Heat and water and making nappies

In the lab, my summer student has been working on a small device to keep a small piece of equipment at a stable temperature. It uses a Peltier device – in essence it's a solid-state heat pump. Pass through current one way, and heat is drawn from the top surface to the bottom; pass current through the other, heat is drawn from the bottom to the top. Therefore, by putting the equipment on the top surface of the peltier device, we can control how much we heat or cool it by via how much electric current (and in which direction) we pass through. 

There are a few things we need to consider, however, to get this to work well. One is the thermal resistance between the peltier device and the object. We need there to be a good thermal contact between the two, otherwise the flow of heat is going to be hampered. It would be rather like putting insulation around radiators in your house. It will keep the radiators nice and warm but it won't do much to the temperature inside your house. We need to ensure that the glue we use to hold our equipment to the Peltier has high thermal conductivity.

But also we are interested in knowing how quickly the equipment changes its temperature in response to heat input. This is quantified by its heat capacity – how much energy (heat) is required to raise its temperature by a given amount. Something with low heat capacity will change its temperature quickly, something with a high heat capacity will change its temperature only slowly.  A large lump of something, like the water in the university swimming pool, has a large heat capacity, and therefore takes a long time to heat up once it's been filled (and consequently remains very cold until January). Do we want our equipment to have a high or low heat capacity? That's not  entirely obvious. Our aim is for something that remains at fairly stable temperature – that neither heats up nor cools down quickly. Otherwise controlling the temperature becomes very difficult. That would suggest a high heat capacity for it. But we don't want it too big or our Peltier Device would never be able to bring it up to the temperature that we'd like. There's a bit of a balance to be had here.

What struck me this week was the obvious parallel with nappies. Well, I guess it's obvious to any physicist who changes nappies on a regular basis. The perfect nappy needs to take urine away from the skin quickly, and also have a high capacity to hold it. The first task is equivalent to the thermal conductivity, but with water. The fluid needs to be able to flow quickly from the skin to the absorbing bit of the nappy. The second task is the equivalent of the heat capacity – we need the material to absorb lots of water while not getting very wet (equivalent to absorbing lots of heat but not raising its temperature very much). The cloth nappies we use have a two different material textures. The first bit, that is in contact with the skin, sucks water away very quickly. The second part holds onto the water very well. Working together, they keep baby dry for longer, which sounds like a rather corny tag line for a nappy brand. 

And, yes, I've taken a clean dry nappy to the bathroom with a measuring jug and slowly poured water in to see exactly how much one would hold. Could you expect a physicist to do anything else?



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