A simple filtration process helped researchers create flexible, wafer-scale films of highly aligned and closely packed carbon nanotubes.
Scientists at Rice University have made inch-wide films of densely packed, chirality-enriched single-walled carbon nanotubes through a process revealed in Nature Nanotechnology.
In the right solution of nanotubes and under the right conditions, the tubes assemble themselves by the millions into long rows that are aligned better than once thought possible, the researchers report.
NANOTUBES LIKE PICKUP STICKS
The lab discovered the filtration technique in late 2013 when graduate students and lead authors Xiaowei He and Weilu Gao inadvertently added a bit too much water to a nanotube-surfactant suspension before feeding it through a filter assisted by vacuum. (Surfactants keep nanotubes in a solution from clumping.)
The film that formed on the paper filter bore further investigation. “Weilu checked the film with a scanning electron microscope and saw something strange,” He says. Rather than drop randomly onto the paper like pickup sticks, the nanotubes—millions of them—had come together in tight, aligned rows.
A scanning electron microscope image shows highly aligned and closely packed carbon nanotubes gathered into a film. (Credit: Kono Lab/Rice) Click/tap for larger image.
“That first picture gave us a clue we might have something totally different,” He says. A year and more than 100 films later, the students and their colleagues had refined their technique to make nanotube wafers up to an inch wide (limited only by the size of their equipment) and of any thickness, from a few to hundreds of nanometers.
Further experiments revealed that each element mattered: the type of filter paper, the vacuum pressure, and the concentration of nanotubes and surfactant. Nanotubes of any chirality and diameter worked, but each required adjustments to the other elements to optimize the alignment.
The films can be separated from the paper and washed and dried for use, the researchers say.
They suspect multiwalled carbon nanotubes and non-carbon nanotubes like boron nitride would work as well.
The films can be separated from their backgrounds and show potential for use in electronic and photonic applications. (Credit: Jeff Fitlow/Rice)
HOW DO THE TUBES LINE UP?
Coauthor Wade Adams, a senior faculty fellow at Rice who specializes in polymer science, says the discovery is a step forward in a long quest for aligned structures.
“They formed what is called a monodomain in liquid crystal technology, in which all the rigid molecules line up in the same direction,” says Adams, senior faculty fellow in materials science and nanoengineering.
“It’s astonishing. (The late Rice Nobel laureate) Rick Smalley and I worked very hard for years to make a single crystal of nanotubes, but these students have actually done it in a way neither of us ever imagined.”
Why do the nanotubes line up? Kono says the team is still investigating the mechanics of nucleation—that is, how the first few nanotubes on the paper come together.
“We think the nanotubes fall randomly at first, but they can still slide around on the paper,” he says. “Van der Waals force brings them together, and they naturally seek their lowest-energy state, which is in alignment.” Because the nanotubes vary in length, the researchers suspect the overhangs force other tubes to line up as they join the array.
The researchers found their completed films could be patterned with standard lithography techniques. That’s yet another plus for manufacturers, says Kono, professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering.
“I gave an invited talk about our work at a carbon nanotube conference, and many people are already trying to reproduce our results,” he says. “I got so much enthusiastic response right after my talk. Everybody asked for the recipe.”
Coauthors are from Rice, Zhejiang University, and Los Alamos National Laboratory. The Department of Energy and the Robert A. Welch Foundation supported the research which is published in Nature Nanotechnology.
Filter-feeding fish accomplish a feat that human technologies cannot: species including goldfish, menhaden and basking sharks filter tiny algal cells or shrimp-like prey from huge volumes of water without clogging their oral filters.
Since fish have been filtering particles for more than 150 million years longer than human beings, we suspected fish may have evolved filter designs that use unknown processes to remain unclogged. So we decided to investigate.
Our research, recently published in Nature Communications, combines approaches from biomechanics, medicine and ecology to explore how these fish retain and transport prey inside their mouths. Our goal is to provide ideas and data that could improve aquaculture, conservation and industrial filtration.
Crossflow filtration works for fish and industry
Until 15 years ago, we thought that most filter-feeding fish used oral structures called gill rakers in the same way that we use coffee filters or spaghetti strainers. These so-called dead-end sieves force water to pass straight through the pores of the mesh. But dead-end sieves always clog as particles accumulate over time to cover the filter surface.
The water flows right through a colander and leaves the spaghetti trapped on the mesh, but a fish needs to move the food from the gill raker filter to the back of its mouth for swallowing. Dead-end sieves would cause problems for fish, since their gill rakers would clog and fish don’t have a tongue to move food particles off the gill rakers. So we knew they must be using some other filtering technique.
