CNT INVISIBILITY CLOAK

Metamaterials are no longer the only option for invisibility. For years that technology has been the source of hope for some future invisibility cloak, but so far only certain frequencies of light can be affected. Carbon nanotubes, on the other hand, can be used to bend all frequencies of visible light right now. Researchers at the University of Texas have used a sheet of carbon nanotubes to artificially create the mirage effect.

The mirage effect can often be seen on hot days when the sun heats paved roads. Sometimes you can see a puddle of water far down a road, but you know it is just an illusion. How does it happen? The air in contact with the pavement is much hotter than surrounding air. This temperature difference bends light so that instead of seeing the road, you see a warped image of the sky (which closely resembles reflecting water). Wherever there is an extreme temperature difference, there will be this mirage effect. The metallic bodies of cars can effectively heat the air around them, but carbon nanotubes (CNTs) are better. When an electric current passes through a sheet of CNTs, heat is quickly produced and dissipated. In the demonstration video, water was used instead of air. Like flicking a switch, the mirage effect can be activated and deactivated at will. This is a very convenient quality of the technology.

The CNTs seem a preferable alternative to metamaterials. The main drawbacks are excessive heat and possibly limited angles of cloaking. On the positive side, CNTs are much more durable than metamaterials and can bend all colors at once. Metamaterials have difficulty bending more than one color of light at a time. Indeed, it is hard to make a metamaterial that works with visible light at all. Rather than a small sheet, imagine a blanket of CNTs. With the press of a button that blanket becomes a personal wall of invisibility. With some advanced insulation on the inner side, the blanket might be able to hide a person, small car, or other item of interest. The main obstacles to this dream are the high cost of producing CNTs and supplying the energy necessary for what is essentially a high-tech electric heater.

GLOWING PLASTIC THAT STRETCHES

Finally, a major step towards flexible display screens has been made. UCLA has developed a stretchy sheet of plastic that glows blue when given an electric current. It is a combination of carbon nanotubes and polymer layers. The future possibilities for this kind of technology are endless. The first thing that comes to my mind is a super-portable, resilient computer screen. Imagine a newspaper that plays videos, and you get the idea. As smartphones become more and more capable, it might be convenient to keep an external monitor in your pocket. As you sip your morning coffee, you could pull out a folded-up sheet of plastic. Unfold it, plug it into your iPhone, and have the iPhone download the latest news stories to be displayed like an old-fashioned newspaper. Better yet, go to YouTube, and you have videos playing on a high-definition display. If touch-sensitivity was included, then you could fold the sheet in half for a makeshift laptop. The flexible screen would serve as keyboard and monitor, and the iPhone would do all the computing.

There are other exciting applications, too. Think about safety—construction workers and other people exposed to traffic dangers. Instead of vests that reflect light, they could wear ones that glow in the dark like neon signs. Traffic cones that glow, too. Clothes with computer displays would come in handy. Again, smartphones might be crucial. A jacket covered with the flexible screen could enable a user to access a smartphone without taking it out of its pocket. Connected to a smartphone, a [smart-jacket?] could show the time of day on its sleeve, like a watch. If the user wants to dial a number, then the sleeve shows a keypad. For self-expression or advertising purposes, the front and back of the jacket become personal billboards. The jacket might even be used for camouflage. Imagine wallpaper and curtains that, once wirelessly connected to your computer, can illuminate a room from all angles. What strange lighting effects that might produce! Selecting a picture or video instantly turns an entire wall into a panoramic vista. Interior design would be radically changed by walls that can change color and pattern at the press of a button. Do you have any ideas?

A FOUNDATION OF BUCKYTUBES

Carbon has a wide range of macroscopic forms—coal, ash, graphite, diamond. A relatively new form of carbon, nanotubes, is becoming a revolutionary material in many ways. Carbon nanotubes (CNTs), or buckytubes, are microscopic cylindrical cages of carbon atoms. Each nanotube is essentially a giant molecule with a diameter of about one nanometer and usual length of several millimeters. Multi-walled CNTs (which consist of nanotubes inside larger ones) have very little friction between layers. Individually a CNT has a greater strength to weight ratio than steel and can conduct heat and electricity better than copper.

So far the main use for CNTs has been to improve other materials. Adding them to mixture-based materials improves strength and durability, such as making crack-resistant concrete. On their own, however, carbon nanotubes can be much more impressive. Imagine a cable made of carbon nanotubes that extends many miles from the ground to a space station in geosynchronous orbit. Such a lengthy cable made of any other material would break from its own weight if not the tension of securing the space station. The incredible tensile strength of CNTs might make a cable of that magnitude possible. The cable would allow a “space elevator” to climb up into the heavens without need of rockets. Wouldn't it be incredible to ascend into outer space without the huge rocket engines but with a hi-tech elevator?

The electrical properties of CNTs make them attractive for improving electronic products. Recent research indicates that nanotubes could be used to speed up DNA sequencing. Chinese researchers have discovered that a sheet of CNTs can be used as a speaker. When a varying electrical current is applied, the CNTs heat up nearby air accordingly. The changing temperature causes rapid changes in pressure which we hear as sound. Normal speakers use vibration to produce changes in air pressure, and they can be rigid, fragile, and bulky. This new speaker, however, is as thin as paper, flexible as fabric, and turns transparent when stretched. Stretching, bending, and even cutting the CNT sheet into different shapes will not significantly affect sound quality.

Solar panels made with carbon nanotubes are also thin, flexible, and lightweight. These unique properties may one day lead to the replacement of today's thick, heavy, and fragile silicon solar panels. CNTs can not only improve how we obtain energy, but they can help us store it. Paper embedded with CNTs constitutes a “paper battery”. It looks and feels like black paper, but it can store electricity and function at greater temperature extremes than many conventional batteries. Medical researchers are interested in this technology because paper batteries can be powered by blood or urine. Imagine a pacemaker that is charged by blood passing through the heart.

While CNTs have astounding properties, there are some major hurdles in the way of success. The extraordinary capabilities are undermined by defects which can be difficult to prevent at the nano-scale. Over the last ten years the cost (per gram) has gone from $500+ to $50. This decreasing trend is promising, but for now it is still too expensive to produce CNTs in large quantities. About large quantities: it is still very difficult to make nanotubes longer than a few centimeters.