Stephen Tindal asked:
I sometimes like a nice warm bath in the winter.† After a while, the bath gets colder.† I know there are a few ways that the bath can lose heat, but does a thick layer of bubbles on the top make any significant†difference to the heat retention?
We put this to Professor Eugene Terentjev from the Cavendish Laboratory at Cambridge University.
Eugene - Basically Steven asks this working on the assumption that a layer of bubbles would thermally insulate the top surface of the water, like a bubble wrap would insulate a hot or cold object. This is correct - it will reduce the heat loss through the top surface, both in straight diffusion of heat into the air and in water evaporation, which is a very efficient mechanism of cooling by phase transition. Of course, one has to have very stable long lived bubbles in this form covering the water surface, which can be achieved by the use of good surfactants and salts, although I don't know how good for your health all that would end up being! But my instinct tells me that both of these mechanisms of heat transmission and water evaporation off the top surface, whether insulated or not, are minor in the overall process of a bath cooling down. Letís conduct an imaginary experiment and cover the top surface of a hot bath with many layers of bubble wrap. I don't think the rate of its cooling will much diminish. I think the direct contact of water with the solid mass of the bath tub body is a much more efficient sink of heat. This is because the transfer of heat into the low density air is generally very poor. In fact, it is air filling the bubbles that is supposed to be the thermal insulator in Stevenís original question, and we know how thin air cavities inside wool hairs make wool clothes so warm.
Diana - As techmind said on our forum, a layer of bath bubbles does indeed keep the bath warmer for longer but be wary of the old style cast iron bars. They're elegant, and will tend to conduct heat away from your water faster than a more modern acrylic bath will. But from pondering to practice, hereís Dave Ansell to explain what he found experimentally.
Dave - To test this on a slightly smaller scale, I took two identical washing up bowls, filled them up 10 centimetres deep with water from a hot water tap, then I took some bubble bath, put it in one of them, frothed it up so you have a nice layer of bubbles over the surface, and left the other one. I then put two thermocouples, one in each bowl in exactly the same place then I waited to see what happens to the temperature. They both started at 44 degrees centigrade and after an hour, the one with bubbles was at 38 degrees centigrade, but one without any bubbles was down to 34 degrees centigrade. And when I looked at all the results, pretty much all the time, the one with bubbles was losing heat at just slightly more than half the rate of the one without any bubbles, which means that the bubbles are actually working as a very, very effective insulator. I think this result should work on a bath as well: on a bath, the surface is probably going to be slightly more important because relatively, there's less side compared to top and bottom on a bath, and I was using about the same depth of water as in a shallow bath. So you should find exactly the same thing happening when you bathe.
I would have thought that the bubbles would significantly reduce heat loss from the top surface... but if you have an (older type) enamelled metal bath then you lose a lot of heat to the bath metal - which is very significant in a poorly heated house in winter! (it helps to rinse the bath out with a smaller quantity of hot water -and wipe it around a lot- before emptying and filling the bath "for real")
http://www.thenakedscientists.com/forum/index.php?topic=26133.msg279920#msg279920 RD, Mon, 8th Aug 2011
The bath has been always in a container that loses heat. I have created a thermos sheet that creates a bath on the bed. This with my micro Carbonated Bubbles Gives a 8 hour bubble bath. Why would you want that, incase you fall asleep. morning will come with you still warm and comfortable in a nourishing bubble bath. made of custom ingredients, like coconut oil, cherry oil, and mild sea salts. Whether you choose a 30 minute or an al nighter the comfort is amazing. Never heard of this product,? It is because we are just in the process of launching it into review. I have designed this for the bedridden, but everyone that tries it wants to do it again. Nursesbath will be soon. Rich Cordero, Fri, 19th Aug 2011