Linear stage. Normalised finger-bubble height h/h₀ as a function of the reduced time t̄ = ω_tht for three injected wavelengths. The curves show the numerical results, and the solid green line corresponds to the theoretical exponential growth. The shaded green region indicates the time interval identified as the linear growth regime. Top panel: nocoupling (NC) case. Bottom panel: intermediate-coupling (IC) case.