Interfacial instability stability

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... 

The mosl important early work on interfacial instabilities is that of Saffman and Taylor (1958) who considered the stability of an interface between two immiscible fluids moving vertically through a porous medium. Wooding (1959,... 

Moreover, the transfer of surfactant between the liquid phases can also have an impact on the interface stability. In fact it has been shown (170, 171) that, depending on the ratio between the diffusion coefficients in the two phases, the transfer of matter across the interface can give rise to interfacial instabilities. 

Abstract Among the noncontinuum-based computational techniques, the lattice Boltzman method (LBM) has received considerable attention recently. In this chapter, we will briefly present the main elements of the LBM, which has evolved as a minimal kinetic method for fluid dynamics, focusing in particular, on multiphase flow modeling. We will then discuss some of its recent developments based on the multiple-relaxation-time formulation and consistent discretizatirai strategies for enhanced numerical stability, high viscosity contrasts, and density ratios for simulation of interfacial instabilities and multiphase flow problems. As examples, numerical investigations of drop collisions, jet break-up, and drop impact on walls will be presented. We will also outline some future directions for further development of the LBM for applications related to interfacial instabilities and sprays. 

In the remainder of this book, we present information on phenomena where dynamic interfacial effects are important. We begin in this chapter with interfacial stability and the closely related subject of interfacial oscillation or wave motion. It is frequently of great interest to know the conditions for interfacial instability. We may ask, for instance, how far a fluid jet leaving a circular orifice travels before it breaks up into drops. Or when we can expect spontaneous convection to arise near an interface across which one or more species diffuse. 

Interface stability in co-extrusion has been the subject of extensive analysis. There is an elastic driving force for encapsulation caused by the second normal stress difference (56), but this is probably not an important mechanism in most coprocessing instabilities. Linear growth of interfacial disturbances followed by dramatic breaking wave patterns is observed experimentally. Interfacial instabilities in creeping multilayer flows have been studied for several simple constitutive equations (57-59). Instability modes can be traced to differences in viscosity and normal stresses across the interface, and relative layer thickness is important. 

Multilayer coextrusion has been widely used, studied and understood in the domain of synthetic polymers for its effect on the viscosity of polymers, die geometry, layer distribution, encapsulation, and interfacial instabilities, influencing the quality and functionality of the multilayer products. Despite the number and diversity of studies on multilayer flow and stability, only some articles report the use of biopolyester in coextrusion processes. Different stratified structures were processed by coextrusion and studied. But very few studies have been carried out with PHA. Most of these are based on the association between PHA e.g. PHBV, and plasticized starch. Applications of such PHA-based multilayers as commodities are primarily limited by PHA cost and have been until now by PHA availability, and thus attention is being focused on products with plastics constituting only a minor part, such as paper coatings like the plastic film moisture barrier in food and drink cartons and in sanitary napkins. 

Interfacial instability due to fluctuations of the electric potential is investigated by Felderhof [493]. A theoretical description of the stability of an evaporating liquid surface is given by Prosperetti and Plesset [510]. They established that at large evaporation flow rates, the instability is very strong with growth time of a millisecond or less. This theory... 

Stability may be inherent or induced. In the latter case, the original system is in a condition of metastable or neutral eouilibrium. External influences which induce instability in a dispersion on standing are changes in temperature, volume, concentration, chemical composition, and sediment volume. Applied external influences consist of shear, introduction of a third component, and compaction of the sediment. Interfacial energy between solid and liquid must be minimized, if a dispersion is to be truly stable. Two complementary stabilizing techniques are ionic and steric protection of the dispersed phase. The most fruitful approach to the prediction of physical stability is by electrical methods. Sediment volumes bear a close relation to repulsion of particles for each other. 

The instability of these chiral monolayers may be a reflection of the relative stabilities of their bulk crystalline forms. When deposited on a clean water surface at 25°C, neither the racemic nor enantiomeric crystals of the tryptophan, tyrosine, or alanine methyl ester surfactants generate a detectable surface pressure, indicating that the most energetically favorable situation for the interfacial/crystal system is one in which the internal energy of the bulk crystal is lower than that of the film at the air-water interface. Only the racemic form of JV-stearoylserine methyl ester has a detectable equilibrium spreading pressure (2.6 0.3dyncm 1). Conversely, neither of its enantiomeric forms will spread spontaneously from the crystal at this temperature. 

The primary effect of the anode modification on the enhancement in luminous efficiency and the increased stability of OLEDs can be attributed to an improved hole-electron current balance. By choosing an interlayer with a suitable thickness of a few nanometers, anode modification enables engineering of the interface electronic properties. The above results indicate that conventional dual-layer OLEDs of ITO/NPB/Alq3/cathode have an inherent weakness of instability that can be improved by the insertion of an ultrathin interlayer between ITO and HTL. The improvements are attributed to an improved ITO-HTL interfacial quality and a more balanced hole electron current that enhances the OLED performance. 

The factors determining the appearance of ordered cell-like motions were first investigated by Sternling and Scriven (S33) who considered the two-dimensional stability of a plane interface separating two immiscible semi-infinite fluid phases with mass transfer occurring between the phases. This system was shown to be unstable for mass transfer in one direction, but stable for transfer in the opposite direction. For an interfacial tension-lowering solute, instability... 

Any conclusion that a low intcrfacial tension per sc is an indication of enhanced emulsion stability is nut reliable. In fact, very low interfacial tensions lead to instability. The stability of an emulsion is inlluenced by the charge at the interface and by the packing of the emulsifier molecules, but the interfacial tension at the levels found in the common emulsion has no influence on stability. 

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