ABSTRACT

Chapter 1 of this book discussed the thermodynamics of interfaces, in equilibrium with their adjoining bulk phases. Multiphase systems such as foams and emulsions are often produced or processed under conditions far from equilibrium, where they are subjected to high deformation rates and significant temperature gradients. Their response to these gradients can be highly nonlinear as a result of an intricate coupling of dynamic interfacial processes and subphase processes. For example, inhomogeneities in the flow along an interface can cause compositional gradients, where regions that are compressed coexist with expanded regions (Figure 2.1). The difference in concentration along the interface drives diffusion of mass along the interface (surface diffusion) and diffusive exchange between the interface and the subphase. But the concentration gradients also induce inplane gradients in the surface rheological properties (such as the surface shear viscosity, or surface dilatational modulus), leading to a nonlinear rheological response, which affects the deformation of the interface.