Interfacial instability thermodynamics

Mullins and Sekerka (88, 89) analyzed the stability of a planar solidification interface to small disturbances by a rigorous solution of the equations for species and heat transport in melt and crystal and the constraint of equilibrium thermodynamics at the interface. For two-dimensional solidification samples in a constant-temperature gradient, the results predict the onset of a sinusoidal interfacial instability with a wavelength (X) corresponding to the disturbance that is just marginally stable as either G is decreased... 

An interdisciplinary team of leading experts from around the world discuss recent concepts in the physics and chemistry of various well-studied interfaces of rigid and deformable particles in homo- and hetero-aggregate dispersed systems, including emulsions, dispersoids, foams, fluosols, polymer membranes, and biocolloids. The contributors clearly elucidate the hydrodynamic, electrodynamic, and thermodynamic instabilities that occur at interfaces, as well as the rheological properties of interfacial layers responsible for droplets, particles, and droplet-particle-film structures in finely dispersed systems. The book examines structure and dynamics from various angles, such as relativistic and non-relativistic theories, molecular orbital methods, and transient state theories. 

In the examples of interfacial stability considered thus far, the systems have been at rest in their initial states. Hence the predictions of when instability can be expected are, in fact, conditions for thermodynamic stability. We have chosen not to emphasize this point and to carry out the analyses in terms of perturbations of the general equations of change because we obtain in this way not only the stability condition but also the rates of growth of unstable perturbations and the appropriate frequencies of oscillation and damping factors for stable perturbations. Also, the basic method of analysis used above is applicable to systems not initially in equilibrium, as we shall see later in this chapter and in Chapter 6. 

The thermodynamic approach shows clearly that it is the molecular layer at the interface which largely determines the value of 7 and, more generally, the potential stability or instability of suspensions. Therefore, in the following section, we shall discuss some simple ways of altering interfacial characteristics. 

All foams are thermodynamically unstable, due to their high interfacial free energy, which decreases on rupture. For convenience, the instability has been expressed... 

In the above systems AAa TAS and AAy TAS and hence AG > 0. This implies thermodynamic instability and the production of suspension or emulsions by the dispersion process is nonspontaneous, i.e. energy is required to produce the smaller particles or droplets from the larger ones. In the absence of any stabilization mechanism (that will be discussed below), the smaller particles or droplets tend to aggregate and/or coalesce to reduce the total interfacial area, hence reducing the total surface energy of the system. Prevention of aggregation and/or coalescence of suspensions or emulsions requires fundamental understanding of the various interaction forces between the particles or droplets and these will be discussed in subsequent sections. 

Instability of immiscible polymer blends. An immiscible polymer blend is thermodynamically unstable. The state of dispersion of one phase in another is governed by both thermodynamics (interfacial tension) and thermo-mechanics (agitation). It is a... 

The first chapter by J. Jiao and D. J. Burgess discusses the thermodynamic instability of multiple emulsions as a result of the excess of free energy caused by the formation of the emulsion droplets. In multiple emulsions consisting of three distinct liquid phases, counteracting the effect of the Laplace pressure by electrolyte addition to the inner dispersed aqueous phase will increase the destabilization of the system owing to osmotic pressure. In addition the authors discuss the effects of both osmotic and Laplace pressure as well as the interfacial rheological properties of these complex systems and their stability. 

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