By putting a biomedical endoscope inside the mouths of feeding fish, colleagues and I discovered in 2001 that several common fish species use crossflow filtration instead of trapping particles directly on a dead-end sieve.
During crossflow filtration, small secondary streams of fluid pass through each filter pore – perpendicular to the filter surface, like in dead-end filtration. But the main stream of fluid – the “crossflow” – is directed to travel across (parallel to) the filter surface, lifting particles off the filter and preventing the pores from clogging with particles.
A tilapia illustrating the current model of crossflow filtration, from Sanderson et al., doi: 10.1038/ncomms11092. The mainstream flow (MF) enters from the right and passes across the gill rakers (GR) that are attached to the branchial arches (BA). The mainstream flow carries concentrated particles to the back of the mouth for swallowing. The smaller secondary flows (the filtrate, Fi) pass through the pores of the gill raker filter. Virginia Greene, virginiagreeneillustration.com, CC BY-NC-ND
Through the endoscope, we could see that the main flow of water heading toward the back of the mouth was transporting concentrated particles parallel to the gill raker filter. Less forceful streams of particle-free water exited between the gill rakers. All of these fluid dynamics are caused by the interaction of the water with the physical structures in the fish’s mouth.
We hadn’t expected to see crossflow filtration in fish, though this mechanism had been independently developed by industry a few decades earlier. Crossflow filtration avoids clogging and is often used to filter wastewater, pharmaceuticals, dairy foods and beverages such as beer and fruit juices.
Unfortunately, even industrial crossflow filters still clog eventually. Over time, as water exits through the filter pores, it deposits some particles on the filter. The filters must then be backflushed or cleaned with chemicals, causing a major operating expense.
So we turned again to fish, to see whether millions of years of evolution might have come up with unique crossflow filter designs.
Biomimetic designs from fish mouths
We started our study by examining basic structures inside fish mouths, familiar to fishermen and aquarium hobbyists. Fish gill rakers – the “feeding filters” – are attached to the branchial arches. These arches are bone or cartilage “ribs” inside the mouth that also support the bright red gills for gas exchange. The arches are typically positioned one after another from the front of the mouth back toward the esophagus, where food is swallowed. Scientists hadn’t previously considered the effects these branchial arches could have on patterns of water flow.
For our latest research, we made our own filters by using computer-aided design (CAD) software and 3D printing to create cone-shaped plastic models of fish mouths. We covered the branchial arch “ribs” with a fine nylon mesh.
We based our physical models on paddlefish and basking sharks because their branchial arches form a series of tall ribs that are separated by deep grooves. In our models, each rib served as a backward-facing step that interacted with the crossflow of water traveling over the step.
Almost anywhere that water flows over a backward-facing step, a vortex is created automatically. For this reason, the closely-spaced tall ribs (“d-type ribs”) in these fish mouths aren’t often used by engineers because of the disruptive vortices that form continuously in the grooves between the ribs.
We designed many models with different versions of these backward-facing steps to test the effects of varying characteristics like height and distance between the steps. Interestingly, designs for some microfluidics devices that are used in labs for cell sorting have similar rib-like structures.
Both paddlefish and basking sharks are ram filter feeders that swim forward with a completely open mouth to capture prey. To simulate this kind of feeding, my three undergraduate student coauthors, Erin Roberts, Jillian Lineburg and Hannah Brooks, and I conducted experiments in a flow tank. We submerged our stationary models in a constant stream of water inside the tank. The models “fed” on particles as we adjusted the speed of the water in the flow tank and added particles of different sizes, shapes and densities to the water.
A paddlefish illustrating the new vortical cross-step filtration model, from Sanderson et al., doi: 10.1038/ncomms11092. The mainstream flow (MF) enters from the right and interacts with the series of backward-facing steps that are formed by the branchial arches (BA), causing vortical flow (Vo). The vortex interacts with the gill rakers (GR) to concentrate particles for transport towards the back of the mouth to be swallowed. Virginia Greene, virginiagreeneillustration.com, CC BY-NC-ND
Unique vortical cross-step filtration in fish
Like the spinning of a mini-tornado, water passed over the backward-facing steps inside our models and formed a distinct vortex in the groove between each pair of ribs. We designed accessory structures to control the movement of the vortices by creating regions of the model where the flow couldn’t escape easily. High shear rates around the vortices scoured particles off the mesh, preventing clogging.
