Water color additive that will maintain color when water is a thin film

I'm producing thin sheets of liquid (50 to 100 microns) from a nozzle and trying to photograph them. I think it would help if I could color the water. However, typically as water becomes more of a thin film it tends to lose color. I'm wondering if anyone has any thoughts on this. I think white would be a particularly good color. Food coloring containing titanium dioxide might work. I know that just using milk will work, but it's a mess to deal with because I am dealing with sprays traveling at a minimum 5 meters per second.




Use [fluorescein](https://en.wikipedia.org/wiki/Fluorescein) or other [fluorescent dye](https://en.wikipedia.org/wiki/Category:Fluorescent_dyes), along with an [effective UV filter](https://www.bhphotovideo.com/explora/photography/buying-guide/a-guide-to-filters-for-lenses) (AKA haze or skylight filter) on the lens and an ultraviolet source, such as a xenon strobe (speed light) or UV flashlight. [A UV-transmission filter on the strobe](https://www.diyphotography.net/make-diy-uv-filter-2-cents-actually-works/) can increase contrast, and can be made cheaply, but might not be needed. Because fluorescence makes the fluid become a *source* of light, it is quite obvious against a dark background. You can even use most laundry detergents, which contain blue-fluorescing dyes -- though foaming might be a nuisance. 


See this [image of fluorescent soap bubbles](https://www.ephotozine.com/articles/6-freakishly-awesome-uv-photography-tips-29569/images/Markus_Berger_UV_COOPH_005.jpg) or [this one](https://www.growingajeweledrose.com/2013/08/paint-and-play-glowing-bubble-painting.html). [Soap bubble film may be just a few hundred nm thick](https://soapbubble.fandom.com/wiki/Color_and_Film_Thickness), yet show clearly with fluorescence. Also [watch a demo of strobe illumination](https://www.youtube.com/watch?v=OtxlQTmx1LE) of water droplets using sodium fluorescein for contrast. In the video, notice, in particular, that the stream tends to break into *pairs* of droplets, one perhaps a millimeter in diameter, and the sibling, just a few microns across, but all clearly visible! 


Choose a near-UV light source and fluorescent dye for safety, since shorter wavelengths are more damaging to eyes and skin. A search for [UV flashlight](https://www.amazon.com/s?k=uv%20b%20flashlight) returned items from US$10 and up, some of which might be suitable. Most were in the 395 to 365 nm spectrum, relatively innocuous.


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Methylene Blue produces a very intense colour at quite low concentrations. It's also very cheap from places like eBay.It is also relatively non toxic


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