Low-reflection, nanostructured wings make butterflies nearly invisible

Researchers of KIT under the direction of Hendrik Hölscher found that irregular nanostructures on the surface of the butterfly wing cause the low reflection. In theoretical experiments, they succeeded in reproducing the effect that opens up fascinating application options, e.g. for displays of mobile phones or laptops. The results are published in the current issue of (DOI: 10.1038/ncomms7909). Irregularity of the size and distribution of nanostructures on the surface of the butterfly wing causes low reflection of light at all view angles. (Photo: Radwanul Hasan Siddique, KIT) Transparent materials such as glass, always reflect part of the incident light. Some animals with transparent surfaces, such as the moth with its eyes, succeed in keeping the reflections small, but only when the view angle is vertical to the surface. The wings of the glasswing butterfly that lives mainly in Central America, however, also have a very low reflection when looking onto them under higher angles. Depending on the view angle, specular reflection varies between two and five percent. For comparison: As a function of the view angle, a flat glass plane reflects between eight and 100 percent, i.e. reflection exceeds that of the butterfly wing by several factors. Interestingly, the butterfly wing does not only exhibit a low reflection of the light spectrum visible to humans, but also suppresses the infrared and ultraviolet radiation that can be perceived by animals. This is important to the survival of the butterfly. For research into this so far unstudied phenomenon, the scientists examined glasswings by scanning electron microscopy. Earlier studies revealed that regular pillar-like nanostructures are responsible for the low reflections of other animals. The scientists now also found nanopillars on the butterfly wings. In contrast to previous findings, however, they are arranged irregularly and feature a random height. Typical height of the pillars varies between 400 and 600 nanometers, the distance of the pillars ranges between 100 and 140 nanometers. This corresponds to about one thousandth of a human hair. In simulations, the researchers mathematically modeled this irregularity of the nanopillars in height and arrangement. They found that the calculated reflected amount of light exactly corresponds to the observed amount at variable view angles. In this way, they proved that the low reflection at variable view angles is caused by this irregularity of the nanopillars. Hölscher’s doctoral student Radwanul Hasan Siddique, who discovered this effect, considers the glasswing butterfly a fascinating animal: “Not only optically with its transparent wings, but also scientifically. In contrast to other natural phenomena, where regularity is of top priority, the glasswing butterfly uses an apparent chaos to reach effects that are also fascinating for us humans.” The findings open up a range of applications wherever low-reflection surfaces are needed, for lenses or displays of mobile phones, for instance. Apart from theoretical studies of the phenomenon, the infrastructure of the Institute of Microstructure Technology also allows for practical implementation. First application tests are in the conception phase at the moment. Prototype experiments, however, already revealed that this type of surface coating also has a water-repellent and self-cleaning effect.

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Surface matters: Huge reduction of heat conduction observed in flat silicon channels

