Consider what happens with boiling water where bubbles of steam form within the liquid and rise to the surface.These bubbles don't form everywhere in the water each one needing a "seed" so the water is able to evaporate.The seed could be something like a jagged bit of the container or a particle of dust.A similar thing happens in the bubble chamber but it is the ions that form the seeds.
An analogous thing happens if you drop something like sand into a glass of beer.You will see a line of bubbles because the sand grains act as seeds for the CO2 to come out of solution.In fact it is said that Glaser,the inventor of the bubble chamber,got the idea for his invention after puzzling over and then working out where the bubbles came from in his beer.He won the nobel prize for his work.Cheers.
