Instability phenomenon

G.H. Markstein. Instability phenomena in combustion waves. Proceedings of the Combustion Institute, 4 44-59, 1952. 

Fig. 12.4, the melt is forced into a converging flow pattern and undergoes a large axial acceleration, that is, it stretches. As the flow rate is increased, the axial acceleration also increases, and as a result the polymer melt exhibits stronger elastic response, with the possibility of rupturing, much like silly putty would, when stretched fast. Barring any such instability phenomena, a fully developed velocity profile is reached a few diameters after the geometrical entrance to the capillary. 

There are two important multilayer flow instability phenomena. The first is an interface that changes, migrating spatially and progressively downstream in both the x and y directions, as shown in Fig. 12.31. It has been established that this instability becomes more pronounced with increasing viscosity ratios (64). The second manifests itself with the onset of wavelike irregularities at the interface, which, because of the prevailing periodicity of below 1 pm, result in loss of see-through optical clarity (65). 

We have surveyed the most recent progress and presented a new molecular level understanding of melt flow instabilities and wall slip. This article can at best be regarded as a partial review because it advocates the molecular pictures emerging from our own work over the past few years [27-29,57,62,69]. Several results from many previous and current workers have been discussed to help illustrate, formulate and verify our own viewpoints. In our opinion, the emerging explicit molecular mechanisms have for the first time provided a unified and satisfactory understanding of the two major classes of interfacial melt flow instability phenomena (a) sharkskin-like extrudate distortion and (b) stick-slip (flow discontinuity) transition and oscillating flow. 

F. Rosenberger, in J. Zierep and H. Oertel, eds., Convective Transport and Instability Phenomena, Vedag G. Braun, Kadsruhe, Germany, 1982, p. 469. 

Table 3. Instability Phenomena in Amorphous Polymers Comparison between the Extension Ratio, X,", at the Initiation of Stress-Whitening and the Maximum Extension Ratio, X, of Chains between Entanglement Points...

Some important similarities to crazes II have also been described by Pakula and Fischer studied instability phenomena associated with self-oscillations... 

Modern theories and concepts describing the initial dynamic evolution of fluid motions leading to the onset of turbulence, explaining experimental observations of instability phenomena, are t3rpically rather theoretical and usually not considered by chemical reaction engineers. The interested reader is referred to texts on chaos theory and non-linear d3mamics (e.g., [155], chap 5). 

Surface evolution—There is limited fundamental knowledge of how surfaces are created and destroyed at the atomic level. New surface science and mathematical tools should be used to develop a theory for electrocrystallization. Better understanding is needed of instability phenomena that lead to shape evolution, such as dendrites, surface roughness, anisotropic chemical and plasma etching, and patterning. 

Unfortunately, however, for Gragg variant, there is always the chance of encountering instability phenomena due to the side solution. 

It is quite obvious that in this situation it is very important to take the fluctuations into consideration. Such kinds of formal reactions are used to describe the chain reactions in nuclear reactors (e.g. Williams, 1974). In this context it is clear that the fluctuations have to be limited, since they could imply undesirable instability phenomena. 

Electrohydrodynamic instabilities in nematics could be classified according to the dependence of the threshold voltage (or field) on the physical parameters of the liquid crystal, cell geometry, field firequency, etc. Arising domain patterns also differ by the period of the structure and its orientation with respect to the initial director. We hope that this classification proves to be useful, both for finding similar instability phenomena in other liquid crystals (cholesteric, smectic, polymer liquid crystals, etc.) and for practical purposes in avoiding parasitic scattering and hysteresis effects which are undesirable in many applications. 

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