Origin of Surface Tension Gradients at Gas-Crude Oil Interfaces

2 Origin of Surface Tension Gradients at Gas-Crude Oil Interfaces 

As we have seen, crude oil foams are usually transient (see, e.g., reference [31]). The absence of significant positive contributions to disjoining pressures means that foam collapse is inevitable when the constituent films drain to their so-called critical thickness. This means that the process of film drainage largely determines foam persistence. In tnrn, that process is usually dominated by the tangential stress boundary condition, Eqnation 1.1, which equates the viscous shear stress to the surface tension gradient in the draining foam film. 

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