Abstract
Self-cleaning windows are well known for their ability to function with airborne pollutants, but there is a growing industry for semi-permanent subaquatic optical devices, where the performance of such windows should be considered. Here sol-gel technology is explored as a means of producing self-cleaning, subaquatic, sapphire windows. We demonstrate removal of marine bacteria and, in the worst-case contamination scenario, dead North Sea crude oil (API 35). This greasy contaminant was smeared across the windows to effectively reduce optical transmission strength to just 54%. The titania-based sol-gel-coated windows can restore transmission to within 10% of the clean value in less than one day, unlike standard sapphire windows, which lose 68% transmission following contamination and aquatic submergence over the same duration. A range of theories to enhance the self-cleaning performance of the sol-gel coating were explored, but none of the tested variables were able to provide any enhancement for subaquatic performance.
Highlights
TiO2 in the anatase polymorph is a semi-conductor with a band gap energy of 3.2 eV, corresponding to a wavelength of 390 nm [1]
This self-cleaning process is well established, with window manufacturers presently implementing the technology on domestic products [4], but this is the first example of the technology being applied to sapphire windows for subaquatic applications
With the inclusion of dopants, there is a tradeoff between the energy required to activate coating under subaquatic conditions
Summary
TiO2 in the anatase polymorph is a semi-conductor with a band gap energy of 3.2 eV, corresponding to a wavelength of 390 nm [1]. TiO2 surface [2,3] This self-cleaning process is well established, with window manufacturers presently implementing the technology on domestic products [4], but this is the first example of the technology being applied to sapphire windows for subaquatic applications. The motivation for testing the windows under water is due to the increasing range of subaquatic system monitoring devices which rely on optics and require clean windows to function effectively [5,6,7,8]. Anyone who has ever cleaned a greasy frying pan in the kitchen sink will be aware of the difficulty in trying to wipe oil clear of a surface: oil is immiscible in water, making mechanical wiping and ultrasonic agitation not effective for subaquatic application [9].
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