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A pink object under the cloak becomes invisible. Credit: SMART Center.

Last week I mentioned that someone has found a new, cheap way to optically “hide” objects (other than the metamaterials route). Via Gizmag, I heard about the the work of two groups who, in parallel, are using calcite crystals to make objects seem to disappear — and its one of those things where you immediately say, “Why didn’t I think of that?”

Calcite, boron nitride, silicon carbide and other crystals (and some plastics) are known for having birefringence (aka, double refraction). Briefly put, birefringence causes a ray of light to split into two rays. The property is already used in LCDs and other optical and electronics applications.

What’s novel is the two groups — one from the SMART Center and the other collaboration between researchers at University of Birmingham (U.K.), Imperial College, London and Technical University of Denmark — is to put two prism-shaped pieces of calcite next to each other, aligning their optical axes. If the resultant wedge (the SMART group used a 38mm X 10mm X 2mm wedge) is then put over an object, it appears to disappear when viewed from either side of the wedge, and the two-dimensional effect works for macroscopic objects “larger than 3500 free-space wavelengths, inside a transparent liquid environment.  Its working color range encompassing red, green and blue light has also been demonstrated.”

The U.K./Denmark collaborative group also were able to achieve the effect in air. One of these researchers, Shuang Zhang, lead investigator from the University of Birmingham’s School of Physics and Astronomy, predicted bigger things ahead. He says:

“By using natural crystals for the first time, rather than artificial metamaterials, we have been able to scale up the size of the cloak and can hide larger objects, thousands of times bigger than the wavelength of the light. Previous cloaks have succeeded at the micron level (much smaller than the thickness of a human hair) using a nano- or micro-fabricated artificial composite material. It is a very slow process to make these structures and they also restrict the size of the cloaking area. We believe that by using calcite, we can start to develop a cloak of significant size that will open avenues for future applications of cloaking devices.”

The SMART groups work is published in Physical Review Letters, and the U.K./Denmark group’s work is published in Nature Communications.

Physics World was particularly impressed with the SMART Center’s work, naming it one of the “Top 10 breakthroughs of 2010.”

Here is a video from featuring Zhang showing how the calcite crystal can make part of a panda disappear:

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A recent paper from GE Global Research and MIT mechanical engineering researchers casts doubt on the effectiveness of superhydrophobic surfaces ability to block ice formation on aircraft, wind turbines, communications towers and other applications where frost frequently appears.

The authors of the paper, which is published in Applied Physics Letters, note that tests done during ice formation studies on how supercooled water acts on superhydrophobic surfaces appear to have been based on spraying or pouring the water. While this provides some important information, they it is incomplete and may mask more serious dangers, viz., it doesn’t take into account the process of frost formation (i.e., ice formation without going through a liquid phase).

The bad news is that, based on their microscopy experiments looking at frost nucleation, growth and adhesion, they believe that the icephobic properties of superhydrophobic surfaces are questionable. In fact, they say that ice adhesion can actual increase “wherever frost can form indiscriminately on the surface.”

“In-flight ice accretion on aircraft surfaces is usually attributed to the freezing of supercooled water droplets suspended in clouds that come into contact with aircraft surfaces. However, recent studies show that icing clouds could be unexpectedly supersaturated resulting in heterogeneous ice nucleation. Hence, frost formation could also be an important in-flight ice accretion mechanism on aircraft surfaces. Therefore, it is important to consider frost formation while designing icephobic surfaces and extreme caution must be exercised in the use of superhydrophobic surfaces for icephobic surface treatments on ground and in-flight applications.”

The good news is that they say the insights from their studies suggestion new designs for better anti-icing surfaces.

“[A]pproaches that can spatially control nucleation (e.g., promote nucleation on top portions of the texture to form [Cassie-Baxter state] ice) . . . could reduce ice adhesion and improve the robustness of textured surfaces for icephobicity.”

The reference to Cassier-Baxter state is explained in the diagram below. A water droplet on a solid surface and surrounded by a gas forms a characteristic contact angle θ. If the surface is rough, and the liquid is in close contact with the droplets, the droplet is in what is known as the “Wenzel” state, which promotes ice formation. If the liquid rests on the tops of the asperities, it is in the “Cassie-Baxter” state, which discourages ice formation.

