In a low pressure (near vacuum), -20°C vessel, would sublimated moisture rise to the top or fall to the bottom?

Question

If I have a strong vessel that I've cooled to say -20°C, and lowered the pressure below 0.006 atmospheres, the water in food placed inside will sublimate and freeze dry the food.

But what happens to the water vapor? Does the moisture rise to the top or fall to the bottom of the vessel itself? Or does it just expand to fill the container equally?

Answer 1

I do believe that freeze-dryers have to have a condenser that actively removes the water vapor. It would depend on the size of the vessel, but the water vapor from the food is likely to be enough to raise the pressure to near atmospheric, and certainly enough to destroy the low pressure required. So a condenser is actively removing the moisture by exposing a surface at a much lower temperature than the atmosphere. But come to think of it, I'm not quite sure how this works considering that the condensed water would be ice.

Directly to your question, I am fairly positive it fills the entire vessel. It is, after all, a gas. There is little in the way of other competing gases and there are negligible effects from things like gravity. I would imagine that the gas would be a near homogeneous mixture of the water vapor and other sparse gases.