Published online 4 July 2008 | Nature | doi:10.1038/news.2008.935

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Hot rods make boiling better

Kettle lined with tiny copper rods enables faster bubbling.

copper nanorodsThe nanorods trap air, allowing tiny bubbles to form easily.Rensselaer/Koratkar

For a faster, more bubbly boil, try adding a layer of copper nanorods to the inside of your kettle.

Researchers led by Nikhil Koratkar at the Rensselaer Polytechnic Institute in Troy, New York, have discovered that lining a copper pot with copper nanorods makes water boil much more quickly. The discovery is described in the journal Small1.

Water can only boil at the point at which it meets air. This interface allows a phase transition — in this case a change from a liquid to a gas — to take place. If the interface isn’t there, the hot liquid, despite being at or above its boiling temperature, has nowhere to go, and so becomes 'superheated'.

It is possible to make a pot boil from the bottom; if a tiny defect is introduced in the base of a vessel, small bubbles can form there. These bubbles, or air pockets, provide the interface needed for a phase transition to happen, and the liquid can boil. If there is also access to air at the top, this means that the water can boil from both top and bottom.

Bubblin' hot

This bubbling from the bottom effect is normally achieved with micrometre-sized defects in the metal. Nanometre-scale objects were thought to be too small to let stable bubbles form. “Classical theories for boiling predict that bubbles should not nucleate from nanopores due to the very high surface tension forces at that scale,” says Koratkar.

The team's experiments used a copper surface covered in nanorods up to 50 nanometres in diameter and placed this in a liquid chamber. The rods had a dramatic effect on bubble formation: Koratkar saw 30 times more bubbles forming on his copper nanorod-lined surface than on a surface made from just copper. Consequently the time taken for the liquid to boil plummeted.

Koratkar and his team didn’t measure the time to boiling directly, rather he measured the amount of water that was converted into vapour per unit time at the heating wall. This was 6 times greater for the copper nanorod-coated surface than the plain copper surface.

Working together

It is the interaction between the micro- and nano- that causes the improved boil, says Koratkar. Between the nanorods are billions of tiny cavities, these nanopores can trap air in nanobubbles and feed them into the slightly larger microcavities present in the copper base layer. Here larger bubbles form, allowing the water to boil.

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The nanopores constantly feed the microcavities, preventing them from being flooded by water and becoming inactive. “The mechanism for the enhanced boiling is fascinating since it involves a synergistic coupling between two entirely different length scales,” says Koratkar.

The technology could have many applications, says David Kenning, an expert in heat transfer from Brunel University in Uxbridge, UK. He suggests that the technology could be used in chemical processing to prevent 'bumping' when solvents are boiled in very smooth surfaced flasks. Koratkar says the nanorods could be used in kettles to dramatically reduce energy usage. More hi-tech applications could include cooling integrated circuits and computer chips, which use copper as connects. 

  • References

    1. Li, C. et al. Small advance online publication, doi:10.1002/smll.200700991 (2008).
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