Evaporation of water from non polar surfaces

[William Jackson]

I'm puzzled by a phenomenon that my daughter pointed out to me. If you have no plastic ware in the dishwasher, your glass and ceramic dishes will dry faster. Slow evaporation from plastic is easy to understand; the water beads up and presents a smaller surface area.
What I'm not clear on is why plastic ware increases the drying time, even for the glassware, when there doesn't seem to be any difference in the overall amount of water remaining after the wash.

 

[DaveC426913]

I think I'd need to hear some details about your experiments and some quantitative results before I'd be willing to grant that it is even happening, let alone speculate about what might be causing it.

Not that I doubt you, it's just that I'm a guy who likes details. eg. How much longer?

 

[William Jackson]

I've never done a quantitative measurement. It would be necessary to use the identical load (except for exchanging the plastic ware with a similar surface of non-plastic), same water temperature and same ambient R.H. Humidity obviously varies with kitchen activity, outside conditions, and whether the forced air heat is on. But it's easy to measure with a couple of thermometers on a string.
It would be interesting to do that. Thanks for the suggestion.

 

[DaveC426913]

That's exactly what I was thinking - it's not an easy thing to analyze.

So, my question is: how can you be sure it's true? How can your daughter be sure it's true?

 

[Nugatory]

I'm puzzled by a phenomenon that my daughter pointed out to me. If you have no plastic ware in the dishwasher, your glass and ceramic dishes will dry faster. Slow evaporation from plastic is easy to understand; the water beads up and presents a smaller surface area.
What I'm not clear on is why plastic ware increases the drying time, even for the glassware, when there doesn't seem to be any difference in the overall amount of water remaining after the wash.

Here's a hypothesis:
- The amount of water that has to be removed by evaporation during the drying process is proportional to the total surface area (dishes plus interior surfaces of the dishwasher) because water clings to these surfaces.
- The rate at which the water is removed depends on the temperature inside the dishwasher
- The ceramic and glassware retain the heat of the wash water more effectively than plastic, both because they have more mass and a higher specific heat, so maintain the interior at a higher heat for longer.
Therefore, adding plastic items to the load will increase the amount of water that has to be evaporated without commensurately increasing the heat that speeds the evaporation along.

This is a hypothesis, may or may not be right. Indeed, as DaveC suggests, the first step should be to test the null hypothesis, namely "adding plasticware to an otherwise fixed dishwasher load has no effect on the drying time"; if you cannot falsify that hypothesis, subsequent experiments will be futile.

This is the sort of thing that you can have a lot of fun with.