The study of purely elastic flow instabilities is now a burgeoning field having its modern origins in the examination of viscometric bulk flow instabilities a decade ago. The initial examinations of Taylor-Couette, Dean, and Taylor-Dean flow, where the deformation gradient is constant along streamlines, have given way to the study of more complex flows such as eccentric cylinder flows and closed cavity flows. In the latter instances, global stress effects are important in the stability picture, and large scale numerical solutions of the flow field using reasonable constitutive equations for the polymeric fluid are critical. With knowledge of the mechanisms of elastic bulk flow instabilities, much of the recent work has been focussed on interfacial instabilities created or seriously enhanced by elastic effects. Certainly, in terms of the practical implications of these instabilities, the presence of interfacial instabilities in extrusion processes, coating applications, etc. can be the most important limiting factor in processing rates. The prototype for the study of these instabilities is the fluid-fluid displacement fingers associated with Hele-Shaw, or alternatively the "ribbing instability" often discussed in coating applications. Careful experiments demonstrate that fluid elasticity dramatically destabilises these flows and this is manifested by markedly reduced critical penetration speeds at which "fingers" or "ribs" form. Moreover, the wavelength of instability at the critical conditions is qualitatively changed by fluid elasticity. This occurs over a wide spectrum of such flows, and it is even found that displacement, which would be stable for Newtonian fluids at ALL speeds, may be unstable for elastic fluids. Experimental evidence is given that elastic stresses generated very near the interface created by local recirculation flows play a key role in the mechanisms of these elastic displacement instabilities.
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