It’s actually pretty similar to regular drying. Molecules in a liquid are more tightly bound than in a gas, but at the surface, there are always some that get bumped enough by their neighbours to jump clear – like a snooker ball getting bounced off the table by a powerful shot.
Drying is just molecules getting progressively jostled out of the liquid until they’re all gone. The same thing happens with the molecules in a solid too but, since they’re bound even more tightly, it happens more slowly. A vacuum speeds the process up dramatically, because there are no air molecules to ricochet the ‘jumpers’ back down, and so any that do break free can make good their escape.
Freeze drying is very useful for food preservation. The food is first frozen to prevent the food from spoiling, and it is then dried slowly in a vacuum for several days, with carefully controlled heat, to gently remove the water without affecting the food structure.
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Asked by: Jonathan Neville, Luton
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