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Abstract

Near-surface flow patterns and their influence on local transport processes are investigated in a horizontal plane with visualization techniques. For this, a new method was developed that produces tracer particles with high effective Schmidt number (Sc = O(10^6) ) by chemical reaction directly at the water surface. While different chemical systems were tried, best results were achieved using the precipitate AgCl, formed by AgNO 3 in the water body and a controlled influx of HCl gas. Trace amounts of precipitate are made visible by scattering laser light. Using the new method, the influence of wind induced turbulences at the water interface can be investigated with previously unachieved sensitivity. Illumination of the wavy water surface leads to shadowing and lensing effects that become apparent in the image data and cannot fully be compensated for with image processing. The results are qualitatively compared to data from active thermography (heat, Pr ≈ 7) and boundary layer imaging (gas, Sc ≈ 600), showing a close resemblance of the flow pattern. With higher Schmidt number of the tracer, the structure becomes more pronounced and higher sensitivity to surface convergence is observed. Experiments with glass spheres (d = 70 μm-2 mm, Sc → ∞) were conducted for further comparison. Tracking of individual particles allows for a precise measurements of the surface velocity and investigations of the local momentum transfer.