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At the end of each week, I end up with a list of a bunch of stories I started to write about, or started to investigate or didn’t even get that far even though the topic looked intriguing, but, I had a meeting to go to …

Anyway, it’s Friday, and rather than have these stories evaporate into the ether, I’ve close out each week by providing some raw links to some of these orphan tales. Check ‘em out:

Ceramic foam for efficient thermal insulation

Scientists create a multi-tool for working with nanoparticles

Needle-free, painless vaccinations with nanopatches

Spheric Technologies produces lithium battery materials
cheaper, faster with microwave; filed for two patents (PDF)

Making Better Photonic Devices (video)

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PV panels on the Mars rover Spirit were covered with dust over a two-year period. Credit: NASA/JPL.

Self-cleaning surfaces aren’t a particularly novel idea, and self-cleaning glass commercial products made by companies, such as Saint Gobain, have been around in various products for at least five years – but at a premium price. These technologies use a TiO₂ coating to photocatalyze organic dust that is then washed away by humidity and rain. However, for some dusty (e.g., nonorganic) materials, even this self-cleaning system doesn’t work.

The cost and performance problems with these existing systems are unfortunate, especially when it comes to photovoltaic solar panels and mirrors, particularly when one considers that many utility-scale solar energy systems are being located in desert areas that are prone to large amounts of non-organic dust. In some of these regions, even dragging out a hose or water truck to rinse off PV panels and mirrors is not practical nor economically feasible.

The effects of the dust on these solar energy system are tangible. “A dust layer of one-seventh of an ounce per square yard decreases solar power conversion by 40 percent,” explains MIT visiting professor Malay K. Mazumder. “In Arizona, dust is deposited each month at about four times that amount. Deposition rates are even higher in the Middle East, Australia and India.”

Mazumder knows something about dust. He has worked with NASA on a similar but more difficult problem: Extraterrestrial dust. When the problem is dust on surfaces somewhere lacking Earth’s atmosphere and weather – say, Mars or the moon – terrestrial technology just won’t cut it. Lunar dust was nasty stuff for astronauts to deal with and is described as tiny pieces, sharp and interlocking pieces of glass or coral that is everywhere on the lunar surface.

According to NASA, Mars dust isn’t quite so bad, but still a big problem:

Dust is also ubiquitous on Mars, although Mars dust is probably not as sharp as moon dust. Weathering smooths the edges. Nevertheless, Martian dust storms whip these particles 50 m/s (100+ mph), scouring and wearing every exposed surface. As the rovers Spirit and Opportunity have revealed, Mars dust (like moon dust) is probably electrically charged. It clings to solar panels, blocks sunlight and reduces the amount of power that can be generated for a surface mission.

NASA knew that dust interference with solar panel function could be catastrophic for Mars missions. Working with the agency, Mazumder and other researchers developed a novel self-cleaning solar panel technology for use in lunar and Mars missions.

Now, Mazumder says the time has come to apply the same technology on earth. “Solar panels powering rovers and future manned and robotic [NASA] missions must not succumb to dust deposition. But neither should the solar panels here on Earth,” he says

Mazumder describes the technology he has in mind as having three parts. The first part is thin layer of transparent, electrically sensitive material on the glass or plastic covering of a solar panel. The second part is a sensor to monitor dust levels on the surface of the panel. The third part is a system to send a brief electric charges over the surface of the panel. Because, like the stuff on moon and Mars, most Earth dust carries an electrical charge, delivering alternating electric fields acting through the thin layer on the panel dislodges, carries and deposits dust particles off and away from surfaces.

According to a news release, Mazumder says a two-minute process removes about 90 percent of the dust deposited on a solar panel. Further, his approach requires only a small amount of electric power, which can easily be supplied by the panel.

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At the end of each week, I end up with a list of a bunch of stories I started to write about, or started to investigate or didn’t even get that far even though the topic looked intriguing, but, I had a meeting to go to …

Anyway, it’s Friday, and rather than have these stories evaporate into the ether, I’ve close out each week by providing some raw links to some of these orphan tales. Check ‘em out:

Retrograde melting property of silicon demo’d by MIT team may yield new purification process

Haier Power Pad takes energy from shower water and returns it to hot water system

Study: Solar power is cheaper than nuclear

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