Interfacial effects in the flow of viscous and elasticoviscous liquids

Abstract

A technique of flow visualization by means of an expanded laser beam and trace amounts of particulate additives is used to study the behaviour of newtonian and nonnewtonian elastic liquids in complex geometries. Differences in response are highlighted between the newtonian and non-newtonian fluids when these flow separately in certain contraction flows and also in a two-dimensional T geometry. This information is then used to interpret the behaviour when both types of fluid flow together in the same geometry with a well-defined interface between them. Of major interest is the observation that a newtonian liquid in the two-liquid situation (the other liquid being non-newtonian) can behave as if it were highly elastic. We are led to associate this behaviour with the boundary conditions existing at the interface between the newtonian and the non-newtonian liquids. Powerful finite-element numerical techniques are used in an attempt to simulate the observed flow characteristics. The techniques are able to meet the challenges posed by the two-liquid situation when both liquids are newtonian. They are also able to simulate the distinctive vortex structure observed when only one highly elastic liquid is used in the experiments. They are, as yet, unable to meet the difficult challenge where one of the two liquids is highly elastic. The experimental and theoretical work has an obvious potential application to the important practical problem of ‘coextrusion’.