Characterisation of the sea surface microlayer - using Langmuir films and ellipsometry

Abstract

The sea-surface microlayer (SSM) is a hugely complex system comprised of many different organic materials that has a considerable influence on the physical and chemical properties of the ocean surface. Under certain conditions, involving wind, internal waves and currents, the SSM is compressed and forms biogenic slicks. These biogenic slicks have a wave dampening effect and when using satellite data to monitor the ocean surfaces look indistinguishable from anthropogenic slicks. It is important to understand the physical properties of these biogenic slicks to identify oil pollution. For this project the physical properties of biogenic slicks from the Solent (UK) and the Black Sea (Ukraine) are being investigated. Model compounds that display similar properties to the slicks are also being characterized; these simpler systems are easier to interpret and parallels to the biogenic slicks can be made increasing understanding. The tools used to characterize these Langmuir films involve the analysis of the phase and amplitude change of polarized light upon surface reflection (ellipsometry). The film can be compressed and expanded monitoring changes in surface pressure. In conjunction, ellipsometry can be used to determine optical properties, thickness, phase changes and hysteresis effects of the film, offering both spectroscopic information and images. The changes of the film on compression are important as they mimic natural events that have the potential to be of use in satellite data interpretation. Results show, in images, the domain separation of different chemical species within the SSM and co-existence of phases. Upon compression and expansion of the films, aggregation of the different phases along with hysteresis effects are seen – phenomena that has only been hypothesized so far. Work on the model compounds demonstrates that it is possible to determine thicknesses of thin films at different surface pressures offering valuable information on phase changes.

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Identifiers

ORCID for Jeremy G. Frey: orcid.org/0000-0003-0842-4302

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