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Viscous solitons are strongly non-linear surface deformations generated byblowing wind over a liquid beyond a critical viscosity. Their shape anddynamics result from a balance between wind drag, surface tension and viscousdissipation in the liquid. We investigate here the influence of the liquidviscosity in their generation and propagation. Experiments are carried outusing silicon oils, covering a wide range of kinematic viscosities $\nu_\ell$between 20 and 5000~mm$^2$~s$^{-1}$. \modif{We show that, for $\nu_\ell>200$~mm$^2$~s$^{-1}$, viscous solitons are sub-critically generated from anunstable initial wave train at small fetch, where the wind shear stress islarger. The properties of this initial wave train are those expected fromMiles's theory of the Kelvin-Helmholtz instability of a highly viscous fluidsheared by a turbulent wind: the critical friction velocity and criticalwavelength are independent of $\nu_\ell$, and the phase velocity decreases as$\nu_\ell^{-1}$.} We demonstrate the subcritical nature of the transition toviscous solitons by triggering them using a wavemaker for a wind velocity belowthe natural threshold. Finally, we analyze the flow field induced by a viscoussoliton, and show that it is well described by a two-dimensional Stokesletsingularity in the far field. The resulting viscous drag implies a propagationvelocity with a logarithmic correction in liquid depth, in good agreement withour measurements.

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