Effects of microplastics and surfactants on surface roughness of water waves

TL;DR

This study investigates how microplastics and surfactants affect ocean surface roughness, revealing that surfactants significantly dampen waves at realistic concentrations, impacting remote sensing detection of microplastics.

Contribution

It provides experimental insights into the distinct effects of microplastics and surfactants on wave damping and surface roughness, clarifying their roles in remote sensing applications.

Findings

Microplastics damp waves only at unrealistically high surface coverage.

Surfactants increase wave damping with concentration, without an optimal point.

Surfactants reduce wind stress and suppress wave generation.

Abstract

In this paper, we study the flow physics underlying the recently developed remote sensing capability of detecting oceanic microplastics, which is based on the measurable surface roughness reduction induced by the presence of microplastics on the ocean surface. In particular, we are interested in whether this 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 and surfactants 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 the surface area fraction of coverage, irrespective of the particle sizes in the test range. The damping by particles is observed only for fractions above $O(5-10\%)$, which is much higher than the realistic ocean condition. For surfactants, their damping effect on mechanically generated irregular waves generally increases with the concentration of surfactants, but no optimal concentration corresponding to maximum damping is observed, in contrast to previous studies based on monochromatic waves. In wind-wave experiments, the presence of surfactants suppresses the wave generation, due to the combined effects of reduced wind shear stress and increased wave damping. For the same wind speed, the wind stress is identified to depend on the concentration of surfactants with a power-law relation. The implications of these findings to remote sensing are discussed.