Convective linear stability analysis of two-layer coextrusion flow for molten polymers

Valette, Rudy; Laure, Patrice; Demay, Yves; Agassant, Jean‐François

doi:10.1016/j.jnnfm.2004.04.002

Cited by 42 publications

(45 citation statements)

References 26 publications

(50 reference statements)

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“…Since these instabilities set severe limits to the industrial processes such as film or fiber fabrication, they have been extensively studied before [7,8]. Although previous experiments and theory agree reasonably well [7,9], a comprehensive description of the flow is still lacking [10].In this Letter we propose a quantitative explanation for various flow patterns observed in purely elastic interfacial instabilities. We perform a set of original experiments on co-flow of a polymer solution and water and map the full flow diagram.…”

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Elastic instability in stratified core annular flow

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We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic -it is caused by viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation captures quantitatively the flow diagram. Suprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure rheological properties of dilute polymer solutions that are difficult to assess otherwise.PACS numbers: 83.80. Rs, 83.60.Wc, 83.85.Cg Polymer solutions exhibit purely elastic flow instabilities even in the absence of inertia [1]. The almost ubiquitous ingredient of such an elastic instability is the curvature of streamlines: polymers that have been extended along curved streamlines are taken by fluctuations across shear rate gradient in the unperturbed state which, in turn, couples the hoop stresses acting along the curved streamlines to the radial and axial flows and amplifies the perturbation [2,3]. Flat interfaces between two fluids with different viscoelastic properties can also become unstable [4-6] due to the normal stress imbalance across the interface. These instabilities often occur in coextrusion where different polymers are melted in separate screw extruders and then flown simultaneously in the extrusion nozzle. Undesirable wavy interfaces are sometimes observed between the adjacent polymer layers both during the flow and in the final product [7]. Since these instabilities set severe limits to the industrial processes such as film or fiber fabrication, they have been extensively studied before [7,8]. Although previous experiments and theory agree reasonably well [7,9], a comprehensive description of the flow is still lacking [10].In this Letter we propose a quantitative explanation for various flow patterns observed in purely elastic interfacial instabilities. We perform a set of original experiments on co-flow of a polymer solution and water and map the full flow diagram. In contrast to previous experimental studies dealing with macroscopic flows of molten polymers, we focus here on flows of dilute polymer solutions and water in microfluidic devices. The advantage of using dilute polymer solutions is that their elastic properties can easily be tuned by dilution and that most of the theoretical work has been performed for models representing dilute polymeric fluids. The use of microfluidic flow geometries offers simple control and visualisation of the flow.We observe that above some flow rate the interface between the polymer solution and water becomes wavy. Surprisingly, for relatively low velocities...

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Elastic instability in stratified core annular flow

et al. 2011

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“…To the best of our knowledge, this type of analysis, which has been performed previously for jets, mixing layers, wakes, boundary layers etc, has not been carried out for miscible channel flows; such an analysis, however, was conducted for immiscible two-layer flows to determine the occurrence of defects in the co-extrusion of polymers [47].…”

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Linear stability analysis and numerical simulation of miscible two-layer channel flow

Sahu¹,

Ding²,

Valluri³

et al. 2009

Physics of Fluids

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The stability of miscible two-fluid flow in a horizontal channel is examined. The flow dynamics are governed by the continuity and Navier-Stokes equations coupled to a convective-diffusion equation for the concentration of the more viscous fluid through a concentration-dependent viscosity.Our analysis of the flow in the linear regime delineates the presence of convective and absolute instabilities and identifies the vertical gradients of viscosity perturbations as the main destabilizing influence in agreement with previous work. Our transient numerical simulations demonstrate the development of complex dynamics in the nonlinear regime, characterized by roll-up phenomena and intense convective mixing; these become pronounced with increasing flow rate and viscosity ratio, as well as weak diffusion. *

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“…Valette et al [22][23][24] have investigated the convective nature of interfacial instability of a coextruded PE/PS flow. The experiments were performed under both industrial and laboratory conditions to point out the general behavior of such stable/unstable transitions.…”

Section: Introductionmentioning

confidence: 99%

Role of the interphase in the flow stability of reactive coextruded multilayer polymers

Lamnawar

Maazouz

2009

Polymer Engineering & Sci

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Coextrusion technologies are commonly used to produce multilayered composite sheets or films for a large range of applications from food packaging to optics. The contrast of rheological properties between layers can lead to interfacial instabilities. Important theoretical and experimental advances regarding theses defects have, during the last decades, been made using a mechanical and numerical approach. This study deals with the influence of the physicochemical affinity between the neighboring layers on interfacial instabilities for functionalized incompatible polymers. It was experimentally confirmed, in this case, that weak disturbance can be predicted by considering an interface of nonzero thickness (corresponding to an interdiffusion/reaction zone interphase) instead of a purely geometrical interface between the two reactive layers. According to the rheological investigations, an experimental strategy was here formulated to investigate the parameters that controlled the stability of the reactive multilayer flows. The role of the viscosity ratio, elasticity ratio, and layer ratio of the stability of the interface was also investigated coupling to the reaction rate/compatibilization phenomenon. Hence, based on this analysis, guidelines for a stable coextrusion of reactive functionalized polymers can be provided coupling the classical parameters and the physicochemical affinity at the polymer/polymer interface. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

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Convective linear stability analysis of two-layer coextrusion flow for molten polymers

Cited by 42 publications

References 26 publications

Elastic instability in stratified core annular flow

Elastic instability in stratified core annular flow

Linear stability analysis and numerical simulation of miscible two-layer channel flow

Role of the interphase in the flow stability of reactive coextruded multilayer polymers

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