Pollution makes one big din

LIKE a line of bicycle racers 'drafting' behind one another against a headwind, tiny pollutant particles get into line behind larger ones in a whirlwind of sound.

Researchers have long known that really loud noises can force tiny particles to merge together, but they have not had a good idea why. Now scientists have made video pictures of tiny particles falling through a strong acoustic field that may settle the question.

The smaller particles seem to get behind larger ones and accelerate until they merge. This is important in designing systems working in smokestacks to co-join the particles into bigger ones so they can be filtered out.