Interfacial surface elasticity

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

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... 

Vogler 31) developed a mathematical model to derive semiquantitative kinetic parameters interpreted in terms of transport and adsorption of surfactants at the interface. The model was fitted to experimental time-dependent interfacial tension, and empirical models of concentration-de-pendent interfacial tension were compared to theoretical expressions for time-dependent surfactant concentration. Adamczyk (32) theoretically related the mechanical properties of the interface to the adsorption kinetics of surfactants by introducing the compositional surface elasticity, which was defined as the proportionality coefficient between arbitrary surface deformations and the resulting surface concentrations. Although the expressions to describe the adsorption process differed from one another, it was demonstrated that the time-dependent interfacial tensions mirrored the change of surface-active substances at the interface. 

It has been suggested that a spreading film of antifoam may simply displace the stabilising surfactant monolayer. In this case, as the oil lens spreads and expands on the surface, the tension will be gradually reduced to a lower uniform value. This will eliminate the stabilising effect of the interfacial tension gradients - that is, the elimination of surface elasticity. 

Fig. 4. The variation of static surface elasticity (s0) with surface pressure (II) for air-seawater interfacial films with compositions corresponding to the FI extracts and mass spectra of Figure 3. The highest elasticities at a given film surface pressure were observed for films dominated by lipid surfactants lowest elasticities were observed for films dominated by polyoxy surfactants...

The stability of a foam is determined through the interplay of a number of factors that involve bulk solution, interfacial properties, and also external forces. We have summarized some of the effects on foam stability of gravity drainage, capillary suction, surface elasticity, viscosity, electric doublelayer repulsion, dispersion force attraction, and steric repulsion. Foams are such complex systems that Lucassen (56) has stated that any attempt to understand their properties in terms of a simple all-embracing theory is doomed to failure. Nevertheless, we have attempted to provide an introduction to the occurrence, properties, and importance of foams as they relate to the petroleum industry. More detailed aspects are taken up in the subsequent chapters of this book. 

Rapid spreading of a drop of oil that has a low surface tension over the lamella can cause rupture by providing a weak spot (46). The spreading oil lowers the surface tension, increases the radius of curvature of the bubbles, alters the original surface elasticity, and also changes the surface viscosity. Thus the interfacial film loses its foam-stabilizing capability. If S is negative, then the oil should not spread at the interface. 

Capillary Ripples Surface or interfacial waves caused by perturbations of an interface. When the perturbations are caused by mechanical means (e.g., barrier motion), the transverse waves are known as capillary ripples or Laplace waves, and the longitudinal waves are known as Marangoni waves. The characteristics of these waves depend on the surface tension and the surface elasticity. This property forms the basis for the capillary wave method of determining surface or interfacial tension. 

Here. Fe is defined as an elastic Hertzian contact-force. K is a reduced bulk modulus, and yi2 is the interfacial surface energy between surfaces 1 and 2. By substituting r In- the term R. where R is the radius of a spherical surface in contact with a flat surface, which is displaced by some small distance J just before detachment of the two surfaces from each other, the pull-off force is re-derived as... 

Noiiy Ring methods, pendant and spinning drop methods, for surface and interfacial dilational elasticity, thin-film techniques, and surface lateral diffusion when using fluorescence recovery after photobleaching (FRAP) methods. 

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... 

When the interfacial shear elasticity is moderate, at moderate surface viscosity, the thinning velocity will be greater than the Reynolds velocity. An increased surface viscosity means decreased surface mobility and a longer drainage time. These are all factors to be considered in decisions toward positive steps of destabili-zation. 

Here, is the surface gradient operator and Ep is the coefficient of interfacial thermal elasticity, [cf Eq. (1)]. The drops moving with different thermocapillary velocities can collide and flocculate or coalesce this is the thermal coagulation. 

Interfacial rheology - Dynamic surface elasticity Interfacial potential/ Intermolecular cohesion Oil particle size... 

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