Interface instabilities

M. Rettenmayr, O. Pompe. Interface instabilities on solidifying globulitic particles. J Cryst Growth 775 182, 1997. 

Thornton, B. H. and Bogy, D. B., "A Parametric Study of Head-Disk Interface Instability Due to Intermolecular Forces, IEEE Trans. Magn., Vol. 40, No. (1), 2004, pp. 337-343. 

The problem of morphological instability was solved theoretically by Mullins and Sekerka [20], who proposed a linear theory demonstrating that the morphology of a spherical crystal growing in supercooled melt is destabilized due to thermal diffusion the theory dealt quantitatively with and gave linear analysis of the interface instability in one-directional solidification. 

Figure 3.18. The process of changes in morphology due to interface instability. (a)-(e) Starting from a smooth interface, it is seen that the morphology changes to a cusp array, to a rod structure, to a dendritic structure.

The main effeots involved in the use of high-intensity US for deliquoring are (a) alternating aooustio stresses, (b) radiation pressure, (c) acoustic streaming, (d) interface instabilities and (e) cavitation. 

The craze front velocity v can be governed by one of two distinct mechanisms of craze matter production. As Argon and Salama have discussed in detail, under the usual levels of service stresses or stresses under which most experiments are carried out, craze matter in single phase homopolymer is produced by the convolution of the free surface of the sohd polymer at the craze tip. This occurs by a fundamental interface instability present in the flow or deformation of all inelastic media when a concave, meniscus-like surface of the medium is being advanced locally by a suction gradient. This is the preferred mechanism of craze advance in homopolymers. In block copolymers with uniform distributions of compliant phases of a very small size, and often weaker interfaces than either of the two phases in bulk, craze advance can also occur by cavitation at such interfaces to produce craze matter as has been discussed by Argon et al. Both of these mechanisms of craze advance lead to very similar dependences of the craze front velocity on apphed stress and temperature that is of the basic form... 

The importance of the interface instability has led to many models being developed to examine the deformation of the interface and the evolution of that interface. Most such models can be placed into one of the two categories (1) fundamental studies of the instability of the interface based on idealization of the cell and (2) computationally intensive investigations where models akin to those of the last section, incorporating most features and phenomena of the cell, are extended to time dependent calculations. 

Fig. 23 Growth rates of interface instabilities computed by Segatz and Droste for various modes of oscillation of the metal-electrolyte interface as a function of current [76].

In their studies of interface instability, most of the investigators listed above have employed a Fourier analysis of the coupling between the fluid motion and the electromagnetic body force. Davidson [81] and, in simpler form, Davidson and Lindsay [82] have pursued an alternative approach whereby a global energy balance... 

In addition to the statistical nature of the interface instabilities active in diamond CVD, the orientation effect and anisotropic growth of crystals (i.e., evolutionary selection) play an important role in the observed instability phenomenon. Surface chemical reactions that occur preferentially between the growing diamond surface and oxidizing species in the combustion synthesis ambient also influence the development of the microstructure and morphology of crystals in diamond films. For example, in combustion CVD,... 

This system has been studied by Goldstein [294] and Levine [295], and seems to be an example of very site-specific adsorption in which the Cs atoms occupy four-fold coordination sites above the uppermost Si atoms. There is some similarity with other systems in that the Si dangling bond states are removed by Cs deposition to be replaced by Cs-induced gap states. There is, however, no evidence for interface instability effects. 

We may conclude, however, that probably as the result of a strong ionic contribution to the bonding, this system appears more straightforward than other metal—semiconductor combinations, at least with regard to interface instability. 

It is evident from the preceding sections that, with a few exceptions, adsorption of a metal on a clean semiconductor surface leads to pronounced solid state reactivity. We may reasonably ask, therefore, whether it is possible to establish a general mechanism for this interface instability. Specifically, an explanation is required for solid state reactions and transport processes which are occurring at comparatively high rates at room temperature, where they might have been expected to be negligible. Two plausible models have been advanced, one by Tu et al. [324, 325] and the other by the Spicer group [298, 326, 327]. 

Interface instabilities, known as myelins, are an example of exotic nonequilibrium behavior present during dissolution in a number of surfactant systems. Although much is known about equilibrium phase behavior much still remains to be understood about nonequilibrium processes present in surfactant dissolution. In this chapter nucleation and growth, self and collective diffusion processes and nonlinear dynamics and instabilities observed in various polymeric systems are reviewed. These processes play an important role in our understanding of myelin instabilities. Kinetic maps and the concept of the free energy landscape provide a useful approach to rationalize some of the more complex behavior sometimes observed. 

In this chapter I will review problems related to dissolution kinetics and nonlinear process that occur in surfactant systems. First we discuss the role of phase kinetics in surfactant dissolution. Then the diffusive processes are discussed where it is important to appreciate the difference between self and collective diffusion. Finally, interface instabilities will be discussed which includes some of the most recent and significant observations. These studies are extremely interesting in the context of industrial problems such as detergency. 

Inverted Device Structures The conventional device structure for PSCs is indium tin oxide (ITO)/PEDOT PSS/polymer blend/Al, where a conductive high-work-function PEDOTPSS layer is used for anode contact, and a low-work-function metal as the cathode. Both the PEDOTPSS layer and the low-work-function metal cathode can cause the degradation of PSCs [110-112]. The acidic PEDOTPSS was reported to etch the ITO and cause interface instability through indium diffusion into the polymer active layer. Low-work-fiinction metals, such as calcium and aluminum, are easily oxidized when exposed to air, increasing the series resistance at the metal/BHJ interface and degrading device performance. 

Up to now, most of the efforts to develop a high-performance insitu composite have been focused on the binary LCP blends, and a number of papers have been disclosed during the last decade. The binary insitu composites, however, frequently lead to unsatisfactory mechanical properties (8,9). It is largely because of poor dispersion and poor elongational deformation of the LCP domains, along with interface instabilities between LCP and matrix phases resulting finm phase separation. 

Recently, several researchers (8,10-12) have reported that compatibilized LCP blends exhibit much enhanced mechanical properties, and they have attributed it to the improved interface adhesion between LCP and matrix phases. In this study, an isotropic polymer was incorporated to the binary LCP blends as a third component to solve the problems caused by interface instabilities and poor deformation of LCP domains. The third component was selected from the commercially available block copolymers on the experimental basis, or prepared by synthesizing block copolymers by molecular design. 

Aqueous solutions of didodecyldimethylammonium bromide (DDAB) exhibit an interesting self-assembly phenomenon." Each amphiphilic molecule of DDAB has two hydrophobic tails, so that DDAB molecules spontaneously form inverted micelles in hydrocarbons and nano-size spherical vesicles in dilute aqueous solutions. However, in aqueous solutions, at the DDAB concentration of about lO moldm , the spherical vesicles self-assemble into a multilayer onion-like structure shown in Figure 7.3. It is also notable that at the same DDAB concentration, the interfacial tension between water and a hydrocarbon (octane) almost vanishes, becoming less than 0.1 mNm and suggesting that the formation of the multilayer structure may be accompanied by water-oil interface instability. 

A crucial point in this time sequence is that the small undershoot in the a(t) curve appears as the mechanical signature of the interface instability. Since the undershoot has been detected on other semidilute systems (see Sect. 3.2.2), this suggests that the interfacial instability is presumably not inherent to this particular solution. 

---