Kelvin-Helmholtz instability and formation of viscous solitons on highly viscous liquids

When wind blows over a highly viscous liquid, localized liquid bumps, of height the order capillary length, are generated and propagate downwind. These strongly nonlinear coherent structures, called ``viscous solitons'', contrast with weakly waves observed at low viscosity. Our experiments performed different viscosities demonstrate that solitons arise subcritically from unstable Kelvin-Helmholtz small fetch.