Green dye helps visualize the vortices generated in model paddlefish and basking shark mouths. S. Laurie Sanderson, CC BY-ND
We manipulated the vortices to carry particles to the floor of the models, showing that fish could be using this highly adaptable filtration system like a “hydrodynamic tongue” to move particles inside their mouths.
We manipulated the vortices in our models to transport concentrated particles along the vortex axis, downstream from each backward-facing step. The vortices lifted particles from the mesh and carried them toward the floor of the model.
Small preserved paddlefish from an aquaculture company, placed in the flow tank in filter-feeding position, also formed vortices that concentrated particles inside the mouth. This suggests that we’ve correctly identified and modeled structures that are important for generating vortices inside real fish mouths.
This new filtration method, which we term “vortical cross-step filtration,” is effective even when the mesh is damaged or missing from a large portion of the models. Just like fish can continue to feed even when their gill rakers are still growing or are torn, our models can capture particles even when there are large holes in the mesh.
Although we’d identified vortices as a potential mechanism for fish filtration as early as 2001, data on particle capture by vortical flow in fish mouths haven’t been published previously.
Rhodamine dye traces the path of a vortex that forms downstream from a backward-facing step. The step mimics a branchial arch inside a fish’s mouth.
The future of cross-step filtration
Our biomimetic models of paddlefish and basking shark mouths use novel arrangements of engineering structures that harness vortical flow to retain and transport tiny food particles. Cross-step filtration could also apply to filter-feeding ducks, baleen whales and the gill rakers of filter-feeding fish such as manta rays.
Understanding these vortices in fish opens new research directions for engineering improved filters with less clogging, as well as the rapid separation of cells for biomedical tests.
Imagine an aircraft that could alter its wing shape in midflight and, like a pelican, dive into the water before morphing into a submarine.
The key to making this Transformer-like fantasy a reality is a hybrid material featuring stiff metal and soft, porous rubber foam that combines the best properties of both: stiffness when it’s called for, and elasticity when a change of shape is required.
The material also has the ability to self-heal following damage.
“It’s sort of like us—we have a skeleton, plus soft muscles and skin,” says Rob Shepherd, a Cornell University engineering professor. “Unfortunately, that skeleton limits our ability to change shape—unlike an octopus, which does not have a skeleton.”
The idea blends the rigidity and load-bearing capacity of humans with the ability to dramatically alter shape, like an octopus can.
“That’s what this idea is about, to have a skeleton when you need it, melt it away when you don’t, and then reform it,” Shepherd says.
His group’s work was published recently in Advanced Materials.
The metal/foam compound can be heated in order to change its shape, then cooled to regain stiffness. It has potential applications in soft robotics and aeronautics. (Credit: Rob Shepherd group)
HOW IT’S MADE
This hybrid material combines a soft alloy called Field’s metal with a porous silicone foam. In addition to its low melting point of 144 degrees Fahrenheit, Field’s metal was chosen because, unlike similar alloys, it contains no lead.
“In general, we want the things we make in this lab to be potentially biocompatible,” says Ilse Van Meerbeek, a graduate student in the field of mechanical engineering and the first author of the paper.
The elastomer foam is dipped into the molten metal, then placed in a vacuum so that the air in the foam’s pores is removed and replaced by the alloy. The foam had pore sizes of about 2 millimeters; that can be tuned to create a stiffer or a more flexible material.
In testing of its strength and elasticity, the material showed an ability to deform when heated above 144 degrees, regain rigidity when cooled, then return to its original shape and strength when reheated.
AIRCRAFT AND ROBOTS
Shepherd says this material would be the skin for a morphing wing, giving a micro air vehicle (MAV) the ability to become an underwater vehicle on the fly.
“If you have a wing that’s really broad, you can’t do that because the wing will break off when it hits the water,” he says. “So you need to sweep it back, similar to what a puffin does, and then go under water. And using that new shape, it could be a propeller-driven ship.”
In addition to a morphing-wing application, Van Meerbeek sees this material being used in soft robots that must negotiate tight spaces.
“It could be used in search-and-rescue robots,” she says. “It would be able to go into dangerous and/or unpredictable environments, and be able to go through narrow cracks, which rigid robots can’t do.”
“Sometimes you want a robot, or any machine, to be stiff,” adds Shepherd, whose group recently published a paper on electroluminescent skin, which also has applications in soft robotics. “But when you make them stiff, they can’t morph their shape very well. And to give a soft robot both capabilities, to be able to morph their structure but also to be stiff and bear load, that’s what this material does.”
