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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
When I open my dishwasher, everything is dry except the plastic items, which remain covered with water droplets. Why is this?
David Jackson
Gosport, Hampshire, UK
When my dishwasher has finished washing and starts the drying cycle, there are two ways the water removes itself from the items: first, it simply runs off, and second, it evaporates.
There is a difference in the rate at which water runs off various items due to their surface properties, with water tending to adhere less strongly to glass (including the glazed surfaces of ceramics) and metals than it does to plastics.
As for evaporation, the final rinse of my dishwasher leaves the items very hot and then pauses for a while in the drying cycle. During this pause, the plastics experience less evaporation than the non-plastics, which hold a lot more heat energy due to their higher thermal mass.
Rinse aid is an additive that helps a lot with drying; it lowers the water surface tension so it runs off more easily, and it is also very effective if you open the machine after the final rinse and shake the water off the plastics before resuming drying. I also tip up my casserole pan and empty the water out of the hollow handles, being careful not to burn myself on the hot metal.
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Due to differing surface properties, water tends to adhere less strongly to glass and metals than it does to plastics
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David Muir
Edinburgh, UK
My dishwasher finishes its cycle with a hot rinse, which heats the dishes to help the water evaporate when the dishwasher is opened. As this rinse lasts about 5 minutes, all of the dishes will be about the same temperature (roughly 65°C or 149°F).
Two things are obvious if you open the dishwasher as soon as the cycle is completed. Metal and ceramic (including glass) tableware is very hot to the touch and plastic items feel hardly warm at all, despite being at the same temperature. The metal and ceramic dishes show little evidence of water drops, but the plastic is covered with beads of water.
Metal and ceramic surfaces are relatively attractive to water molecules, so water easily forms a thin layer over these surfaces. Plastic is non-polar, so it is unattractive to water molecules, causing them to prefer to cling to each other, forming droplets.
A material’s ability to transfer heat is called its thermal conductivity. Metals have a higher thermal conductivity than plastics. The dishes that feel hot when the dishwasher is opened are very efficient at transferring heat to your fingertips, whereas plastics are not, so plastics keep their heat to themselves. This is why Styrofoam cups keep your coffee hot on the inside but don’t burn your fingers on the outside.
It may sound like sacrilege, but if you drink beer from a plastic “glass” on a hot day, it will stay cold for longer. Or maybe you should just drink it straight from the bottle or can as soon as you get it from the fridge.
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