It is not your imagination. And your dishwasher is not about to pack up.
Modern dishwashers are pretty effective at drying what's inside them after the end of the cycle, except when it comes to one thing: plastic.
Though hand-powered dishwashers have been around since the middle of the 19th Century, the dishwasher's true renaissance came after World War Two, first in commercial kitchens and then the home, alongside the growth of standardised kitchen cupboards and counters. Come the 1970s, they became much more affordable, and much more common.
But so did something else: plastic utensils and food storage containers.
Ever since, emptying a dishwasher has meant dealing with dripping plastic containers, no matter how hot the temperature of the dishwasher. It is an issue which had confounded dishwasher builders for decades. And good old-fashioned thermodynamics is at least partly to blame.
Because most plastic cookware isn't as dense as crockery or metal utensils, it loses heat much quicker.
"Plastic plates and cutlery are significantly lighter than ceramic or stainless steel items and the specific heat capacity is round about the same, so there is less stored heat in the plastic," says Roger Kemp, a fellow of the Royal Academy of Engineering and emeritus professor of engineering at the University of Lancaster. The stored heat remaining in the metal or porcelain keeps them at a slightly higher temperature than the air, aiding evaporation.
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"With plastic, there is not enough stored heat to keep it a significantly higher temperature than the general temperature of the dishwasher, so the water on it doesn’t evaporate. There are similar problems with washing thin aluminium food trays because they are very light and, consequently, are poor at storing heat.”
There's another physical force at work here as well – "surface energy". Water placed on a plastic surface will form a bead, whereas it will form a thin layer on glass. The lower surface energy of glass means the water will be more attracted to it. As it spreads it becomes thinner, and more likely to lose some of its mass to evaporation.
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