Dishwasher drying

Why do plastic containers come out of a dishwasher wet when the glasses beside them are dry?

Open the machine an hour after it finishes. The glasses are dry enough to put straight in the cupboard. The plastic tub sitting on the same rack, through the same wash, the same rinse, the same hour of waiting, is covered in droplets.

The usual answer is that plastic is water-repellent. That is true, and it is not the main thing. If it were, the plastic would come out with a few large beads on an otherwise dry surface — which is not what you see. You see something wetter than it has any right to be, sometimes wetter than the cabinet wall next to it. So something is putting water onto it.

Ask first where the drying energy comes from. Most European machines have no fan and no heater running at the end; the cycle simply stops after a hot final rinse and the door stays shut. There is no external supply. The only energy available to turn the film of water on a plate into vapour is the heat that plate carried out of the rinse in its own body.

So the whole question becomes an accounting one: how many joules did each item bank, per square centimetre of wet surface it must dry?

And evaporation is expensive. Turning a gram of water to vapour at these temperatures takes roughly 2,400 joules, about 550 times what it costs to warm that same gram by one degree. Even a thin film is a real bill.

Here the obvious answer fails a second time. "Plastic has a low heat capacity" is, as usually stated, wrong: polypropylene stores about 1.9 kilojoules per kilogram per degree, glass only about 0.8. Per kilogram, plastic is the better store. But glass is roughly three times denser, so per unit volume the two land close together — and volume is not the right unit either.

The right unit is thickness behind the wet surface. A tumbler carries three or four millimetres of glass behind every wetted square centimetre. A food container carries about one millimetre of plastic. Multiply through and the glass leaves the rinse holding several times the heat per unit of water it has to evaporate, which is enough to boil its own film away while the plastic runs out of energy almost immediately.

Now the second half, and this is what makes the plastic actively wet rather than merely undried. The cabinet is a sealed box full of vapour, mostly evaporated off the glass and crockery. Vapour condenses on whatever is coldest. The stainless steel tub is meant to be that surface — it is cooled from outside by room air and what condenses on it runs down and drains away. But the plastic, having banked almost no heat, reaches cabinet temperature faster than anything else in there. It becomes a condenser too. Water leaves the glasses and lands on the plastic.

Only now does surface chemistry earn its place. On clean glass the film sheets out and runs off under gravity; on a low-energy plastic surface it beads, and a bead has a small footprint and no inclination to run. That is exactly what rinse aid addresses — it is a surfactant, lowering the water's surface tension so the film sheets instead of beading.

Three things worth pinning down.

The first is that "heat capacity" as a bare phrase misleads here, and the tag on this idea should be read carefully. What matters is stored energy per unit of wetted area, which is heat capacity times density times wall thickness. Two of those three favour glass, and the third — thickness — favours it heavily. This is also why thin-walled stainless cutlery dries well despite a modest specific heat: it is hot, it is dense, and it holds very little water.

The second is that this is not the same effect as a tiled floor feeling colder than a carpet. That one is about how fast a surface pulls heat out of your skin, a question of conductivity. This is about how much heat an object banked and how much water it must spend it on — a stock, not a rate.

The third is a set of honest limits. Machines with a heating element in the base, or with a zeolite drier that releases heat as it adsorbs moisture, do supply external energy at the end, and on those the plastic problem is much reduced though rarely absent. Position matters too, and one common complaint is not this mechanism at all: water pooled in the upturned rim of a lid never evaporated or condensed anywhere, it simply failed to drain, and that is gravity rather than thermodynamics.

• How a zeolite drier releases usable heat simply by adsorbing water vapour.
• Why the same stored-heat argument decides which windscreen in a car park frosts first.

Every term the collection defines is gathered in the glossary.