[nudephil (OP)]

Here's a question from Naked Scientists listener John:

When I put sheets or duvet covers in my tumble dryer, they inevitably wind themselves up into a big ball! Thus seems to happen however carefully I place them in the dryer. Why is this?

Any insights into sheet/duvet physics? There's probably some overlap with the famous headphones in pockets effect...

[Soul Surfer]

I presume your annoyance is that If they form a hard ball and roll around in the dryer they will not dry properly or take much longer.

I can see three possibilities. 

Washing machines can cause things to form a ball which is flattened out by the spin drying process but is still in effect a knotted mess.  It is important to separate them out and put them in individually or this could continue.  This is particularly easy if you use a washer dryer and run the process straight through from washing to drying without a quick sort out before starting the drying.

The second possibility is that you are overloading the dryer.  For a washer dryer or a domestic sized dryer the drying load is usually half that for the washing process.  This is because the clothes need more room to fall through the air than be agitated when heavy and wet during the washing process.  Commercial dryers are much bigger and give the material much more room to fall through the warm air.

Finally you have a very badly designed dryer which should include reverse rotation processes to help to prevent forming lumps like this.

[keyjpb]

Sheet/duvet cover put in dryer on its own.

Tumble dryer is made by Miele so should be well designed. It's happed with previous machines as well (AEG, Creda).

You're right that if a sheet/duvet cover is dried in a commercial tumble dryer with a much bigger drum, the sheet can 'fall' through the air more easily, and therefore doesn't get tangled, but why, in a smaller drum does the 'balling' occur?