Interfacial instability profile

A second classic type of interfacial instability is mot with liquid cylinders. Consider, for instance, a fiber (a hair, with a typical diameter of 100 pm, is a good example) encased in a liquid sheath. Generally, such a film proves unstable. It develops a wavj profile (Figure 5.8). 

However, it seems that this instability may be more complicated than just a failure of adhesion between layers in shear flow. In many cases the region of fully developed shear flow is small or nonexistent and the analysis developed in Section 7.6.2 may not be applicable. The origin of the interfacial instability could be at the die exit where large stresses arise as the velocity profile undergoes a rapid rearrangement. It could also be associated with converging flow upstream of the die lips. In other words, differences in the extensional viscosity of the two polymers as well as differences in the relaxation behavior could lead to interfacial instability. 

Fig. 2. Solute distribution and transport phenomena at the interface of a growing crystal (a) Instability of the crystal-liquid interface and formation of a nonplanar pattern (schematically), (b) Faceted growth. It is assumed that the solute concentration in the liquid far from the interface (Cq) is constant due to forced and natural convection (stirring) whereas a thin solute diffusion layer (S) is quiet and possesses a solute distribution profile depending on the crystallization process type (a) interfacial control or a surface reaction (interfacial kinetics), Ce < C RJ C a difference between Cj and Cj is responsible for the driving force to buUd up the crystal surface (c) diffusion control Cj < Cj, providing a driving force for bulk diffusion in the liquid (b) mixed control.

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