Grants from the US Air Force Office of Scientific Research, the National Science Foundation, and the Alfred P. Sloan Foundation funded the work. The research was published recently in Advanced Materials.
Scientists have discovered a novel way to make plastic from carbon dioxide (CO2) and inedible plant material, such as agricultural waste and grasses. They say the new technology could provide a low-carbon alternative to plastic bottles and other items currently made from petroleum.
“Our goal is to replace petroleum-derived products with plastic made from CO2,” says Matthew Kanan, an assistant professor of chemistry at Stanford University. “If you could do that without using a lot of non-renewable energy, you could dramatically lower the carbon footprint of the plastics industry.”
Kanan and colleagues described their results in the journal Nature.
A PROMISING ALTERNATIVE
Many plastic products today are made from a polymer called polyethylene terephthalate (PET), also known as polyester. Worldwide, about 50 million tons of PET are produced each year for items such as fabrics, electronics, recyclable beverage containers, and personal-care products.
PET is made from two components, terephthalic acid and ethylene glycol, which are derived from refined petroleum and natural gas. Manufacturing PET produces significant amounts of CO2, a greenhouse gas that contributes to global warming.
“The use of fossil-fuel feedstocks, combined with the energy required to manufacture PET, generates more than four tons of CO2 for every ton of PET that’s produced,” Kanan says.
For the Nature study, he and his collaborators focused on a promising alternative to PET called polyethylene furandicarboxylate (PEF). PEF is made from ethylene glycol and a compound called 2-5-Furandicarboxylic acid (FDCA).
“PEF is an attractive replacement for PET, because FDCA can be sourced from biomass instead of petroleum,” Kanan says. “PEF is also superior to PET at sealing out oxygen, which is useful for bottling applications.”
Despite the many desirable attributes of PEF, the plastics industry has yet to find a low-cost way to manufacture it at scale. The bottleneck has been figuring out a commercially viable way to produce FDCA sustainably.
One approach is to convert fructose from corn syrup into FDCA. The Dutch firm Avantium has been developing that technology in partnership with Coca-Cola and other companies. But growing crops for industry requires lots of land, energy, fertilizer and water.
“Using fructose is problematic, because fructose production has a substantial carbon footprint, and, ultimately, you’ll be competing with food production,” Kanan says. “It would be much better to make FDCA from inedible biomass, like grasses or waste material left over after harvest.”
DRAMATIC RESULTS
Instead of using sugar from corn to make FDCA, the team has been experimenting with furfural, a compound made from agricultural waste that has been widely used for decades. About 400,000 tons are produced annually for use in resins, solvents, and other products.
But making FDCA from furfural and CO2 typically requires hazardous chemicals that are expensive and energy-intensive to make. “That really defeats the purpose of what we’re trying to do,” Kanan says.
They solved the problem using a far more benign compound: carbonate. Graduate student Aanindeeta Banerjee, lead author of the Nature study, combined carbonate with CO2 and furoic acid, a derivative of furfural. She then heated the mixture to about 290 degrees Fahrenheit (200 degrees Celsius) to form a molten salt.
The results were dramatic. After five hours, 89 percent of the molten-salt mixture had been converted to FDCA. The next step, transforming FDCA into PEF plastic, is a straightforward process that has been worked out by other researchers, Kanan says.
‘THIS IS JUST THE FIRST STEP’
The approach has the potential to significantly reduce greenhouse emissions, Kanan says, because the CO2 required to make PEF could be obtained from fossil-fuel power plant emissions or other industrial sites.
Products made of PEF can also be recycled or converted back to atmospheric CO2 by incineration. Eventually, that CO2 will be taken up by grass, weeds, and other plants, which can then be used to make more PEF.
“We believe that our chemistry can unlock the promise of PEF that has yet to be realized,” Kanan says. “This is just the first step. We need to do a lot of work to see if it’s viable at scale and to quantify the carbon footprint.”
Kanan and colleagues have also begun to apply their new chemistry to the production of renewable fuels and other compounds from hydrogen and CO2. “That’s the most exciting new application that we’re working on now,” he says. The researchers published their findings in the journal Nature.
Some potholes are like zombies – they never die. Or at least that’s the perception of much of the driving public, especially as we enter peak pothole season: late winter and early spring.
At a minimum, potholes create rough roads and poor driving conditions. Too often they degenerate into vehicle-damaging safety hazards that incur the wrath of drivers, attract negative attention from the news media and adversely affect commerce by disrupting or slowing traffic. Our nation’s deteriorating roads – of which potholes are an obvious manifestation – are a drag on our economy.
