 Here I've got the tracks from the radioactive source, leaving their trails as they streak through the saturated vapour. But on the small scale, let's follow one of those particles as it ionises the air on its way through. In slow motion, let's have a look and watch the alpha particle getting close to one of the air particles ripping an electron from the outer shells and you'll see them scattering away either way. So it gets close, it strips the electron and carries on moving. So as soon as the particle is ionised, the alcohol starts to be drawn towards it, clumping together and starting to form the track that we've seen in the cloud chamber. If we zoom out slightly from that, you'll see this is happening around every single particle that's been ionised and that will leave a trail of the alcohol.