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Metal at 400 degrees underwater

Sun, 16th Sep 2012

Chris Smith

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Adding a water-repelling layer to a metal ball means it can sit submerged without boiling explosively, scientists have discovered. 

Writing in Nature, Northwestern University researcher Neelesh Patankar and his colleague Ivan Vakarelski, coated 20mm diameter stainless steel balls with a commercially-available material called "Glaco Mirror Coat".

This superhydrophobic layer repels water and is stable to temperatures of over 400C. Compared with control, untreated balls heated to this temperature and dropped into cold water where they produced a fierce explosive boiling, the superhydrobic balls - in Petankar's own words - "did nothing".  In fact, fast-frame video footage of the experiments show that the balls become surrounded with a layer of water vapour.

Image of a 2 cm heated steel sphere cooling in boiling water.Apart from being a relatively poor heat conductor, this prevents the liquid touching the ball surface, so it cannot boil off. 

The researchers think that the process works because the applied coating consists of nanoscale ridges and grooves.  Liquid, they speculate, sits atop the ridges but cannot flow down into the grooves owing to the hydrophobicity of the surface.  So water vapour fills the groove instead, keeping the liquid at bay.

Why this is important is that explosive boiling occurring when liquids contact hot surfaces can be problematic in some industries.  Being able to prevent this could be a major processing advantage.



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Could this tech also be used to recover water from exhaust gases, without having to cool the total gas flow? ... I know the effect appears to be the 'wrong way round' for this, but the having the selectively 'move around' heat - must have many uses....

Perhaps micropore filters could be selectively coated with this type of surface, then H20 can go through some holes but not others (?). peppercorn, Mon, 17th Sep 2012

I would do the experiment on sub hulls and ship hulls.

Perhaps the hydrophobic channels would make it like running the vessel in an air pocket, and reduce he water resistance.  Also, potentially making an unpleasant environment for barnacles.

Would it reduce the sonar signature?

Repainting would be a pain though. CliffordK, Mon, 17th Sep 2012

Actually, the paper explores both superhydrophobic and superhydrophilic surfaces and Neelesh Patankar, the co-author, told me that what has been aluded to above is precisely what they are seeking to do. The interesting thing here is the ability to control the phase changes, in both directions. chris, Mon, 17th Sep 2012

The more I think about this means for selectivity in gas flows (with mixed phase-transition), the exciting it seems in improving a whole host of thermodynamic processes! peppercorn, Thu, 20th Sep 2012

No burn cookery? yamo, Sun, 23rd Sep 2012

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