Combining state-of-the-art realistic atomistic modelling and experiments, the paper describes how thermal conductivity of ultrathin silicon membranes is controlled to large extent by the structure and the chemical composition of their surface. A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. The ability of materials to conduct heat is a concept that we are all familiar with from everyday life. The modern story of thermal transport dates back to 1822 when the brilliant French physicist Jean-Baptiste Joseph Fourier published his book “Théorie analytique de la chaleur” (The Analytic Theory of Heat), which became a corner stone of heat transport. He pointed out that the thermal conductivity, i.e., ratio of the heat flux to the temperature gradient is an intrinsic property of the material itself. The different circles represent the studied surfaces of the Si membranes: crystalline, rough, flat with native SiO2, and rough with native SiO2. The right image shows a representative thermal map on the membranes upon a localized thermal excitation used to measure the thermal conductivity. The advent of nanotechnology, where the rules of classical physics gradually fail as the dimensions shrink, is challenging Fourier's theory of heat in several ways. A paper published in ("Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering") and led by researchers from the Max Planck Institute for Polymer Research (Germany), the Catalan Institute of Nanoscience and Nanotechnology (ICN2) at the campus of the Universitat Autònoma de Barcelona (UAB) (Spain) and the VTT Technical Research Centre of Finland (Finland) describes how the nanometre-scale topology and the chemical composition of the surface control the thermal conductivity of ultrathin silicon membranes. The work was funded by the European Project Membrane-based phonon engineering for energy harvesting (MERGING). The results show that the thermal conductivity of silicon membranes thinner than 10 nm is 25 times lower than that of bulk crystalline silicon and is controlled to a large extent by the structure and the chemical composition of their surface. Combining state-of-the-art realistic atomistic modelling, sophisticated fabrication techniques, new measurement approaches and state-of-the-art parameter-free modelling, researchers unravelled the role of surface oxidation in determining the scattering of quantized lattice vibrations (phonons), which are the main heat carriers in silicon. Both experiments and modelling showed that removing the native oxide improves the thermal conductivity of silicon nanostructures by almost a factor of two, while successive partial re-oxidation lowers it again. Large-scale molecular dynamics simulations with up to 1,000,000 atoms allowed the researchers to quantify the relative contributions to the reduction of the thermal conductivity arising from the presence of native SiO2 and from the dimensionality reduction evaluated for a model with perfectly specular surfaces. Silicon is the material of choice for almost all electronic-related applications, where characteristic dimensions below 10 nm have been reached, e.g. in FinFET transistors, and heat dissipation control becomes essential for their optimum performance. While the lowering of thermal conductivity induced by oxide layers is detrimental to heat spread in nanoelectronic devices, it will turn useful for thermoelectric energy harvesting, where efficiency relies on avoiding heat exchange across the active part of the device. The chemical nature of surfaces, therefore, emerges as a new key parameter for improving the performance of Si-based electronic and thermoelectric nanodevices, as well as of that of nanomechanical resonators (NEMS). This work opens new possibilities for novel thermal experiments and designs directed to manipulate heat at such scales.

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Nanoscientists model atomic structures of three bacterial nanomachines

esearchers at UCLA’s California NanoSystems Institute have become the first to produce images of the atomic structures of three specific biological nanomachines, each derived from a different potentially deadly bacterium — an achievement they hope will lead to antibiotics targeted toward specific pathogens. The scientists used a leading-edge technology called cryo electron microscopy, or cryoEM, to reveal the form and function of these important structures. Papers on their findings were published in three top-tier journals: , , and . Two of the nanomachines are structures called contractile ejection systems, which their bacteria use to transfer toxic molecules into healthy cells to usurp them for their own purposes, to attack rival bacteria by delivering toxins into them, and other functions. These structures have sheath–tube assemblies that create openings in the outer membranes of target cells through which they can insert toxic molecules. The third nanomachine — different from the other two — is a pore structure that delivers deadly anthrax toxin into mammalian cells, once the anthrax bacteria is in the bloodstream. This mechanism is how anthrax bacteria activate the disease in an infected animal or person. How the nanomachines work had been poorly understood, but the UCLA researchers used a cryoEM equipped with a special camera called a direct electron detector to produce highly detailed images. The scientists hope the new information about how they function will enable them to create antibiotics that target bacterial pathogens. The team, led by Hong Zhou, professor of microbiology, immunology and molecular genetics, and of chemistry and biochemistry, runs the Electron Imaging Center for Nanomachines laboratory, which is based at CNSI and houses UCLA’s Titan Krios electron microscope — a highly sophisticated and rare cryoEM. “As the centerpiece of our electron microscopy core lab, the cryo electron microscope is enabling exploration of new territory in molecular biology,” said Jeff Miller, director of the California NanoSystems Institute. “These unprecedented images enable us to understand the actual workings of these remarkable structures.” Anthrax toxin In a paper published online by ("Atomic structure of anthrax protective antigen pore elucidates toxin translocation"), Professor Zhou and his team reported that they were the first to determine the atomic structure of the anthrax toxin pore, the major disease molecule of Bacillus anthracis, the bacterium that causes the disease anthrax in humans and animals. The anthrax toxin pore’s atomic structure is mushroom-shaped with a gate inside the “shaft.”

