology. Colloids and Surfaces B Biointerfaces, 20, 197-210. 

Malhotra, A.K. Wasan, D.T. Interfacial Rheological Properties of Adsorbed Surfactant Films with Applications to Emulsion and Foam Stability in Thin Liquid Films, Ivanov, I.B. (Ed.), Dekker New York, 1988, pp. 829-890. 

It can be considered from the scheme that one has to distinguish between the foam kinetics, i.e. the rate of generation of foam under well defined conditions (air input and mechanical treatment) and the stability and lifetime of a foam once generated. The foam kinetics is also sometimes termed foamability in the literature. These quantities can be related to interfacial parameters such as dynamic surface tension, i.e. the non-equilibrium surface tension of a newly generated surface, interfacial rheology, dynamic surface elasticity and interfacial potential. In the case of the presence of oily droplets (e.g. an antifoam, a... 

Dispersion behaviour in systems with liquid/liquid or liquid/gas interfaces (i.e. droplet or bubbles) has traditionally been described in terms of rheological properties, wetting properties, including contact angle and interfacial tensions, or phase behaviour and stability measurements. Direct force measurements provide a means to fundamentally probe the interactions between deformable interfaces that significantly impact the dispersion (or emulsion) behaviour. 

A lower interfacial tension will lead to a more stable emulsion. Temperature affects physical properties of oil, water, interfacial films, and surfactant solubilities in the oil and water phases, which can all affect emulsion stability. Further, the rheology of the emulsion itself is affected significantly by temperature. 

Interfacial rheology is a very important tool in understanding the formation, stability and other properties of emulsions and foams. It also contributes to the characterization of monolayers, in addition to spectroscopic, electric and other methods. Hence, there is a clear motive for considering it in some detail. 

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