Stable core-annular flows of viscoelastic fluids using the visco-plastic lubrication technique

Abstract

We give an experimental demonstration that stable core-annular flows can be achieved when lubricating a viscoelastic core fluid with a yield stress fluid. We use Carbopol as the lubricating yield stress fluid and Polyethylene Oxide (PEO) as the core fluid. The yield stress in the lubricating fluid preserves a ring of unyielded material around the interface, restricting the growth of instabilities. When the inlet radius is smaller than that of the established flow the core fluid stream expands, resulting in a net relaxation of the elastic normal stresses as the flow becomes fully developed. At low flow rates ( Q 1) of the core fluid this relaxation does not break the surrounding plug. At larger Q 1 secondary flows and then instabilities are observed. The secondary flows are interesting in that the elastic instability is frozen into the yield stress fluid at the interface, as the stresses drop below the yield stress in the developing flow. At still larger Q 1 the surrounding plug is broken and the interface may deform. In this case elasticity appears to retard the degree of interfacial mixing, in comparison to Newtonian core fluids. When the inlet radius is larger than that of the established flow a stabilizing effect is observed. The study shows that the visco-plastic lubrication technique is feasible for multi-layer flows with elasticity.