We have investigated both experimentally and computationally the role of elasticity in the dynamics of air-fluid interfaces during fluid displacement flows. Our experimental study of coating flows with gravity stabilization in an eccentric cylinder geometry for both viscous Newtonian fluid and elastic Boger fluids is discussed in terms of three different flow types: the forward-roll coating, the roll-plate coating and the forward-inverse coating. For all three flow types considered, we have uncovered several new features in traditional fingering instabilities in elastic displacement flows. These include a very strong elastic destabilization which can be correlated directly with the elasticity of the coating fluid. Elastic effects are also shown to increase dramatically the hydrodynamic coating film thickness and create two dimensional ‘cusped’ or ‘sharp’ interfaces even under creeping flow conditions. In our theoretical investigation, a Newton-Raphson pseudo-solid domain mapping technique coupled with the DEVSS finite element formulation is applied to study the effects of viscoelasticity on free surface flows. Two distinct flow types are analyzed: the flow induced by a long air bubble steadily displacing a polymeric liquid confined by two parallel plates, i.e. Hele-Shaw flow, and the slot coating of viscoelastic fluids in the low metering rate limit. The Oldroyd-B, FENE-CR, and FENE-P constitutive equations are used to model the viscoelastic fluid. Our study reveals the formation of an elastic boundary layer in the capillary-transition region near the bubble front at moderate Weissenberg numbers while both the pressure and stress fields in the parallel flow region remain largely unaffected for both flows. Our calculations show that the elastically driven film thickening observed in experiments is associated with the onset of these stress boundary layers.
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