An experimental investigation of interfacial instabilities in multilayer flow of viscoelastic fluids: Part I. Incompatible polymer systems

Abstract

The interfacial stability of coextruded polypropylene (PP) and high-density polyethylene (HDPE) has been examined experimentally in a slit-die geometry. Our experimental apparatus makes use of a novel system for introducing temporally regular disturbances with a controllable amplitude and frequency. The growth or decay of the disturbance can be observed via optical windows mounted in the test die and characterized with the aid of digital image-processing techniques. This approach has allowed us to construct experimentally determined stability and growth-rate plots as a function of disturbance wavenumber and layer-depth ratio. Our experimental results are compared to theoretical results based on the linear stability analysis using a modified Oldroyd-B equation. We have found this model to be quantitatively correct in its prediction of the critical layer-depth ratio and the existence of a maximum spatial growth rate in the vicinity of a dimensionless wavenumber of one. The theoretically predicted growth rates agree with our experimental data at low wavenumbers but fail to agree at higher wavenumbers. Overall, our findings provide evidence that in order to achieve quantitative agreement between theoretical and experimental results, a constitutive model should be utilized that accounts for the shear-rate dependence of first and second normal stresses and a spectrum of relaxation times.