Elastic instability in stratified core annular flow

TL;DR

This study investigates elastic interfacial instability in stratified core annular flow using microfluidic experiments, revealing velocity-dependent flow regimes driven solely by polymer solution viscoelasticity and enabling rheological measurements.

Contribution

It provides the first detailed experimental analysis of elastic instability in stratified core annular flow and links flow regimes to viscoelastic properties through linear stability analysis.

Findings

Flow transitions are driven by elasticity, not inertia.

Unstable flows occur at high velocities, stable at low velocities.

The method allows rheological property measurement of dilute polymer solutions.

Abstract

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. Surprisingly, 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.