Instabilities of viscoelastic liquid film coating tube in the presence of surfactant

• The liquid viscoelasticity may stabilize the long wavelength perturbation in the presence of base flow.
• The competition and interaction between liquid viscoelasticity and surface tension will lead a linear stable film flow.
• The interfacial surfactant may strengthen the liquid viscoelastic effect on the flow instability.

The instability characteristics of a viscoelastic liquid film coating the inner surface of a tube are studied by a normal-mode linear stability analysis and a complementary long wavelength approximation. The air–liquid interface is contaminated with an insoluble surfactant. The base flow is driven by an external force (e.g. gravity) and an additional interfacial shearing stress. Influences of liquid viscoelasticity, surface tension, surfactant and base flow on flow instability are investigated comprehensively. For the clean interface, a window of parameters can be found in the presence of base flow, in which the flow is linear stable. Physically, this can be attributed to the competition and interaction between the instability mechanisms caused by liquid viscoelasticity and surface tension. For the contaminated interface, it is found that the surfactant can play a role in strengthening the viscoelastic effect in the presence of base flow. Based on the long wavelength approximation analysis, the mechanism for each unstable zone on τs–Boτs–Bo plane is identified.