River Wissey Lovell Fuller


October 2008

The doctor wonders why plastic does not dry in a dish washer

CONUNDRUM OF THE MONTH: Why does plastic not dry in the dishwasher?

Those of us who are fortunate enough to have dishwashers of the electrical variety must have suffered the plastic problem. You see some Tupperware or plastic food storage boxes on offer and bring them home. You put them in the dishwasher with your other crockery and start the cycle. An hour or two later, you have a dishwasher with clean and sparkling crockery and a lot of plastic boxes half full of very mucky water which spills over the other contents of the dishwasher as you open the door. OK, you realise, the boxes are light and flip over during the wash, half filling with water. The next time, you jam the plastic boxes in the machine such that they cannot flip over and you end up with a dishwasher full of empty but very wet plastic boxes. So, why do plastic items not dry when all the ceramic and metal items end up both dry and shiny?

The problem is explained by heat capacity and heat conductivity. All the items in the machine are subject to the same amount of water and to the same temperatures inside the machine. The difference lies in the character of the materials being washed. When a dishwasher gets hot at the end of the cycle, it relies upon stored heat in the washed items to gradually evaporate the surface water, leaving the items dry. Plastic tends to be light and thin and to have a very low heat capacity; thus, it has little stored heat and cannot evaporate the water lying on its surface. A plate or a mug might have ten or more times as much heat stored in it as a plastic mug, so it has enough heat to evaporate its surface water so that it dries. Heat conductivity is also important. If an upside-down china mug has some water pooled in its base, the heat held within the walls of the cup can be conducted to the base where it will help to evaporate the pool of water. A plastic mug, however, has very low heat conductivity so the water in the base of the mug will not be evaporated by conducted heat.

The thermal conductivity of a ceramic plate is easy to demonstrate another way. Put a piece of bread on a plate and put it in the oven. They both end up at the same temperature but you can easily pick up the bread, which is a poor conductor of heat. However, you cannot touch the plate because it is a good conductor of heat and will easily transfer its heat to your hand! The thermal tiles on a space shuttle are very light and porous - a very poor conductor of heat. The outside can be really hot but the inner surface is cool to touch, thus protecting the shuttle.

I found a website giving suggestions about what could be cleaned in a dishwasher, apart from dishes. Hubcaps and wheel trims, baseball hats (must be an American website) non-electrical toys made of hard plastic, flip-flops, sandals and those new Croc type shoes (I've tried this and it was very successful), shin pads, glass, metal and plastic shelving (the plastic will be wet!) hand tools and garden tools, dustpan and brush and the front of electric fans. They also suggest washing keyboards "Pop off the keys and put them in the utensil container. Wrap the USB in plastic to keep it dry and lay the keyboard face down on the top rack. The keyboard will be squeaky clean and germ free". Having killed a keyboard stone dead with about a quarter of a teaspoon of spilled coffee, I am highly sceptical about this last idea. Perhaps they mean musical keyboards which might be different or perhaps they are just "having a larf".- Don't try this one at home, folks!

DailyCandy, a popular lifestyle website, has created a book of modern words which do not exist but, perhaps, should:

Altarcation - A heated dispute between a couple regarding the details of their wedding.

Amoraphobia - The fear of falling in love.

Biodebatable - The questionable products that claim to be green.

Carbonara footprint - Deleterious effect of over-indulgence in creamy pasta dishes

Drailing - Drunken E-Mailing

E-mnesia - The condition of having sent or received an e mail and having no recollection of it whatsoever.

Haitress / Haiter - Angry waitress / waiter

Jingle Belles - The exposed cleavage of women who dress inappropriately at Christmas parties.

Rockafella - A groom who chooses a ring for its size and ostentation.

SCUM - Self Centred Urban Male who only looks up from his laptop if a pretty girl walks by.

Yellular - The habit of shouting into a mobile 'phone.

John the Laptist - The man in front of you who reclines his aeroplane seat until his head is in your lap.

My favourite, which is not in the above list, is Testiculation - waving your arms around while talking b*ll*cks.

Best wishes to you all

Ian G. Nisbet

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