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Abstract

The linear dynamics and instability mechanisms of double-layered weakly viscoelastic fluid flowing over an inclined plane are analyzed in the presence of insoluble surfactant at both the free surface and interface. The constitutive equation of the non-Newtonian flow field follows the rheological property of Walters' $B^{''}$ model. The Orr-Sommerfeld-type boundary value problem is derived using the classical normal-mode approach and numerically solved within the framework of the Chebyshev spectral collocation method. Numerical analysis detects three distinct types of instabilities: surface, interface, and interface surfactant modes. The viscoelasticity of both the top and bottom layers strengthens the surface wave instability in the longwave region. On the other hand, the behavior of interfacial wave instability depends on both viscosity and density stratification. Stronger top-layer viscoelasticity suppresses interfacial instability, while increased bottom-layer viscoelasticity amplifies it, provided the viscosity ratio $m>1$. However, in the case of $m<1$, top-layer viscoelasticity provides strong interfacial wave stabilization in the longwave region but becomes comparatively weak in the shortwave regime. The viscoelasticity of the bottom layer has a destabilizing/stabilizing effect on the interfacial wave in the longwave/shortwave regions. Meanwhile, top-layer viscoelasticity stabilizes the interfacial surfactant mode. However, this mode can be destabilized in the vicinity of the instability threshold but is effectively stabilized far away from the onset of instability by higher bottom-layer viscoelasticity.