by Dinesh » Sun Jul 24, 2011 3:00 pm
Jeff's right. It depends on a whole bunch of factors like pressure, area of plate, size of the enclosure, rate of evaporation of the water. You might, for example, have a vertical temperature gradient where the ceiling or roof of the room is at a higher temp than the floor(due to a heated room upstairs, for example). Then there'll be a net movement of air to the ceiling, reducing the the svp above the plate. It's also the absolute amount of water, the actual mass of the water above the emulsion, rather than the amount of water any given volume of air could "hold". As far as water penetration is concerned, the water molecules don't actually know how much more water that particular volume of air could have held, they carry out their Maxwellian dance regardless.
The emulsion doesn't actually "suck in" the water vapour above it. The emulsion simply is in the way of the water molecules as they do this Maxwellian dance. The amount of water that goes into the emulsion simply depends on how many water molecules get trapped in the gelatin matrix (unless there's a Maxwell's Demon in the emulsion!). Consider a mass of water above the emulsion. The air doesn't actually "hold" the water, you can have water pressure present in a vaccuum. The molecules of water above the emulsion are whizzing about in every direction with a certain range of speeds, dependent on a whole bunch of factors given above. Some of the water molecules happen to hit the emulsion surface. Some of the ones that hit the surface happen to be going fast enough to penetrate into the emulsion. These then get trapped in the emulsion by intermolecular forces and add to the swell rate. Meanwhile, there's a moist surface on the emulsion with it's own water molecules, also whizzing about but mainly above and away from the surface. Some of these have enough speed to escape completely from the emulsion, removing water from the emulsion. This dries the emulsion. Meanwhile, the gelatin itself is cross linking and hardening the surface, causing fewer and fewer molecules to leave the surface and drying the plate. All of these processes are going on simultaneously. How long? Well, the equilibrium point for any set of conditions probably is in the range of a few milliseconds, but the equilibrium point is also constantly changing and re-adapting as the room conditions change and as the gelatin cross links.
In real terms, there are too many factors to give a simple plot, but, unless you're in a sauna, the swell rate is probably not more than a few percent before the emulsion has dried and is not absorbing water anymore.
Jeff's right. It depends on a whole bunch of factors like pressure, area of plate, size of the enclosure, rate of evaporation of the water. You might, for example, have a vertical temperature gradient where the ceiling or roof of the room is at a higher temp than the floor(due to a heated room upstairs, for example). Then there'll be a net movement of air to the ceiling, reducing the the svp above the plate. It's also the absolute amount of water, the actual mass of the water above the emulsion, rather than the amount of water any given volume of air could "hold". As far as water penetration is concerned, the water molecules don't actually know how much more water that particular volume of air could have held, they carry out their Maxwellian dance regardless.
The emulsion doesn't actually "suck in" the water vapour above it. The emulsion simply is in the way of the water molecules as they do this Maxwellian dance. The amount of water that goes into the emulsion simply depends on how many water molecules get trapped in the gelatin matrix (unless there's a Maxwell's Demon in the emulsion!). Consider a mass of water above the emulsion. The air doesn't actually "hold" the water, you can have water pressure present in a vaccuum. The molecules of water above the emulsion are whizzing about in every direction with a certain range of speeds, dependent on a whole bunch of factors given above. Some of the water molecules happen to hit the emulsion surface. Some of the ones that hit the surface happen to be going fast enough to penetrate into the emulsion. These then get trapped in the emulsion by intermolecular forces and add to the swell rate. Meanwhile, there's a moist surface on the emulsion with it's own water molecules, also whizzing about but mainly above and away from the surface. Some of these have enough speed to escape completely from the emulsion, removing water from the emulsion. This dries the emulsion. Meanwhile, the gelatin itself is cross linking and hardening the surface, causing fewer and fewer molecules to leave the surface and drying the plate. All of these processes are going on simultaneously. How long? Well, the equilibrium point for any set of conditions probably is in the range of a few milliseconds, but the equilibrium point is also constantly changing and re-adapting as the room conditions change and as the gelatin cross links.
In real terms, there are too many factors to give a simple plot, but, unless you're in a sauna, the swell rate is probably not more than a few percent before the emulsion has dried and is not absorbing water anymore.