Gradients in interfacial tension

What is the mechanism which results in rapid coalescence if mass transfer occurs from the drops but slow or no coalescence if both phases are mutually saturated Interfacial turbulence caused by local gradients in interfacial tension looks promising. 

Spreading Rate. If a gradient in interfacial tension occurs in a liquid interface, because the interface is suddenly expanded or some surfactant is locally applied to the interface, the interfacial tension will be evened out, i.e., become the same everywhere at the interface. The rate at which this occurs if of considerable importance for the extent of the Marangoni effect. It proceeds as a longitudinal surface wave. The linear velocity of the wave on an A-W interface is given by... 

Rumscheidt and Mason [288] distinguish four classes of deformation and breakup in simple shear flow depending on the viscosity ration p. When p > 1, the deformed drop has rounded ends, while for smaller p values the ends become pointed. When p < 0.1, very small droplets break off and form the sharply pointed ends-this is called tipstreaming. This is caused by gradients in interfacial tension due to convection of surfactants along the drop surface. The interfacial tension is lowered at the tip, causing very small droplets to break off. 

Surface tension on the surface of a liquid at gas-Uquid or vapor-liquid interfaces can vary due to a variation in temperamre or species concentration. The components of the tangential stress, Tj Ty and are related to the corresponding gradients in interfacial tension, between the liquid phase, j = 1, and the gas/vapor phase, y = 2 by (Bird et al, 2002) ... 

Surface elasticity in the sense under consideration cannot exist in a system of pure liquid phases. In a system containing surfactant molecules, gradients in interfacial tension can arise as a result of the formation of new area, as mentioned above, or because of the loss of interfacial area. In the former case, the time lag between the formation of new interface and the diffusion of surfactant to that interface will produce an interfacial tension that is higher than equilibrium. The local value of the surface excess T, will fall and the value of a, will approach that of the pure system. The net effect will be a tendency for the interface to contract, providing a healing effect to reduce the chance of droplet coalescence. In the case of loss of interfacial area, there will be a time lag from the point of compression of the interfacial film until the excess surfactant molecules can desorb and diffuse away from the interface. 

Finally, we consider the problem of Marangoni instability, namely convection in a thin-fluid layer driven by gradients of interfacial tension at the upper free surface. This is another problem that was discussed qualitatively in Chap. 2, and is a good example of a flow driven by Marangoni stresses. 

In addition, the presence of temperature and concentration gradients results in interfacial tension gradients and influences the phase distributions and flow rates. In dynamic systems, the history of the flows and the surface conditions also play a role and lead to the observed hysteresis in the phase distributions. 

This teehnique was used to show that demulsifica-tion effeetiveness ean be correlated with a low dynamic interfacial tension gradient, especially at low interfacial shear viscosity and this is evidenced by a sharp rise in interfacial tension at low frequencies for ineffective demulsifiers. 

Gradients in surface (or interfacial) tension can accelerate the spreading of fluids, enhance the stability of surfactant-laden films of liquid, emulsions, and foams, and increase rates of mass transport across interfaces. The motion of fluid driven by a gradient in surface tension is referred to as a Marangoni flow . We have demonstrated that electrochemical reduction of IF to IF at an electrode that... 

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