Interfacial instability thickness

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. 

This type of interfacial instability can be reduced or eliminated by increasing skin-layer thickness, increasing die gap, reducing total rate, or decreasing skin-layer polymer viscosity. These methods may be used singly or in combination. These remedies reduce interfacial shear stress, and stable flow results when it is below the critical stress for the polymer system being coextruded. Most often skin-layer polymer viscosity is decreased. In feedblock coextrusion the resultant viscosity mismatch imposed by this remedy can cause variations in layer thickness as discussed earlier. Shaped skin layer feedslots are then used to compensate. 

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. 

The dynamics of thin liquid films are often dominated by viscous friction. As such, the lubrication approximation is entirely appropriate. In this section, we discuss four examples in order of increasing difficulty. They are the thinning of a vertical liquid film, the levelling of a wavy film, and two examples of interfacial instability (suspended film and liquid cylinder). We restrict our attention here to films of thickness e > 100 nm, for which the long-range interactions described in chapter 4 can be neglected [P e) -> 0]. 

Calculate the film thickness for the Marongoni instability shown in Fig. Xlll-2 using the relationship in Eq. XIIl-3, assuming that the interfacial tension is 20 mN/m. 

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. 

FIGURE 1.8. (a) Schematic representation of the device used to study capillary surface instabilities. A polymer-air bilayer of thicknesses /ip and /ia, respectively, is formed by two planar silicon wafer held at a separation d by spacers. A capillary instability with wavelength k = 27t/q is observed upon applying a voltage U or a temperature difference AT. (b) Dispersion relation (prediction of Eq. (1.6)). While all modes are damped (r < 0) in the absence of an interfacial pressure pei, the application of an interfacial force gradient leads to the amplification of a range of k-values, with /.m the maximally amplified mode. 

Figure 13 shows that the stability at 50 °C of concentrated emulsions is affected by the volume fraction of the dispersed phase. As the volume fraction of the dispersed phase increases the stability decreases. The increased instability of the concentrated emulsion is caused by the decreased thickness of the interfacial film that surrounds the cells of the dispersed phase. 

A more elaborated theoretical model based on interfacial Taylor instability triggering the surface wave was developed by Peskin and Raco [39]. A thin layer of a liquid, wetting the surface of a solid resonator wbch vibrates to its plane, forms a chessboard-like pattern of stationary capillary waves. This phenomenon occurs when the vibration amplibde exceeds a threshold value. Further on, ligament breakup of the liquid occurs and droplets are hurled from the crests of the capillary waves. Together with the wavelength, they introduced wave amplibde and the sheet thickness as parameters to determine the droplet size [39]. 

This instability develops in the die land, and its onset can be correlated with a critical interfacial shear stress for a particular polymer system (2). The most important variables influencing this instability are skin-layer viscosity, skin-to-core thickness ratio, total extrusion rate, and die gap. Although the interfacial shear stress does not cause instability, elasticity is related to shear stress, and interfacial stress is used to correlate variables for a particular system. 

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