Breakup into Droplets

A jet emerging from a nonciicular orifice is mechanically unstable, not only with respect to the eventual breakup into droplets discussed in Section II-3, but, more immediately, also with respect to the initial cross section not being circular. Oscillations develop in the Jet since the momentum of the liquid carries it past the desired circular cross section. This is illustrated in Fig. 11-20. 

Liquid Breakup into Droplets There are four basic mechanisms for breakup of liquid into droplets ... 

Figures 7.18(b) and 7.18(c) show the breakup into droplets of an extended filament of high density polyethylene in a polystyrene matrix. In Fig. 7.18(b) the distance between the extruder die and the quenching bath is short and the fiber freezes before breaking up, whereas in Fig. 7.18(c) the distance was increased, giving the filaments sufficient time for breakup. As the filament extends, its diameter is reduced until shear forces no longer dominate the surface tension cohesive forces and the filaments breaks into droplets, just like a stream of water from a faucet breaks up into droplets.

The inviscid jet must be stabilized in about 10 seconds or it will breakup into droplets. Yet another point in regard to this process is the fact that small diameter fibers are difficult to make. For example, the smallest diameter of an alumina-calda fiber produced by this technique is 105 xm. These fibers, according to the authors, have tensile strengths close to 1 GPa. 

Surface tension-driven breakup into droplets is rarely important in melt spinning, where the large viscous and elastic forces overwhelm the surface tension forces, ft is an important mechanism in the formation of the dispersed phase in polymer blends, and it is important in solution processing. The surface tension-driven breakup of a viscoelastic filament has been analyzed using both thin filament equations and a transient finite element analysis, but we will not pursue the topic here because it is not relevant to our present discussion. 

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