Existence of pure capillary solitary waves in constant vorticity flows

We prove that the finite-depth pure-capillary rigidity mechanism in the irrotational water-wave problem is destroyed by a suitable constant-vorticity critical shear.

More precisely, we construct small-amplitude finite-depth pure capillary solitary waves for the two-dimensional free-boundary Euler equations with nonzero constant vorticity and zero gravity.

The waves bifurcate from a critical shear flow whose relative horizontal velocity vanishes at the bed, so that the standard Dubreil--Jacotin no-stagnation formulation is singular at the asymptotic state.

We therefore formulate the traveling-wave problem directly as a Hamiltonian spatial-dynamics system in flattened Euler variables, remove a nonlinear boundary condition from the domain of the vector field, and verify the spectral and resolvent hypotheses needed for a two-dimensional center-manifold reduction.

A parameter-dependent Darboux transformation and a cubic expansion of the reduced Hamiltonian yield, under a long-wave scaling, a stationary KdV equation.

Its reversible homoclinic orbit persists under the full reduced dynamics and gives a family of small-amplitude waves of depression.