We investigate experimentally the influence of the liquid viscosity on the problem of the generation of waves by a turbulent wind at the surface of a liquid, extending the results of Paquier et al. [A. Paquier et al., Phys. Fluids 27, 122103 (2015)] over nearly three decades of viscosity. The surface deformations are measured with micrometer accuracy using the free-surface synthetic schlieren method. We recover the two regimes of surface deformations previously identified: the wrinkle regime at small wind velocity, resulting from the viscous imprint on the liquid surface of the turbulent fluctuations in the boundary layer, and the regular wave regime at large wind velocity. Below the wave threshold, we find that the characteristic amplitude of the wrinkles scales as ${\nu }^{-1/2}{u}^{*3/2}$ over nearly the whole range of viscosities, whereas their size is essentially unchanged. We propose a simple model for this scaling, which compares well with the data. We show that the critical friction velocity $u^{*}$ for the onset of regular waves slowly increases with viscosity as ${\nu }^{0.2}$. Whereas the transition between wrinkles and waves is smooth at low viscosity, including for water, it becomes rather abrupt at high viscosity. A third wave regime is found at $\nu >(100–200)\times {10}^{-6}{\mathrm{m}}^2{\mathrm{s}}^{-1}$, characterized by a slow, nearly periodic emission of large-amplitude isolated fluid bumps.

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• Capillary waves
• Interfacial flows
• Surface gravity waves