That’s why transportation department maintenance crews and pavement engineers continue to stress the need for more effective and efficient pavement repair and maintenance. From their perspective, the ideal repair would last at least a year, could be performed in all seasons, and could be installed easily and relatively quickly – all while keeping traffic delays to a minimum and repair costs down.
Recurring “zombie” potholes are too often a reflection of the type of method that’s used to patch or “fix” them – many of which are short-lived and only marginally effective. Researchers around the world, including my colleagues and me, are working to develop better and longer lasting repair alternatives. At the same time, researchers and pavement engineers in academia and in the private and public sectors are developing improved construction techniques and innovative pavement formulations – such as “self healing” materials – that will help prevent potholes from forming in the first place.
But we still have to deal with the millions of miles of roads – and their potholes – we already have.
Animation describing the birth of a pothole.
Pothole repair is patchy
Potholes are symptomatic of underlying structural and/or pavement problems.
Potholes occur in all climates. Here’s a nasty one on Kauai, where a temporary ‘fix’ was putting large rocks in the hole. Larry Zanko, CC BY-ND
They form when water penetrates a crack. In cold climates, that water freezes and expands, pushing up on the overlying pavement. Repeated freezing and thawing is a great way to create a pothole. Water also softens and weakens the base material below a pavement, making it susceptible to deformation by passing traffic loads. As the base material deforms, the pavement loses structural support and breaks up. And another pothole is born!
Bottom line: a roadway marred by multiple potholes means the pavement is probably failing and should be replaced. But before that can happen, potholes still need to be repaired.
Traditional repair techniques are, at best, temporary fixes. Take “throw-and-go” cold patching methods, for example. These use repair mixtures that are worked when cold. Some cold patch repairs don’t bond well to the edges of the hole, a shortcoming made worse when attempting a repair under challenging wintertime conditions. The same pothole repaired with cold mix patches may have to be redone several times a winter.
Without a good interface bond, the freezing and thawing cycles of late winter and early spring can further weaken the repair and enlarge the crack between the patch and the surrounding pavement.
Another way a repair gets weakened is analogous to hydraulic fracturing, or “fracking.” The tires of vehicles passing over the repair forcefully push liquid water and fine aggregate particles down into the crack, progressively widening any cracks. Road salt further enhances the latter effect by keeping water in liquid form in subfreezing conditions.
Together, it’s all a prescription for a repair’s early demise – hello again, zombie pothole.
Search for better repairs heats up
In an effort to address the ongoing need for better repair mechanisms, my colleagues at the Natural Resources Research Institute – University of Minnesota Duluth, outside project collaborators, and I recently completed a study to evaluate promising innovative pothole repair tactics. Our emphasis is on all-season approaches that use the iron oxide mineral magnetite (Fe3O4).
Magnetite content enhances microwave heating rates. Hopstock, 2009, CC BY-ND
More than 10 years ago, we showed that magnetite and magnetite-containing rock were excellent microwave energy absorbers. The mineral is contained in iron ore rock mined and processed on Minnesota’s Mesabi Iron Range, and has the ability to readily absorb microwaves and heat very quickly. We started thinking: when combined with portable microwave technology, could magnetite-containing materials be an effective solution to cold-weather pothole repair?
We mixed small amounts (1 to 2 percent) of magnetite into patching compound, typically made of recycled asphalt pavement (RAP) augmented with ground-up recycled asphalt shingles (RAS); the RAS adds a little more asphaltic binder to the overall mix. Then we pack the mixture into a pothole and microwave until the binder softens and is compactible, thanks to the magnetite.
Here’s our cookbook recipe (emphasis on cook) for the microwave repairs we performed.
First, find a pothole (easy).
Clean loose debris and/or blow water from pothole.
In subfreezing temperatures, preheat pothole and pavement adjacent to hole with microwave unit to melt or debond any ice or snow in the hole, and to soften the surrounding pavement. This warming contributes to a good bond since the edges of the pothole are able to plasticly intermix – that is, smush together – and meld with the patch material.
Remove or blow out loosened/melted ice/snow.
Place mixture of RAP, microwave-absorbing taconite materials, and RAS into the pothole. Overfill the hole by about two inches to allow for final compaction.
Heat mixture until temperature reaches at least 100°C (212°F) at base of mixture in the hole. Sufficient heating takes place in about 8 to 12 minutes at a 40kW power level.
Tamp down heated mixture with portable gasoline-powered compactor.
