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A panel that can turn water into steam, without boiling, through the magic of plasmonics

2016-04-11
Reading the paper this morning was depressing.  Politics and politicians, big business, war, insurgency, and terrorism.  That is why I prefer to read upbeat news sources like Science News, New Scientist, Scientific American, etc.  Immersing oneself in science helps one to take the long view, which is always optimistic because of entropy, which always seems to be running in reverse.  In fact, I have a theory that we have the theory of entropy backwards.  But enough about me.

Science News has an exciting item about what I believe is called a “metamaterial”– in this case, an alumina screen with of 300 nanometer (average) pores 650 nanometers apart partially filled with gold nanoparticles of a range of sizes, about 90 nanometers thick.  The size range is chosen so that the gold absorbs light from 400 angstroms (near-ultraviolet) to 10 micrometers (mid-infrared) with 99% effectiveness.  The material is said to be self-assembling by “physical vapor deposition” of gold ions in a very low pressure vapor yielding a random range of gold nanoparticle sizes, which saves greatly on manufacturing costs.  It is so light that it floats on water.

This material appears to be very dark, blacker than any printed black.  When a body of water covered by a layer of this material is exposed to sunlight, it heats water to the point that it evaporates– without boiling.  Its greatest advantage besides its relative cheapness of production is that it can be used with seawater and the evaporated water is essentially pure.  The potential uses of this material are tremendous, most especially to desalinate seawater.

The work that went into creating this material did not proceed in a vacuum.  Much development preceded it; gold, silver, platinum, and other metals were etched with thin parallel lines to a very fine grid using focused ion beams or electron beam lithography.  Many different configurations were tried in an attempt to achieve maximum absorption of light at minimal thickness.  None of those materials were as efficient in absorbing light as this one, which has the added advantage of being self-assembling.  The work is described as creating “ideal absorbers” and the effect produced by these very tiny holes (400 angstroms wide and 165 angstroms apart) partially filled with gold nanoparticles is called “plasmonics.”  The new material changes light into heat to vaporize water with 90% efficiency, much better than the 30% efficiency of current photovoltaic cells.  This is not quite as efficient as some of the other materials that have been tried including carbon nanotubes, but it is much cheaper to manufacture.

The steam produced by sunlight can be condensed into fresh water, or it can be used to operate a steam engine, or even to generate electricity.  There is no mention of what happens to the salt left behind when saltwater is evaporated by this process.  If it stays in the water, then eventually the brine left behind would be too concentrated to boil.  If it is left on the surface of the panels, then it could clog up the pores very quickly.  Clearly, this is a subject for further evaluation.

The effect that absorption of light produces, localized heating at the pores of the material, is called “localized surface plasmon resonance.”  A plasmon is a “quasiparticle” consisting of a quantum of plasma oscillation, an oscillation of the electron density in a conducting medium, described as an instability in the dielectric function of a free electron gas.  Yes, I was unprepared for the complexity of this “explanation” but I found it in Wikipedia so it must be true (insert irony here.)  Plasmons were first described in 1952 by physicists David Pines (the founding director of the Institute for Complex Adaptive Matter and still living at age 91) and his doctoral advisor David Bohm.

This quote from Wikipedia reveals more hidden depths; Bohm worked on the Manhattan Project during WWII but left the US after he was investigated for his Communist affiliations: “Bohm warned of the dangers of rampant reason and technology, advocating instead the need for genuine supportive dialogue which he claimed could broaden and unify conflicting and troublesome divisions in the social world. In this, his epistemology mirrored his ontological viewpoint.  Due to his youthful Communist affiliations, Bohm was the subject of a federal government investigation in 1949, prompting him to leave the United States. He pursued his scientific career in several countries, becoming first a Brazilian and then a British citizen.”

To get back to the subject at hand: The research behind this new material was developed in laboratories at  Nanjing University in China and the State University of New York in Buffalo. The original research publication is in the Science Advances section of Science magazine and is apparently free to read, so you don’t have to have a subscription to Science News if you want to wade into the details.  This is a very exciting development, and even if I don’t understand plasmons, I can see great potential for saving the world if it can be produced at scale in time to ameliorate global warming.

I think Robert Heinlein (or was it Isaac Asimov?) said “Any sufficiently complex process appears to be magic to the uninitiated.”

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