Observations of ribbing instabilities in elastic fluid flows with gravity stabilization

We have investigated the role of elasticity in the stability of air-fluid i nterfaces during fluid displacement flows. Our investigations of the stabil ity of coating flows with an eccentric cylinder geometry for both a viscous Newtonian fluid and ideal elastic Boger fluids are discussed in terms of t hree classes of phenomena. To begin, we have documented several new feature s in traditional fingering instabilities in elastic displacement flows. The se include a very strong elastic destabilization of forward roll coating: a destabilization which can be correlated directly with the elasticity of th e coating fluid and which appears to be present even in the absence of dive rging channel walls, Moreover, elastic effects are shown to create a novel saw-toothed cusped pattern in the eccentric cylinder roll-and-plate geometr y. Secondly, we have found that purely elastic bulk flow instabilities in t he neighbourhood of air-fluid interfaces can cause surface deformations if the secondary flow is of sufficient strength. Finally, hows created by the displacement of less viscous air by a more viscous elastic fluid are found to display a new class of purely elastic instabilities which appear to be i ndependent of traditional viscous fingering instabilities and elastic bulk flow instabilities. Thus interfaces which are stable for Newtonian fluids a re unstable via purely elastic mechanisms. We have found that indeed elasti city has a dramatic effect on the stability of interfaces, not only changin g the critical conditions, but also changing the manifestation of tradition al fingering instabilities, and causing new purely elastic interfacial inst abilities.