A different type of photosensitive material I'm curious as to whether there is any small, solid or gel-like material that dissolves/liquefies/evaporates under brief exposure to bright artificial light? So, some small element or agent that would form a 'wall' between two liquids but then vaporise when exposed to light (preferably artificial)

I think I found a crystal that changed to oxygen awhile ago via google, but I cannot locate it again so any help would be well appreciated.
There are many types of photoresist that might suit your purpose. Many are commercially available from electronics suppliers; even Amazon carries some. A thin film of photoresist could be used to separate two liquids, and dissolve when exposed to light, if at least one of the liquids is transparent to the wavelength needed.

You might also consider a photo-sensitive valve using a plastic that changes shape in light.