When the particles travel in the detector, they leave ions along their path. The supersaturated vapor condense on theses ions, forming a visible track. The track is made of thousands of alcohol droplets coming from the transformation of vapor into droplets on the ions. Theses droplets then fall in the bottom by gravity and the track disappear.
In addition of the creation of supersaturated vapor, there is normal condensation of « normal » vapor into droplets which make the foggy background in the chamber. When the droplets coming from the supersaturated vapor fall to the ground, they also drain to the ground the normal droplets (same process like rain). So in the area where the track just vanished, there is a lack of droplets of any kind thus no refraction of light and a « black track » seems to appear.
You have to wait half a second until normal droplets and supersaturared vapor form in the area, making the foggy background again where the track was.
colomnes di’ions