## 3 Wave Tank and Field Experiments  
### Wave Tank Experiments  
The wave tank experiments were carried out in the Oval Wind Wave Tank of the Institute of Applied Physics, RAS. A sketch of the experiments is shown in Figure 2. The tank was half-filled with freshwater (density of 0.997 g/cm\({}^{3}\)); then, saltwater was slowly poured through special holes in the tank bottom. The thickness of each layer was about 14 cm. IWs were generated by a vertical paddle oscillating around a horizontal axis located at the interface between the salt- and freshwater layers. IWs were absorbed at the end of the working section of the tank using a special absorbing system consisting of three parts. First, two long (1.5 m length) false vertical walls were installed at a small angle to the tank walls in order to smoothly reduce the tank width up to about 2/3 of its initial value; then, there was a system of submerged beaches and, finally, a system of plastic sheets (about 10 sheets) suspended vertically across the tank at distances of about 10 cm between the sheets. The reflection coefficient for the IWs was less than 10%. IW amplitudes were recorded using resistance wire gauges mounted vertically beneath the water surface and centered at the inter, and the amplitudes were varied in the range of 3-5 mm. Surface waves were recorded in the middle tank plane with resistance gauges similar to IW gauges, but adapted for freshwater. Wind waves were generated with a ventilator (1). Wind speeds were measured in the center of the wind channel cross-section using a small propeller sensor; the experiments were carried out at mean wind speeds of 2 and 2.5 m/s. The wind drift velocity was measured using small floats (less than 2 mm thick) from a sponge with neutral buoyancy drifting in the water surface layer. The measured average wind drift velocities at wind speeds of 2 m/s and 2.5 m/s were 5-6 cm/s and 6-7 cm/s, respectively. Two cases were studied in experiments at different density jumps between the upper and lower water layers. The first case corresponded to \"fast\" IWs, propagating with phase velocities essentially exceeding the wind drift velocities (\"non-resonance case\"). The second set of experiments was realized with \"slow\" IWs, for which phase velocities were close to the wind drift velocities and hence a resonance modulation of surfactants occurred (see Section 4). Oleic acid (OLE) was used to produce a monomolecular film on the water surface. Elastic properties of the surfactant were close to those for biogenic films on the sea surface and for thin films of crude oil [34]. OLE was dissolved in ethanol (the solution volume concentration was 10 mg/mL) to improve spreading and was applied on the water through a dropper; its nozzle was positioned at a height of 2-3 mm over the water surface. The dropping period was varied in the range from 2 s to 10 s. The dropper was located at a distance of 10 m from the working area (see Figure 2) and low variations of Ka-band radar return in the absence of IW indicated that OLE formed a uniform film in the study area. Surfactants were continuously skimmed by paper napkins at the end of the tank where the absorbing system was located. Film samples were collected from the water surface using nylon nets. The damping coefficient and the wavelength of capillary standing waves (Faraday ripples) were measured for the sampled films with a method of parametric waves (see [35]) and the surface tension coefficient (STC) and the film elasticity were retrieved. Then, using previously obtained dependencies of these parameters on OLE concentration, the latter was retrieved. These dependences for the film elasticity and film pressure (the difference between STC values for clean and film covered surfaces) are shown in Figure 3 (see also [32]). The water surface in the tank before the experiments was cleaned with paper napkins until the STC for film samples differed from the tabulated values for clean water by at least 3% or less.