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In this paper, we study the flow physics underlying the recently developedremote sensing capability of detecting oceanic microplastics, which is based onthe measurable surface roughness reduction induced by the presence ofmicroplastics on the ocean surface. In particular, we are interested in whetherthis roughness reduction is caused by the microplastics as floating particles,or by the surfactants which follow similar transport paths as microplastics.For this purpose, we experimentally test the effects of floating particles andsurfactants on surface roughness, quantified by the mean square slope (MSS),with waves generated by a mechanical wave maker or by wind. For microplastics,we find that their effect on wave energy and MSS critically depends on thesurface area fraction of coverage, irrespective of the particle sizes in thetest range. The damping by particles is observed only for fractions above$O(5-10\%)$, which is much higher than the realistic ocean condition. Forsurfactants, their damping effect on mechanically generated irregular wavesgenerally increases with the concentration of surfactants, but no optimalconcentration corresponding to maximum damping is observed, in contrast toprevious studies based on monochromatic waves. In wind-wave experiments, thepresence of surfactants suppresses the wave generation, due to the combinedeffects of reduced wind shear stress and increased wave damping. For the samewind speed, the wind stress is identified to depend on the concentration ofsurfactants with a power-law relation. The implications of these findings toremote sensing are discussed.

Effects of microplastics and surfactants on surface roughness of water waves