Steps to an effective microwave pothole repair. Zanko et al., 2016, CC BY-ND
The existing pavement essentially becomes part of the repair itself – a unique and key benefit of this technique. And our repairs showed excellent longevity, with some performing well more than two years after their installation.
Obviously, we’re not just opening the door of a kitchen microwave oven and pointing it down at the road. In the pilot test, we worked with a small company which had previously developed a truck-mounted microwave system to thaw frozen ground to access buried utilities. They adapted their high-power (50kW), vehicle-based microwave system for the project.
And it wouldn’t be much of a stretch for Minnesota’s taconite industry to supply the relatively small quantities of magnetite a repair compound would consume. The industry typically produces about 40 million tons of magnetite concentrate annually, and generates tens of millions of tons of additional byproduct rock that also contains magnetite. The byproduct taconite rock actually represents a potential source of hard and durable high quality aggregate for our nation’s roads and highways.
Benefits of the pilot pothole patch system
One welcome side effect of the in-place heating mechanism is that it drives off moisture, letting the patch more readily adhere to the surrounding pavement. Patch material can be premixed and stockpiled or mixed on site. And you don’t need to keep material hot during transport, making this patching system well-suited for cold weather situations.
Importantly, our project also demonstrated that an effective microwave pothole repair compound can be made almost entirely from inexpensive and abundant recycled materials (such as RAP and RAS) that many maintenance departments have on hand, as opposed to repair compounds that rely on specialized asphalt formulations, virgin asphalt and/or specialty binders.
Microwave technology is not yet a routine method of repair, and it’s best-suited for potholes in asphalt rather than concrete. But this approach merits further consideration, and we are working to advance the technology.
After all, given our nation’s aging network of roads, zombie potholes will continue to flourish. Microwave repair could be an effective method for keeping them at bay.
A new flexible, stretchable and tunable “meta-skin” uses rows of liquid-metal devices to cloak an object from the sharp eyes of radar.
The meta-skin takes its name from metamaterials, which are composites that have properties not found in nature and that can manipulate electromagnetic waves. By stretching and flexing the polymer meta-skin, it can be tuned to reduce the reflection of a wide range of radar frequencies.
The new technology proves an idea: Electromagnetic waves—perhaps even the shorter wavelengths of visible light—can be suppressed with flexible, tunable liquid-metal technologies.
Jiming Song, a professor of electrical and computer engineering at Iowa State University, was a member of the team that developed the technology. He says the meta-skin could one day coat the surface of the next generation of stealth aircraft.
But the researchers are hoping for even more: a cloak of invisibility.
“The long-term goal is to shrink the size of these devices,” says team member Liang Dong, an associate professor of electrical and computer engineering. “Then hopefully we can do this with higher-frequency electromagnetic waves such as visible or infrared light.
“While that would require advanced nanomanufacturing technologies and appropriate structural modifications, we think this study proves the concept of frequency tuning and broadening, and multidirectional wave suppression with skin-type metamaterials.”
HOW IT WORKS
Rows of split ring resonators are embedded inside layers of silicone sheets. The electric resonators are filled with galinstan, a metal alloy that’s liquid at room temperature and less toxic than other liquid metals such as mercury.
Those resonators are small rings with an outer radius of 2.5 millimeters and a thickness of half a millimeter. They have a 1 millimeter gap, essentially creating a small, curved segment of liquid wire.
The rings create electric inductors and the gaps create electric capacitors. Together they create a resonator that can trap and suppress radar waves at a certain frequency. Stretching the meta-skin changes the size of the liquid metal rings inside and changes the frequency the devices suppress.
Tests showed radar suppression was about 75 percent in the frequency range of 8 to 10 gigahertz, according to the paper. When objects are wrapped in the meta-skin, the radar waves are suppressed in all incident directions and observation angles.
“Therefore, this meta-skin technology is different from traditional stealth technologies that often only reduce the backscattering—i.e., the power reflected back to a probing radar,” the engineers wrote in their paper published in Scientific Reports.
Graphene is the “rock star” of recently discovered materials, but efforts to make 3D objects out of graphene have proven very difficult.
The material, discovered in 2004, is 1 million times thinner than a human hair, 300 times stronger than steel, incredibly lightweight, and it’s the best known conductor of heat and electricity. So, it’s easy to image the potential applications, from basic everyday products all the way up to a space elevator.
Recently, scientists poured graphene oxide suspension, a gel-like form of the material, into freezing molds to create 3D objects. The process works, but only with simple structures that have limited commercial applications.
Another option is to use a 3D printer. In this scenario, scientists typically mix graphene with a polymer or other thickening agent. This helps keep the structure from falling apart. But when the polymer is removed via thermal process, it damages the delicate structure.
