The formation of bio-films is a major problem for undersea optical instruments. To prevent fouling, researchers at the Electro-Chemical Systems and Interfaces Laboratory (LISE) at CNRS (France’s leading research establishment) have contributed to the development of a very promising electro-chemical protection process.
The principle is based on the electrolysis of sea water, which produces free chlorine on the surface of the optical window to be protected. With this environmentally-friendly process, the windows of undersea cameras remain clean for up to five months.
The electro-chemical process developed within the framework of a CNRS-IFREMER partnership consists of coating the window to be protected with a film of tin dioxide (SnO2), which acts as a transparent electrode. This is polarised by an amount that enables the localised production of bleach (i.e. hypochlorous acid, HClO) out of the oxidation of chloride ions present in sea water. The CNRS team has optimised the make-up of the bio-film (deposited by spray pyrolysis) to achieve good electrical conductivity and a product lifespan of several months. A counter-electrode and a reference electrode complete the system.
IFREMER adapted the principle to undersea camera windows having the three electrodes at the front. Tests undertaken so far at various sites have shown that anodic protection enables transparency to be maintained for three to five months.
The process is particularly attractive since it makes only a small impact on the environment: the anti-fouling activity only produces very small quantities of bleach around the windows being protected. The process is also being applied in the ANTARES project (aimed at detecting and studying very high-energy cosmic neutrinos beneath the Mediterranean Sea) in co-operation with the Villefranche Oceanology Laboratory (part of CNRS/Université Paris 6). The researchers are testing this method on the windows of high-resolution cameras used for imaging phytoplankton at a depth of 2,600 metres in the Mediterranean.
Management of the power requirements of the active protection system is very important in implementing the process. Tests involving the periodic interruption of the polarisation, with durations ranging from 10 minutes to a few hours, have been successfully carried out. If a film of tin dioxide that has never been polarised is immersed in sea water, it tends to become coated with bio-film. However, if the film has previously been subjected to anodic polarisation of sufficient duration with the production of chlorine, it retains an anti-fouling effect after the polarisation is discontinued. This passive protection has a limited lifespan, which can nevertheless last for up to several days.
In parallel with this work on glass, researchers are studying the possibility of depositing a tin dioxide conductive film onto transparent polymer substrates, such as PMMA (polymethyl methacrylate), using a plasma-assisted chemical vapour deposition (PACVD) process (2). In the medium-term, these studies will open up possibilities for application in self-cleaning aquarium walls.