Questions of wetting

Par Ross Brown, IPREM, UMR 5254 du CNRS de l’Université de Pau et du Pays de l’Adour

Mardi 20 Juin, 14h, Salle des séminaires (215), 2ème étage, Bâtiment A4N

Abstract :

(talk in French, slides in English)

This talk presents two recent studies in which wetting and wettability play key roles:

1) How do gas  hydrates spread over a substrate ?
Clathrate gas hydrates are ice-like solids in which a gas– N2, methane, CO2 or even larger– is encapsulated in molecular-sized pockets in a frozen aqueous matrix. Stable at high pressures (~100bar) and low temperatures (<~10°C), hydrates were long the bane of the petroleum industry, since they may block pipelines. But recently, they have been viewed either as a fabulous source of natural gas, locked in deposits on the ocean floors or under the permafrost, or a frightful hazard, should their accidental or intentional dissociation destabilize ocean sediments, not to mention the contribution to the greenhouse effect. Other applications include gas purification, refrigeration, and water desalination.
The importance of the interaction of the hydrate with a substrate– the container, or a sediment– is recognized, but so far was relatively little examined, and hardly at all from the point of view of mechanisms at microscopic scales. We have applied optical microscopy at ~1µm resolution to elucidate the growth processes. Combining wide field and confocal measurements with phase contrast, fluorescent tracer nano-particles, and a rigidochrome fluorescent dye to highlight interfaces, we show the importance of wetting, specifically a water precursor film and a breath figure (‘dew’), both nm’s thin, in the spreading of gas hydrates over a substrate.

References:
M-L. Martínez de Baños et al, Cryst. Growth Des.  16 (2016) 4360-4373.
N. Hobeika et al, Langmuir 33 (2017) 5179-5187.

 

2) Fluorescent ‘marigolds’ are prettier than ‘coffee-stains’.
A drying droplet containing suspended matter may leave a variety of patterns on the substrate, the most unwelcome being the familiar  ‘coffee-stain’. This part will report, and we hope elicit discussion of preliminary measurements of a spectacular ‘marigold’ stain.