A research team may have solved that problem. In the journal Small, they describe how they used a modified 3D printer and frozen water to create lattice-shaped cubes and a three-dimensional truss with overhangs using graphene oxide.
The structures could be an important step toward making graphene commercially viable in electronics, medical diagnostic devices, and other industries.
“Graphene is notoriously difficult to manipulate, but the structures we built show that it’s possible to control its shape in three-dimensional forms,” says Chi Zhou, assistant professor of industrial and systems engineering at the University at Buffalo and a corresponding author of the study.
[Continue to the next page to learn how they did it and see the VIDEO about the process…]
Since ancient times, people have experimented with light, cherishing shiny metals like gold and cutting gemstones to brighten their sparkles. Today we are far more advanced in how we work with this ubiquitous energy.
Starting with 19th-century experimentation, we began to explore controlling how light interacts with matter.
Combining multiple materials in complex structures let us use light in new ways. We crafted lenses and mirrors to make telescopes to peer out into the universe, and microscopes to explore the world of the small.
Today this work continues, on a much more detailed level. My own research into what are called “metamaterials” explores how we can construct materials in ways that do amazing – and previously impossible – things.
We can build metamaterials to respond in particular ways to certain frequencies of light. For example, we can create a smart filter for infrared cameras that allows the user to easily determine if the white powder in an envelope is baking soda or anthrax, determine if a skin melanoma is benign or malignant and find the sewer pipe in your basement without breaking through the concrete. These are just a few applications for one device; metamaterials in general are far more powerful.
Working with light
What scientists call “light” is not just what we can see, but all electromagnetic radiation – from low-frequency radio waves to high-frequency X-rays.
Normally, light moves through a material at a slower speed. For example, visible light travels through glass about 33 percent slower than it does through air. A material’s fundamental resistance to the transmission of light at a particular frequency is called its “index of refraction.” While this number changes with the light’s frequency, it starts at 1 – the index of refraction for a vacuum – and goes up. The higher the index, the slower the light moves, and the more its path bends. This can be seen when looking at a straw in a cup of water (see below) and is the basis of how we make lenses for eyeglasses, telescopes and other optics.
Scientists have long wondered if they could make a material with a negative index of refraction at any given frequency. That would mean, for example, that light would bend in the opposite direction when entering the material allowing for new types of lenses to be made. Nothing in nature fits into this category. The properties of such a material – were it to exist – were predicted by Victor Veselago in 1967.
These odd materials have properties that look very strange compared with our everyday experiences. In the picture below, we see two cups of water, each with a straw in it. The picture on the left is what happens normally – the section of the straw in the water appears disconnected from the part of the straw that is in the air. The image is displaced because air and water refract light differently.
The image on the right indicates what the straw would look like if the fluid were a material with a negative index of refraction. Since the light bends in the opposite direction, the image is reversed, creating the observed illusion.
At left: normal refraction. At right: with simulated negative refraction. Water glass with straw (normal) from shutterstock.com
While Veselago could imagine these materials in the late 1960s, he could not conceive of a way to create them. It took an additional 30 years before John Pendry published papers in 1996, 1998 and 1999 describing how to make a composite man-made material, which he called a metamaterial.
An early metamaterial using repeating elements of copper split-rings and copper wires. D. R. Smith et al., Left-handed Metamaterials, in Photonic Crystals and Light Localization, ed. C. M. Soukoulis (Kluwer, Netherlands, 2000)., CC BY-ND
Making metamaterials
This work was followed up experimentally by David R. Smith’s group in 2000, which created a metamaterial using copper split-rings on circuit boards and lengths of copper wires as repeating elements. The picture below shows one such example produced by his group. The size and shape of the split-rings and copper posts determines what frequency of light the metamaterial is tuned to. The combination of these components interacts with the incident light, creating a region with an fully engineered effective index of refraction.
At present, we are only able to construct metamaterials that manage interactions with very specific parts of the electromagnetic spectrum.
The electromagnetic spectrum, showing all types of light, including the narrow band of visible light. Philip Ronan, CC BY-SA
Smith’s group worked initially in the microwave portion of the spectrum, because working with larger wavelengths makes metamaterial construction easier, as multiple copies of the split-rings and pins must fit into the space of one wavelength of the light. As researchers work with shorter wavelengths, metamaterial components need to be much smaller, which is more challenging to build.
