Why do plastic cups coming out of the dishwasher usually have water left on them but the glass cups are dry?
In: Chemistry
There are 2 factors at play here:
1. The amount of heat that plastic vs. glass can retain (heat capacity)
2. The way in which water spreads itself on plastic vs. glass (adhesion)
For the first point, glass can retain heat for much longer than plastic. As you know, heat will cause water to evaporate at a faster rate, so as the water stays on the glass it absorbs much more heat from the glass which allows it to evaporate at an increased rate. Similarly, water stays on plastic but the plastic cools down much faster than the glass, and it doesn’t absorb nearly the same amount of heat so more of it will remain in liquid form and not evaporate.
For the second point, water has a tendency to ”stick” to itself because water (H2O) molecules have a special type of intermolecular force called ”hydrogen bonding”, it basically means that the forces between each water molecule is strong and they stick together because of this (this is the reason that water is found in the shape of ”beads” on a leaf for example, water prefers to stick to itself in a sense). Not all molecules have the special property of hydrogen bonding. If the material that water is on also is made of molecules with strong bonds, then the water doesn’t have to adhere to itself, like how it does on a leaf in the form of water ”beads”. Instead, the water will spread itself across the surface into a film of water because it is attracted to the molecules that make up the glass. Glass is made up of these strong bonds (silicon dioxide), so water spreads itself across the surface of glass. Similarly, plastic inst made of very strong bonds, so the water molecules are not attracted to the molecules that make up plastic and therefore the water will remain as ”beads” on the plastic, minimizing surface area.
The point is that surface area is also factor for how fast a liquid can evaporate. Since water spreads itself on glass and doesn’t spread itself on plastic, the water covers more surface area on the glass and will therefore evaporate faster as a result of that.
In conclusion, its a mixture of both factors. Water absorbs more heat and has more surface area on glass, therefore evaporating at a fast rate. Water absorbs less heat from plastic and doesn’t spread itself, which minimizes the surface area it covers, therefore not evaporating as fast as if it were on glass.
Note: I know that’s a very butchered way of explaining polarity.
[2p worth] I suspect rince aid (which I believe is essentially an extremely light oil) is much less effecctive on plastics as a rince aid which also helps on the drying cycle which is essentially a steam bath. I’ve just realised. Nothing is as it seems with dishwashers.
Water likes glass a lot more than it likes plastic. So it spreads out on the glass in a thin film, unlike how it beads up on plastic. (Plastic is essentially oil that is so heavy that it’s always solid, and you know oil and water don’t mix.) Beads of water that are too small cannot fall off of the plastic, while the sheet of water on glass slips right down to the bottom where it can fall off. The tiny amount of water left as a film on the glass dries very quickly: there’s almost no water in it, and it has a huge surface area.
So-called “drying agents” like Jet-Dry are special types of soap that help make all dishes act “wet” like clean glass does. No matter what material your dishes are, a drying agent helps the water stick in a thin film to them. That way, water can sheet off the way it does on very clean glass.
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Glass is denser than plastic, so it retains heat for longer. Glass also transfers heat more effectively than plastic, so even if both reach the same temperature during the wash cycle, when it ends, the glass will more effectively move heat to its surface than the plastic will.
The glass retains a lot more heat and cools very slowly, so the hot surface helps the water stay warm and thus evaporate faster. Plastic on the other hand does not retain heat well so it cools fast and the water cools with it, and dries slowly. The same thing can be demonstrated when you heat things in the microwave. Glass containers often come out very hot and plastic ones not as much.
Does everyone else glasses dry completely? My thicker glasses do but thin glasses and the bottom of wine glasses don’t completely dry. Seems to me that the better comparison would be to ceramic and metal, where the silverware and plates are always completely dry by the end. People are talking about how glass has a higher thermal density and is more thermally conductive than plastic… maybe so but I think still a very low thermal density and conductivity, relative to the other things you put in your dishwasher. There has to be a reason that induction stove tops have glass surfaces. They are cool to the touch a few seconds after taking off the pan.
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