How to avoid dyed liquids staining reflective/transparent surfaces

Question

I'm dealing with some hydrocarbons (diesel, gas oil, kerosene, ethanol) where I'm putting them in a chamber and shining light through them to hit a sensor on the other side in order to measure their colour. Unfortunately what I'm finding is that the lenses and reflective surfaces in the chamber that I'm using for these optical measurements are being stained by the products and subsequently affecting the measurements. When the chamber is emptied, small amounts of product remain clinging to the internal surfaces. As this dries a residue is left which remains on the surface.

I've tried a couple of different materials: stainless steel reflectors, transparent Grilamid plastic lenses, a stain-resistant transparent potting and regular glass. All of them end up with a residue that clings to the surface that needs to be wiped clean. Unfortunately, for my application I'm not going to be able to get access to the internal surface to clean it so I need to find a way to prevent this residue from clinging to the internal surfaces.

The main restriction for me is that the system needs to basically be 100% maintenance free, and unfortunately I can't control the fluid coming in and out and therefore can't rinse the system out.

I was wondering if there are any products (both transparent and reflective) that are basically completely "hydrophobic" but for hydrocarbons and will prevent the product clinging at all to the surface and staining the lenses and reflectors?

Answer 1

You will want to rinse the chamber between uses with something that dries clear and is a non-polar solvent. A typical solvent like pentane or diethyl-ether should do the trick though keep in mind the fire hazard.

Answer 2

A super-hydrophobic coating would work well for you because super-hydrophobic materials are self-cleaning. Sandia National Labs has a nice report on super-hydrophobic coatings which can be found here (associated patent here). In it they report the invention of a coating who's hydrophobicity can be varied by application of UV light. The contact angle of their coating is greater than 150$^\circ$, and they reference two other patents which reach contact angles of 169$^\circ$ and 172$^\circ$ which are both very high, the maximum is 180$^\circ$.

The problem is, all of this research is fairly recent and doesn't seem to have found its way to industry yet. I can't find many coating houses who sell super-hydrophobic optical coatings. Aculon's website says that they sell one, but they don't publish the contact angle, and the picture they show makes it appear to only be ~90$^\circ$.

Aculon also claims to sell an oleophobic (oil repelling) coating. This may work for you, but you would need to test a sample with the materials you plan to use. The video on their website shows someone trying to write on it with a Sharpie and it just won't take to the surface.

The problem with all of these coatings is longevity. Rain-X is an excellent example of a hydrophobic coating, but it must be reapplied once a month or so. My eyeglasses came with a hydrophobic coating which was very impressive when they were new. Now that they are 10 months old, everything sticks to them again. So, whatever you decide to build, you should be able to service it regularly.