Since the first experiments, multiple research groups have made metamaterials that work in the infrared; some are skirting the fringe of the visible portion of the spectrum. For these short wavelengths, circuit boards, copper wires and pins are far too large. Instead the structures have to use micro- and nano-fabrication techniques similar to what is used to make computer chips.
Creating ‘invisibility’
Soon after the first metamaterials were fabricated, researchers began engineering applications for which they would be useful. One application that got a lot of press was the creation of an “invisibility cloak.”
Normally if a microwave radar were aimed at an object, some of the radiation would absorb and some would reflect off. Sensors can detect those disturbances and reconstruct what the object must have looked like. If an object is surrounded by the metamaterial cloak, then the radar signal bends around the object, neither being absorbed nor reflected – as if the object were never there.
By creating a metamaterial layer on the surface of an object, you can change what happens to the light that hits the object. Why is this important? When you look at a still pool of water, it is not surprising to see your reflection. When you point a flashlight at a pond at night, some of that light beam bounces off onto the trees beyond.
Now imagine you could coat the surface of that pond with a metamaterial that worked for all the visible spectrum. That would remove all reflection – you wouldn’t see your own reflection, nor any light bouncing into the woods.
This type of control is very useful for determining specifically what type of light can enter or exit a material or a device. For example, solar cells could be coated with metamaterials that would admit only specific (e.g., visible) frequencies of light for conversion to electricity, and would reflect all other light to another device that collects the remaining energy as heat.
The future of wave engineering
Engineers are now creating metamaterials with what is called a dynamic response, meaning its properties vary depending on how much electricity is passing through it, or what light is aimed at it. For example, a dynamic metamaterial filter might allow passage of light only in the near infrared, until electricity is applied, at which point it lets through only mid-infrared light. This ability to “tune” the responsiveness of metamaterials has great potential for future applications, including uses we can’t yet imagine.
The amazing thing about all of the wondrous possibilities of metamaterials’ interaction with light is that the principle works much more broadly. The same mathematics that predict the structure needed to produce these effects for light can be applied to the interaction of materials with any type of waves.
A group in Germany has successfully created a thermal cloak, preventing an area from heating by bending the heat flow around it – just as an invisibility cloak bends light. The principle has also been used for sound waves and has even been discussed for seismic vibrations. That opens the potential for making a building “invisible” to earthquakes! We are only beginning to discover how else we might use metamaterials and their underlying principles.
These tiny creatures are encased within a hard shell shaped like a wide, flattened cylinder—like a tambourine—that is made of silica. Known as diatoms, they are single-celled algae organisms, around 30 to 100 millionths of a meter in diameter, that are ubiquitous throughout the oceans.
Researchers in the lab of Julia Greer, professor of materials science and mechanics at Caltech, have recently found that these shells have the highest specific strength—the strength at which a structure breaks with respect to its density—of any known biological material, including bone, antlers, and teeth.
The shell, or frustule, of a diatom is porous, perforated by a honeycomb-like pattern of holes. There are several theories about the function of these intricate shell designs, including that they evolved to control fluid flow, for example, or to help the organisms acquire nutrients.
Greer and her group propose that the holes also act as stress concentrators—”flaws” in the material that can suppress the propagation of cracks, which would lead to failure of the entire organism.
“Silica is a strong but brittle material. For example, when you drop a piece of glass, it shatters,” says Greer. “But architecting this material into the complex design of these diatom shells actually creates a structure that is resilient against damage. The presence of the holes delocalizes the concentrations of stress on the structure.”
The group plans to use design principles from diatoms to create resilient, bioinspired artificial structures.
Scientists have created a new composite material that is half liquid, but it doesn’t leak when cracked.
And it can quickly heal, over and over. Like a sponge, it returns to its original form after compression.
The material called SAC (for self-adaptive composite) consists of what amounts to sticky, micron-scale rubber balls that form a solid matrix. Researchers at Rice University made SAC by mixing two polymers and a solvent that evaporates when heated, leaving a porous mass of gooey spheres.
Other “self-healing” materials encapsulate liquid in solid shells that leak their healing contents when cracked.
“Those are very cool, but we wanted to introduce more flexibility,” says Pei Dong, a postdoctoral researcher who co-led the study with Rice graduate student Alin Cristian Chipara. “We wanted a biomimetic material that could change itself, or its inner structure, to adapt to external stimulation and thought introducing more liquid would be a way. But we wanted the liquid to be stable instead of flowing everywhere.”
They think the material could be a useful biocompatible material for tissue engineering or a lightweight, defect-tolerant structural component.
Continue on to see a fascinating video showing how this novel self-healing substance works.
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