Projecting double slit onto fog instead of screen In the quantum mechanical double slit experiment, the resulting pattern is projected onto a screen. This is an important part of the experiment, because the screen represents the quantum mechanical observer. 
What happens when we replace the screen with a fog gradually increasing in density from the slits to the former screen position, such that we can recognize the pattern when looking at the fog at the former screen position?
The fog consists of distinct macroscopic particles suspended in the air. We can not recognize a pattern when looking onto a fog volume that consists of only one particle. We can recognize a pattern looking onto a volume of dense fog at the screen position. What is in between? And is it relevant to the double slit experiment - does it affect the quantum mechanical observation?
 A: The quantum-mechanical interference in the double-slit experiment happens at the slits, not at the screen where the pattern is visible.
If you sent light through a double slit into a fog, the volume of the fog would be illuminated in the same way as in crepuscular rays.
A: It is a good idea trying the use of fog as a trace for light, influenced by edges.
And it's worth an effort to give it a try. Why? Because it could be similar to the deflection of electrons in a lattice, like proposed by Laue or other methods of X-ray crystallography. So your question

does it affect the quantum mechanical observation?

should be proofed by experiment. The analogy to the powder crystallography maybe is an indication that it works.

What is in between?

The chaotic deflection of the photons in the fog will work in the same way as powder from lattices. So the photons will be deflected not only in the direction to the screen but also up and down. This gives the chance to detect some photons with a measurement instrument, located above the edges. Of course the experiment makes sense only if still a intensity distribution on a screen appears. 
Carrying out the experiment with a dimmed beam or even better single photons over a long period, the detector with small detection area above the edges will show, are the photons direct behind the slits still photons or are they wave distributions.
