Static Dark Fluid Thin Shells in Schwarzschild-de Sitter Spacetimes: Stability and Black Hole Shadows

We study the existence and radial stability of static, spherically symmetric thin shells separating two Schwarzschild--de Sitter spacetimes with parameters $(m_\pm,\Lambda_\pm)$. Using the Israel junction formalism and a linear barotropic equation of state $p = \lambda(\sigma - \sigma_1)c^2$, we decouple the sound speed $c_s^2 = \lambda c^2$ from the equilibrium equation-of-state parameter $w_0 \equiv p_0 / (\sigma_0 c^2)$ and derive the effective potential governing radial dynamics. For observationally motivated parameters, stable configurations with $\sigma_0>0$ and $0<\lambda\leq1$ exist only when $m_+/m_- > 1$. Three distinct stability windows emerge, when $\lambda=1$: $-3/7 \lesssim w_0 \lesssim 1/2$ for $\Lambda_+ = \Lambda_-$, $-2/3 \lesssim w_0 \lesssim 1/2$ for $\Lambda_+ > \Lambda_-$ and $0 \lesssim w_0 \lesssim 1$ for $\Lambda_+ < \Lambda_-$. Positive-pressure shells ($w_0>0$) reside near the photon sphere, whereas negative-pressure shells ($w_0<0$) extend outward, reaching either the cosmological horizon or the static radius. Stability relies on the variation of $w(\sigma)$ with the surface energy density. Negative pressure (tension) stabilizes the system because the tension increases during expansion. Conversely, positive pressure stabilizes the system because the pressure increases during contraction. Finally, a static, stable, dark, fluid thin shell acts as a gravitational refractive layer that enlarges the black hole's shadow for a distant, static observer outside the shell. The effect depends on the shell radius $R_0$, the background parameters $(m_{\pm}, \Lambda_{\pm})$, and the equation-of-state. Dark fluid shells can be considered as theoretical toy models that illustrate qualitative effects. Future high-resolution black hole shadow observations could, in principle, use such models to explore how different equations of state might influence observable signatures.