I Dust sticking to fan blade downstream of rivet head

[Swamp Thing]

Why does dust stick to the fan blades just downstream of the rivet heads, forming a comet-tail like effect?

 

[Lnewqban]

Those are areas where pressure and velocity of the air stream are reduced.
Please, see:
http://www.pilotfriend.com/training/flight_training/aero/aero_res.htm

Perhaps you have noted that the rivets of commercial airliners have flat heads.

 

[Swamp Thing]

I'm a bit confused that pressure AND veloctiy are reduced there. From Bernoulli's principle, I would have thought that if one increased, the other would decrease?

Also, flat headed rivets seem to be the exception among all these images: https://www.google.com/search?q=aircraft+rivets&client=img&source=lnms&tbm=isch

 

[Lnewqban]

It is an area of turbulence, into which air is drawn by lower pressure.
Velocity may increase, but in a chaotic way and in circular trajectories.
Same should happen over the thick trailing edge of each wood or plastic blade.

Recessed rivets are more expensive, but those make top speed increase and fuel consumption decrease.
Please, see:
https://en.wikipedia.org/wiki/Hughes_H-1_Racer#Design