Scientists from MIT, including one of Indian origin, have found that water can freeze solid even at high temperatures inside tiny carbon nano-tubes. This study could pave the way for use of ice-filled wires to power electrical devices.
Scientists, including Kumar Varoon Agrawal from Massachusetts Institute of Technology in the US, have found that water can freeze even at high temperatures that would normally set it boiling in carbon nano-tubes whose inner dimensions are not much bigger than a few water molecules.
Water starts to boil at a temperature of 100 degrees Celsius. Scientists have long known that when water is confined in very small spaces, its boiling and freezing points can change, usually dropping by abound 10 degrees Celsius.
The discovery illustrates how even very familiar materials can drastically change their behavior when trapped inside
structures measured in nanometres, or billionths of a meter.
The finding might lead to new applications – such as essentially, ice-filled wires – that take advantage of the unique electrical and thermal properties of ice while remaining stable at room temperature.
In one of the tests, the water solidified at a temperature of 105 degrees Celsius or more.
The way water’s behavior changes inside the tiny carbon nano-tubes – structures the shape of a soda straw, made entirely of carbon atoms but only a few nanometres in diameter – depends crucially on the exact diameter of the tubes.
In the experiments, the nano-tubes were left open at both ends, with reservoirs of water at each opening.
Even the difference between nano-tubes 1.05 nanometres and 1.06 nanometres across made a difference of tens of degrees in the apparent freezing point, the researchers found. Such extreme differences were completely unexpected.
In earlier efforts to understand how water and other fluids would behave when confined to such small spaces, “there were some simulations that showed really contradictory results,” said Michael Strano, professor at MIT.
Part of the reason for that is many teams were not able to measure the exact sizes of their carbon nano-tubes precisely, not realizing that such small differences could produce such different outcomes, he added.
In fact, it’s surprising that water even enters into these tiny tubes in the first place, Strano said.
Carbon nano-tubes are thought to be hydrophobic, or water-repelling, so water molecules should have a hard time getting inside.
The fact that they do gain entry remains a bit of a mystery, he said. The study was published in the journal Nature Nanotechnology.