## 3 Wave Tank and Field Experiments
### Wave Tank Experiments
To study variations of surfactant concentration over an IW profile, net samples were collected simultaneously over different phases of IW, namely, over IW crests, troughs and slopes. Figure 3: Film pressure (**left**) and elasticity (**right**) vs. surface concentration of OLE. Measurement by the method of parametric waves at a frequency of 25 Hz. Solid curves—least squares interpolations. Figure 2: The Oval Wind Wave Tank of IAP RAS (1—a ventilator for wind flow generation; 2—an IW maker; 3—a dropper; 4—wire gauges of IWs and surface waves; 5—Ka-band radar; 6—a wave absorbing system). Variations of surfactant concentration over IW can be retrieved when estimating STC and elasticity values from investigations of film samples using the parametric method. It is also important to know a mean surfactant concentration which can be determined when analyzing net samples taken without IW. To make this determination more reliable, we used additional considerations to estimate \(\Gamma\) values. We assumed that a steady film was formed in the working area as a result of balance between the OLE inflow due to dropping and the outflow due to wind drift. Therefore, we estimated the mean surfactant concentration as \(\Gamma\sim C_{s}V_{d}/(T_{d}V_{d}L_{T})\), where \(V_{d}\) is the volume of droplets, \(C_{s}\) is the volume concentration of surfactants in the alcohol solution, \(V_{dr}\) is the drift velocity, \(T_{d}\) is the period of droplets, and \(L_{T}\) is the tank width. Thus, estimated concentration values were found to be consistent with measurements. The Ka-band radar used in our experiments was a scatterometer operating in a continuous wave regime. The microwave radar frequency was about 35 GHz and was modulated within the band of 100 MHz. A radar is a coherent system measuring the amplitude and phase of scattered microwaves that allows one to investigate the Doppler spectrum of radar return. The radar antenna beam width was about 4\({}^{\circ}\). Although the scatterometer is a single polarized device, it can be utilized for measurements at vertical or horizontal co-polarizations of transmitted/received microwaves, VV or HH polarizations, respectively. VV and HH polarization measurements can be arranged consecutively, by simply turning the instrument around the antenna axis by 90 degrees. Notably, the radar antenna patterns in two orthogonal planes are slightly different, and this leads to small (about 10%) differences between the radar backscatter at VV and HH polarizations; however, this was insignificant in our experiments. The radar was mounted at a height of about 1 m over the tank looking at an incidence angle \(\theta=50^{\circ}\) in the upwind direction. The cross-tank and along-tank sizes of the radar footprint on the water surface were about 17 cm and 30 cm, respectively. This allowed us to analyze ripple variability along the IW profile. In order to reduce parasitic reflections from metal constructions in the laboratory, the top of the tank below the radar was covered with absorbing sheets, and a gap of about 15 cm in width was left between the sheets to limit the radar beam width. An example of the Doppler radar spectrum is shown in Figure 4. The Doppler spectrum could be divided into two parts. One part was characterized by a peak with a frequency offset at about 50-60 Hz, which corresponded to scatterers moving with velocities close to the phase velocities of free gravity-capillary waves with the Bragg wavelength. The low-frequency part was due to side lobes of the radar antenna and due to specular reflections from the walls and metal constructions in laboratory. Notably, the intensity of the low-frequency peak did not depend on the presence of films on the water surface in practice, whereas the Bragg peak could be strongly suppressed in the presence of films. In order to reduce the influence of the low-frequency part of the radar return, we eliminated this part, approximating it in the range of Bragg peak by a smooth curve, as shown in Figure 4 (dashed curves).