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The finding confirms how the disease affects cells. When healthy cells encounter nanoscale objects in the body, they assume the objects are nutrients and absorb them. Like a Trojan horse, the toxin pore appears to the cells as something beneficial — in this case, a nutrient — and is taken in by the cell. But once inside the cell, the pore senses the change to a more acidic environment, which opens the pore’s gate and releases the anthrax toxin molecule into the cell. “This is a very important step toward understanding this mechanism, and it is essential for any anthrax countermeasure,” Zhou said. “It also informs our understanding of the mechanisms of other toxins that function like anthrax, which could lead to other targeted antibiotic drugs.” Tularemia type VI secretion system Another nanomachine was described by Dr. Marcus Horwitz, a UCLA professor of medicine and of microbiology, immunology and molecular genetics, who worked with Zhou’s team. In a study published in the journal ("Atomic Structure of T6SS Reveals Interlaced Array Essential to Function"), the scientists reported the first atomic resolution model of any type VI secretion system, or T6SS, a nanomachine found in roughly 25 percent of gram-negative bacteria. Gram-negative bacteria are responsible for diseases such as cholera, salmonellosis, Legionnaires’ disease and melioidosis, and severe infections including gastroenteritis, pneumonia and meningitis. For the new study, the scientists examined Francisella tularensis, a bacterium that causes tularemia and is of great concern as a potential bioterrorism agent. Built from component proteins, the T6SS nanomachine has an atomic structure that resembles a piston. When F. tularensis is taken up into a type of white blood cell called a macrophage it is surrounded by a bubble-like membrane, a structure known as a phagosome. The T6SS nanomachine then assembles inside the bacterium, where it plunges a tube through the bacterial wall and the membrane of the phagosome into the cytoplasm, the substance inside the macrophage. This enables the bacterium to escape the phagosome into the cytoplasm, where it can complete its lifecycle and multiply. Soon, the macrophage fills with bacteria and ruptures, freeing the bacteria to infect other cells. Thus, the T6SS is a novel target for antibiotics against this bacterium, and against others that use it to survive within host cells or to combat rival bacteria. “We are already identifying drug molecules that target the F. tularensis T6SS,” Horwitz said. “Knowing how this structure works guides us in selecting drug molecules that block its assembly or function. The overall goal is to find new antibiotics that directly target this top-tier bioterrorism agent and other gram-negative bacteria with a T6SS such as Vibrio cholerae, Pseudomonas aeruginosa, Burkholderia pseudomallei, and pathogenic Escherichia coli.” Horwitz and his team could potentially also develop wider-spectrum drugs that work on many different gram-negative pathogens that have in common a T6SS. Pseudomonas aeruginosa In humans and animals, a bacterium called Pseudomonas aeruginosa causes infectious diseases that lead to generalized inflammation and sepsis, a dangerous infection of the blood. A team led by Zhou and Miller discovered the atomic structures of R-type pyocins, contractile ejection systems of Pseudomonas aeruginosa. Their findings were published online by ("Atomic structures of a bactericidal contractile nanotube in its pre- and postcontraction states"). R-type pyocins are used by the bacterium to rapidly insert their nanotubes, like battering rams, into the cell membranes of competing bacteria to kill the competitors, giving Pseudomonas aeruginosa easier access to nutrients. These pyocins appear to create a channel in the outer envelope of the target bacteria, which essentially acts to weaken and kill it. This ability has made R-type pyocins the focus of research into possible antimicrobial and bioengineering applications, and scientists believe they could be engineered to give drugs a powerful antibacterial component.

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“The R2 pyocin is an extraordinary molecular machine that uses energy from its own biological battery to function,” said Miller, who also is a professor of microbiology, immunology and molecular genetics. “It is ideal for engineering targeted antibiotics that kill the bad bacteria without disrupting a patient’s protective gut bacteria.” The scarcity of the technology and the expertise needed to use it make CNSI one of the world’s few facilities capable of imaging atomic structures like these nanomachines at atomic-level resolution, which is why researchers from around the world come to UCLA to use the Electron Imaging Center for Nanomachines, a fee-for-service laboratory open to any scientist in academia or industry.

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