Side-wall wetting and linear stability of falling films
We investigate the influence of side-wall wetting on the linear stability of falling liquid films confined in the spanwise direction. A biglobal stability framework is developed, capturing inertia, viscosity, gravity, capillarity, and geometric confinement. The base flow exhibits a curved meniscus and a streamwise velocity overshoot near the side walls. Linear stability analysis based on the Navier--Stokes equations is performed in two limiting regimes. In \textit{confined} channels, where spanwise confinement stabilizes moderate-wavenumber perturbations via side-wall boundary layers, wetting weakens this stabilization; as the contact angle decreases, the neutral curves shift towards the unconfined one-dimensional limit, thus wetting acts as a relative destabilizing mechanism. In contrast, in \textit{weakly-confined} channels where side-wall boundary layers do not provide confinement-induced stabilization, wetting produces a net long-wave stabilization ($k \rightarrow 0$), significantly increasing the critical Reynolds number. This effect strengthens as the contact angle decreases, indicating a competition between destabilizing inertia and stabilizing wetting-induced capillary forces. The predicted long-wave stabilization effect is compared quantitatively with available experimental measurements, showing consistent trends and comparable magnitudes within the accessible parameter range. Perturbation eigenmode structures show that, in confined channels, the relative destabilization is associated with near-wall vortical structures induced by the meniscus elevation and velocity overshoot, which reduce effective viscous damping. In contrast, in weakly-confined channels, stabilization is consistent with interface tensioning through strong anchoring of the perturbations at the side walls.