Unstable capillary-gravity waves

Abstract: We make rigorous spectral stability analysis for non-resonant capillary-gravity waves as well as resonant Wilton ripples of sufficiently small amplitude. Our analysis is based on a periodic Evans function approach, developed recently by the authors for Stokes waves. On top of our previous work, we add to the approach new framework ingredients, including a two-stage Weierstrass preparation manipulation for the Periodic Evans function associated to the wave and the definition of a stability function as an analytic function of the wave amplitude parameter. These new ingredients are keys for proving stability near non-resonant frequencies and defining index functions ruling both stability and instability near non-zero resonant frequencies. We also prove that unstable bubble spectra near non-zero resonant frequencies form, at the leading order, either an ellipse or a circle and provide a justification for Creedon, Deconinck, and Trichtchenko's formal asymptotic expansion for the Floquet exponent. For non-resonant capillary-gravity waves for the stability near the origin of the complex plane, our stability results agree with the prediction from formal multi-scale expansion. New are our stability results near non-zero resonant frequencies. As the effects of surface tension vanish, our result recovers that for gravity waves. Also new are our stability results for Wilton ripples of small amplitude near the origin as well as near non-zero resonant frequencies.