It's also solubility-dependent. The eye is an aqueous environment and Carfentanyl has shitty water solubility.
Mucous membranes and an ideal carrier (you can super saturate a Propylene Glycol solution, for example) such as the nasal membranes and lung is a far different story.
Scary enough, a glycerine/propylene glycol supersaturated solution is what is used both in vape fluid and FOG MACHINES.
In an enclosed room, a spiked fog machine could kill dozens or more with no way to bring enough Narcan to counteract it.