Instability of the interface in co-extrusion flow of two UCM fluids in the presence of surfactant

Abstract

The effect of an insoluble surfactant on the flow instability of two upper convected Maxwell (UCM) fluid co-extrusion flow in a circular pipe is studied using a linear normal-mode and complementary energy analysis. The influences of surfactant and fluid elasticity on the interfacial instability at low Reynolds number ( ℝ → 0 ) are of principal concern. The results show that the interfacial perturbation flow, which is induced due to elasticity stratification, can drain the surfactant from the interfacial wave trough and push it toward the crest. Thereby, the interplay of the elasticity stratification and the surfactant acts to stabilize the capillary instability. The neutral curve in the Weissenberg number ( We)–axial wavenumber ( k) plane is presented. A window of parameters is identified in which the flow is linearly stable. As the radius of the cylinder tends to infinity, a viscoelastic thread suspended in a quiescent infinite ambient fluid is obtained. Corresponding results show that the influence of fluid elasticity on flow instability is weakened with the increasing of Reynolds number ( ℝ ) . Similar results can also be received for the influence of surfactant.