Physicists at the Fraunhofer Institute have designed a coffee mug that cools your coffee down to the ‘ideal’ temperature and keeps it there for about a half hour. From the article:
The key for this magic trick is physics and PCM—phase change material—an extraordinary substance used in construction and winter clothing. PCM is capable of storing and releasing heat or cold.
In its original state it is a solid but. Then, when it receives heat, it absorbs it like a sponge liquifying into a gooey wax. As the PCM solidifies it releases the energy at a steady pace, keeping any liquid or room at the perfect temperature. In houses, they achieve this by filling hollow walls with PCM, which absorbs heat from the sun, and then releases it as the atmosphere cools down, keeping the room at a perfect temperature.
It seems to me that all materials change phase. If you wanted to keep the liquid at 0 degrees Celsius, you could use water as a ‘PCM’. But that doesn’t sound as impressive…