Instability interfacial

An interfacial instability causes distortion of the streamline where two layers of a coextruded film meet (Fig. 5.4). This can affect both the performance and visual properties of the film. Defects such as thickness inconsistencies, reduction in clarity, and even delamination of layers can result. 

A technique that has shown potential in eliminating interfacial instability is the use of fluoropolymer-based processing aids. These same additives are used to eliminate melt fracture in blown him extrusion. In one study [46], the incorporation of a processing aid into the skin (outer) layer of a three-layer coextrusion suppressed formation of the instability. 

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S. Coriell, M. Cordes, W. Boettinger, R. Sekerka. Convective and interfacial instabilities during unidirectional solidification of a binary alloy. J Cryst Growth 49 13, 1980. 

The changes of lateral force F in forward and backward motions follow the curve 1 and 2, respectively. It can be observed that there is one saddle-node bifurcation for the repulsive pinning center, but two bifurcations for the attractive piiming center. This suggests that the interfacial instability results from different mechanisms. On one hand, the asperity suddenly looses contact as it slides over a repulsive pinning center, but in the attractive case, on the other hand, the... 

Fig. 13.11 Injected droplet model proposed to explain how nonvolatile species become heated in a collapsing bubble. Nanosized liquid droplets enter the bubble as a result of interfacial instabilities...

Tzoganakis, C. and Perdikoulias, J., Interfacial Instabilities in Coextrusion Flows of Low-Density Polyethylenes Experimental Studies, Polym. Eng. ScL, 40, 1056 (2000)... 

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 effective diffusion coefficient is calculated according to the model of Ref. 209, which accounts for interfacial instabilities. This model includes a Handlos-Baron-like correlation (210) and one adjustable parameter, C/P ... 

Schrenk et al. (66) were among the first to report and study this interfacial instability, which they attributed to exceeding a critical value of interfacial shear stress. This criterion... 

As mentioned previously, the see-through clarity of the coextruded films deteriorates with the presence and extent of the wavelike interfacial instability. Quantitative... 

Another role of the surfactant is to initiate interfacial instability, e.g., by creating turbulence and Raykleigh and Kelvin-Helmholtz instabilities. Turbulence eddies tend to disrupt the interface since they create local pressures. Interfacial instabilities may also occur for cylindrical threads of disperse phase during emulsification. Such cylinders undergo deformation and become unstable under certain conditions. The presence of surfactants will accelerate these instabilities as a result of the interfacial tension gradient. 

VanDamme, H., Flow and interfacial instabilities in Newtonian and colloidal fluids (or the birth, life and death of a fractal), The Fractal Approach to Heterogeneous Chemistry, edited by D. Avnir, Wiley, Chishester, 1989, pp. 199-226. 

In a recent work [69], we were able to reveal that the sharkskin merely originates from a local interfacial instability of the boundary condition near the die exit wall. Specifically, the oscillation of adsorbed chains between their coil and stretch states produces a small scale periodic perturbation on the overall die swell and makes the extrudate surface appear rough or sharkskin like. 

The term melt fracture has been applied from the outset [9,13] to refer to various types of visible extrudate distortion. The origin of sharkskin (often called surface melt fracture ) has been shown in Sect. 10 to be related to a local interfacial instability in the die exit region. The alternating quasi-periodic, sometimes bamboo-like, extrudate distortion associated with the flow oscillation is a result of oscillation in extrudate swell under controlled piston speed due to unstable boundary condition, as discussed in Sect. 8. A third type, spiral like, distortion is associated with an entry flow instability. The latter two kinds have often been referred to as gross melt fracture. It is clearly misleading and inaccurate to call these three major types of extrudate distortion melt fracture since they do not arise from a true melt fracture or bulk failure. Unfortunately, for historical reasons, this terminology will stay with us and be used interchangeably with the phase extrudate distortion. ... 

Certain melt processing factors have to be considered in order to eliminate problems. Some of these factors can be compensated by the available plasticator and die/mold adjustments. An unsteady balance of shear forces causes the interfacial instability. Examples of the factors include ... 

Melt flow instabilities, such as interfacial instability, melt fracture, surging, and/or layer nonuniformity, can become problems that could cause quality problems with the coextruded product.200 There are several options to correct these problems. The key to success is to select... 

Waller, P., Melt Fracture or Interfacial Instability Different Ills Needs Different Cures, PT, Mar. 2003. 

This arises when two layers of fluids (may not be of same species or density) are in relative motion. Thus, this is an interfacial instability and the resultant flow features due to imposed disturbance will be much more complicated due to relative motion. Physical relevance of this problem was seized upon by Helmholtz (1868) who observed that the interface as a surface of separation tears the flow asunder. Sometime later Kelvin (1871) posed this problem as one of instability and solved it. We follow this latter approach here. The basic equilibrium flow is assumed to be inviscid and incompressible - as two parallel streams having distinct density and velocity - flowing one over the another, as depicted in figure below. 

The interface is stable in the entire range of the applied potential or, more realistically, the interfacial tension easily becomes zero with applying the potential, possibly resulting in the interfacial instability of another kind at y 0, where the electrocapillary emulsification [27] may take place. 

K.P. Chen, Interfacial instability due to elastic stratification in concentric coextrusion of two viscoelastic fluids, J. Non-Newtonian Fluid Mech., 40 (1991) 155-175. 

Another type of interfacial instability occurred in both systems whenever liquid crystal penetrated the brine to contact the brine-microemulsion interface. At high magnification (40x), rapid convection of liquid crystal particles to the interface was observed at volcano-like instabilities (Figure 11). Reported earlier for the same systems (4), this type of instability forms convection currents in the surrounding brine phase. After times ranging from a few seconds to a few hours, the instabilities choke-off." The mechanism by which this small-scale convection is initiated, maintained, and terminated is as yet unknown. 

Dong, M., Ma, S., Liu, Q., 2009. Enhanced heavy oil recovery through interfacial instability A study of chemical flooding for Brintnell heavy oil. Fuel 88, 1049—1056. 

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